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Is complete linkage found in all four chromosomes or only Y-chromosome of Drosophila?


Male Drosophila shows complete linkage. Is it observed for all four chromosomes or only the Y chromosome?


All chromosomes. There is no crossing over in males in Drosophila. Check out the related question Why doesn't recombination occur in male Drosophila? for sources and context.


Linkage of Genes : Complete Linkage, Incomplete Linkage and Significances | Biology

If linkage is complete, there should be parental combinations only and no recombination. Morgan (1919) reported a complete linkage in Drosophila. When ordinary male wild fly with grey body and normal wings was crossed with female having black body and vestigial wings, in F1, hybrids were all grey bodied and normal winged (with dominant characters).

But when F1, male is back crossed with recessive female parent only two types of individuals in F2 generation were produced instead of expected four. These two types were grey bodied and normal winged and black bodied and vestigial winged in equal number, thus indicating the complete linkage.

But how do genes located on same chromosome assort independently? It may be due to exceptions in Mendel’s second law of independent assortment or due to some mechanism for genes on the same chromosome to separate and recombine during meiosis. Both the cases have been observed.

Incomplete linkage:

Incomplete linkage produces new combinations of the genes in the progeny due to the formation of chiasma or crossing over in between the linked genes present on homologous chromosomes. When in sweet peas a cross is made between blue flower and long pollen (BBLL) with red flower and round pollen (bbll) in F1 expected blue flower and long pollen (BbLl) heterozygous condition is got. (Fig. 5.47).

Fig. 5.47. A case of incomplete linkage.

However, test cross between blue and long (BbLl) and double recessive (bbll) gave blue long (43.7%), red round (43.7%), blue round (6.3%) and red long (6.3%). The parent combinations are 87.4% are due to linkage in genes on two homologous chromosomes, while in case of new combinations (12.6%) the genes get separated due to breaking of chromosomes at the time of crossing over in prophase-I of meiosis. New combinations in the progeny appeared due to incomplete linkage (Fig. 5.47).

Significance of Linkage:

1. Linkage does not permit the breeders to bring the desirable characters in one variety.

2. Linked characters are maintained for generations because linkage prevents the incidence of recombination.

Sometimes a few qualitative characters such as grain colour, leaf shape, corolla colour etc., get linked to some quantitative characters such as number of flowers, weight of seeds, lint length. There genes are called as marker genes. In cotton, corolla colour is marker for lint index. In rice, grain colour is marker for its yield. Marker genes are also found in plants like Sorghum, Zea etc. Marker genes are of great significance to plant breeders.


Is complete linkage found in all four chromosomes or only Y-chromosome of Drosophila? - Biology

The phenomenon of permanent association of genes of a single chromosome that can be inherited in successive generations in same position and proportion without any changes or separation is called linkage.

Mendel did not notice the phenomenon of linkage because he chose characteristics controlled by genes located on different chromosomes. This phenomenon of linkage was noticed by post- Mendelian geneticists. It was firstly discovered by Bateson and Punnet (1906) while working on sweet pea (Lathyrus odoratus)in which they found two pairs of alleles did not assort independently. Sutton and Boveri first suggested linkage in 1903 when they first suggestion linkage when they propounded "Chromosomal Theory of Inheritance." The term 'Linkage' was coined by T.H. Morgan and he proposed "chromosomal theory of linkage" by working on Drosophila 1910. According to this theory, the linkage is defined as the phenomenon of inheritance of genes together and retain their parental combination even in the offspring.

Linked and unlinked genes

The genes present in the single chromosome are inherited together are called linked genes. Unlinked genes are those genes which are found on the different chromosome. Those characters which are controlled by linked genes are called linked characters.

Unlinked Genes

They are found on the same chromosome.

They show independent assortment.

Dihybrid phenotypic ratio is 9:3:3:1.

The test cross ratio in dihybrid cross is 1:1:1:1.

Linkage group

Genes situated on the chromosome are said to be linked. The genes on the single chromosome form a linkage group. A linkage group usually passes into a gamete and are inherited together. Linkage group in a cell equals to the pair of chromosomes present in the cell of an organism. But it should be noted that the number of linkage group is restricted to the haploid number of chromosomes of an organism. The total number of linked genes present in a single chromosome from the linkage group. The linkage group can be determined by:

number of linkage group= number of haploid chromosomes in an organism,

Example- In Drosophila-2n = 8&rArr n=4&rArr n= 3+xy&rArr 3+x+y&rArr 5

In human female- 2n=46&rArr n=23&rArr n=22+xx&rArr 22+x (homomeric/gametic) &rArr 23

Chromosomal Theory of Linkage-

Morgan formulated the chromosomal theory of linkage according to which-

1)Genes lie in linear order in the chromosomes and distance between them is variable.

2)The genes that are linked, stay on the same chromosome.

3)The tendency of genes to remain together in their parental combination is due to their presence on the same chromosome.

4)The strength of linkage is directly related to the distance between the linked genes on a chromosome. The closer the genes are located, stronger is the linkage.

Types of linkage

1) Complete linkage

2) Incomplete linkage

1) Complete linkage -The phenomenon in which the genes present in a single chromosome do not separate and are inherited together in the successive generations due to the absence of crossing over is called complete linkage. Hence it produces only parental combination but not non-parental ones. It is very rare in nature.

A cross between Drosophilawith gray body- normal wings [GGNN] and black body-vestigial wings [ggnn] -

The above cross produces F1 hybrids with the gray body- normal wings [GgNn]. When these F1 hybrids are crossed with the recessive parent having a black body- vestigial wings (test-cross), it produces two types of offspring in equal proportion (50% - 50%) in F2 generation. These F2 offspring resemble their grandparents.

Hence, the gray-body character is inherited with normal wings and black body character is inherited together with vestigial wings. It means these genes are linked genes and no non-parental combination are formed due to the absence of crossing over.

Fig: Complete linkage

From the above cross - F2 ratio= gray body normal wings= 50%

and, black body vestigial wings= 50%

2) Incomplete linkage -The phenomenon in which linked genes present in the same chromosome have a tendency to separate due to crossing over and forms both parental and non-parental combination in the F2generation is called incomplete linkage. It can be illustrated in maize grains.

When colored-full seed [CCFF] of maize is crossed with colorless-shrunken seed [ccff] of maize-

The above cross produces F1hybrids with coloured-full seeds [CcFf] of maize. When F1 hybrid female is crossed with recessive parent i:e colorless shrunken male, they produce both parental and non-parental combination with four phenotypes in the ratio of 1:1:1:1 i:e 1 colored-full : 1 colored-shrunken : 1 colorless-full : 1 colorless-shrunken seed. This is possible due to the incomplete linkage.

Fig: Incomplete linkage

From the cross above the F2 phenotypic ratio is = 1:1:1:1.

Significance of linkage-

The phenomenon of linkage has great significance for living organisms, because-

1) It reduces the possibility of variability in gametes.

2) The number of linkage groups is equivalent to number of chromosomes present in genome,

3) It is useful for maintaining the good characters in new variety,

4) Linked character is preserved for successive generations because linkage prevents the incidence of crossing over.

5) It doesn't permit the plant breeders to bring the desirable character in a variety,

Reasons for selectingDrosophilafor genetic studies-

The reasons for selectingDrosophilafor genetic studies are as follows-

1)Drosophilacan be cultured easily under normal condition.

2)Drosophilais harmless and inexpensive to culture. The culture medium required to rear them is also very simple.

3) A large number of Drosophilacan be accommodated in a small space.

4) Progeny produced after each mating is large.

5) It breeds throughout the year. Its life cycle is short and completed in 10-12 days so that the results of controlled breeding are quickly available.

6) The number of chromosomes is only four pairs and all are of different size and shape. Chromosomes II, III, and IV are autosomes while the chromosome 'I' is sex chromosome. Female has XX and male has XY, Y being characteristically J-shaped. It is, therefore, easy to identify and study each chromosome.

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram. Principle of biology. 2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.

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Things to remember
  • The phenomenon of permanent association of genes of a single chromosome that can be inherited in successive generations in same position and proportion without any changes or separation is called linkage.
  • Linkage was firstly discovered by Bateson and Punnet (1906) while working on sweet pea (Lathyrus odoratus)in which they found two pairs of alleles did not assort independently.
  • Sutton and Boveri first suggested linkage in 1903 when they first suggestion linkage when they propounded "Chromosomal Theory of Inheritance."
  • The term 'Linkage' was coined by T.H. Morgan and he proposed "chromosomal theory of linkage" by working on Drosophila 1910.
  • The genes on the single chromosome form a linkage group.
  • The phenomenon in which the genes present in a single chromosome do not separate and are inherited together in the successive generations due to the absence of crossing over is called complete linkage.
  • The phenomenon in which linked genes present in the same chromosome have a tendency to separate due to crossing over and forms both parental and non-parental combination in the F2generation is called incomplete linkage.
  • It includes every relationship which established among the people.
  • There can be more than one community in a society. Community smaller than society.
  • It is a network of social relationships which cannot see or touched.
  • common interests and common objectives are not necessary for society.

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Linkage of Genes in Plants: Kinds and Theories

Mendels law of independent assortment is applicable both to genes and chromosomes. During meiosis, the maternal and paternal members of each pair of chromosomes are distributed independently to the gametes. It is for this reason that genes carried in different chromosomes undergo independent assortment and produce the ratios of differentiating characters which Mendel discovered and explained so successfully.

It has been found that in most individuals the number of genes exceed the number of pairs of chromosomes. For example, in Drosophila hundred of genes have been studied yet there are only four pairs of chromosomes. About 400 pairs of genes are known in maize, yet there are only 10 pairs of chromosomes.

Stern has estimated that the total number of pairs of genes in man is not less than 5000 or more than 1,20,000 but man has only 23 pairs of chromosomes. Thus, it is clear that numerous pair of genes must be located on each pair of chromosomes and genes located in the same chromosome will not be assorted independently. So, Mendel’s second law is not universal but is limited to genes in different chromosomes.

Coupling and Repulsion Theory:

Bateson and Punnett in 1906 discovered that independent assortment of factors does not take place always as assumed by Mendel in some cases. This difference from the law of independent assortment was first studied by them in sweet peas. In a cross between purple long and red round the F2 progeny did not give the 9:3:3:1 ratio as expected.

On the other hand, purple long and red round were more numerous than expected where as the purple round and red long were less in frequency. Likewise, when a purple round variety was crossed with a red long variety, the parental combinations appeared more frequently in the F2 than the new combinations.

On the basis of the above results Bateson and Punnett formulated a coupling & repulsion theory. They back crossed the hybrid purple long (PpL1) with recessive parent (ppll) and got the phenotypic ratio of about 7 purple long to 1 red long to 1 purple round to 7 red round. It is clear from this that the hybrids produced gametes of types PL and pl about 7 times as frequently as those of types PL and pl.

Hence Bateson and Punnett suggested that these dominant determiners of two pairs must in some way be coupled, so that they tended to pass in the same gametes at gametogenesis. This tendency of same pair of characters (PP or LL) to unite together and to reappear hand to hand in next generation was termed Coupling by them.

Secondly, they crossed the purple round peas (PPll) with red long (ppLL). The F1 hybrid was again heterozygous (PpLl) purple long. When this hybrid was back crossed with recessive parent (ppll), the F2 ratio was 1 purple long to 7 purple round to 7 red long to 1 red round.

Now it is clear that gametes PI and pL appeared seven times more than PL and pl gametes (This is just opposite to the previous experiment). This tendency of unlike pairs (Pl and pL) to remain together and to avoid union with their dominant and recessive partners is termed as Repulsion.

In fact, the nature of coupling and repulsion was not completely understood by Bateson and Punnett. No satisfactory explanation of coupling and repulsion was given until Morgan and his associates Muller, Bridges, Sturtevant and others discovered that coupling and repulsion are essentially two aspects or facts of the same phenomenon, linkage.

T.H. Morgan (1910) postulated by his extensive experiments on Drosophila that those genes which are located on the same chromosome are linked and pass together from generation to generation while those chromosomes on other hand become freely assorted or segregated during gametic formation.

Furthermore, Morgan advanced the basic idea that the degree of strength of linkage depends upon the distance between the linked genes in the chromosome. This proved very fruitful idea and led to the construction of genetic or linkage maps of chromosomes. This tendency of genes to live together in the same chromosome during hereditary transmission is called linkage.

Linkage is the tendency of two or more genes to transmit together in the same gamete due to which the parental types are obtained in greater than expected in the progeny

The tendency of genes inherited in groups is known as linkage.

Linkage is the consequence of the concerned genes being located in the same chromosome. Linked genes do not show independent segregation, as a consequence, the ratios found in F2 and test cross generations are significantly different from the expected ratios of 9:3:3:1 and 1:1:1:1, respectively in the case of two linked genes.

This effect of linkage is more clearly noticeable in a test cross generation. The frequency of parental characters combination are clearly more than expected while those of new or non-parental character combinations are lower.

Linkage in Maize:

A good example is the results obtained by Hutchinson who crossed a variety of maize having seeds that were coloured and full to one with colourless and shrunken seeds.

In other experiments it had been shown that colour gene “C” was a simple dominant over colourless ‘c’ where as full endosperm, gene ‘S’ is dominant over shrunken ‘s’. Accordingly the parents were CCSS and ccss and the F1 as expected, had coloured full seeds that must have the genotype CcSs.

If C and S assort (segregate) independently in accordance with Mendel’s second principle, these plants should produce four types of gametes CS, Cs, cS, cs in equal numbers.

The easiest way to test this gametic ratio is to make a test cross of F1 to the double recessive cross which according to the expectation stated above, would produce four classes of progeny in the ratio of 1:1:1:1. When the cross was made, however, this expectation was not realized, but the following result was obtained.

Coloured shrunken Cs/cs = 149

Colourless full cS/cs = 152

Colourless shrunken cs/cs = 4,035

In the experiment shown, the situation is as follows:

Parental combination CS = 4,032

cs = 4,035/8,067 or about 96.4% of the total

cs = 152/301 or about 3.6% of the total

It is clear that the two pair of genes C-c and S-s have not assorted independently. The parental combinations greatly exceed the expected 50% they remain combined or linked in 96.4% and are recombined in only 3.6% of the gametes.

When another experiment was so arranged that the same genes entered the cross in different associations, that is, when parents with colourless full seeds (ccSS) were crossed with those coloured shrunken (CCss) seeds, it was found that again the parental combination were in excess, although now these parental combinations are just the opposite of what they were in the first experiment.

The results of the second experiment were as follows:

Here the parental combinations are 21,379+21,906 = 43,285 or 97.06 percent of all. Where as the recombination’s, cross-over type or non-parental combinations are 638+672 = 1,310 or 2.94 percent of the total.

The ratio between parental combinations and recombination’s is only slightly lower than it was in the first experiment. It is now clear that whatever the parental combinations of two different pairs of linked genes may be, linkage tends to keep them together in about the same proportion of the gametes of the double heterozygotes.

Linkage in Drosophila:

If an ordinary Drosophila fly with grey body and long wings (BBW) is crossed with another Drosophila having the two recessive characters of black body and vestigial wings (bbyy), the F1 di-hybrids are like the wild parent.

If now a male fly from the F1 generation is back crossed with a double recessive black vestigial female, there should be, as a result of independent assortment, four kinds of offsprings in equal numbers e.g., Grey long Grey vestigial. Black long and Black vestigial.

The actual experiment, however, showed only two types of offsprings which resembled the two grand parents. This suggests that the genes for grey body and long wings and black body and vestigial wings are linked together.

This can be represented diagrammatically as below:

In the above case the two classes of F2 generation individuals were like the parents, i.e., characters responsible for grey body and long wings in Drosophila female and these responsible for black body and vestigial wings in male fly remain unchanged during the process of un-heritance. Therefore, genes for parental characters remained linked in their offsprings showing phenomenon of linkage.

It points out that linkage brings similarity between parents and offspring. It is a strict restriction in the production of variability among organisms.

Linkage Groups:

The genes found in one chromosome are usually linked to particular associations. The group of genes which exist in the same chromosome or the string of genes on a chromosome is known as linkage group. The number of linkage groups is proportional to the haploid number of chromosomes.

Thus in Drosophila, there are four linkage groups corresponding to the four chromosomes. Each linkage group contains many genes responsible for particular characters. Two linkage groups in Drosophila are very long, one is too small and another one is intermediate in size.

In maize there are 10 linkage groups (n = 10) and in barley, there are 7 linkage groups (n = 7). In cereal plants and animals all the linkage groups have not been discovered e.g., in mouse 16 linkage groups for 20 pairs of chromosomes and in rabbit 11 linkage groups for 22 pairs of chromosomes are known so far.

Chromosome Theory of Linkage:

Castle, Morgan with his associates have formulated chromosome theory of linkage as follows:

1— The genes showing linkage are situated in the same pair of chromosomes. The substance of chromosomes bind these linked genes together during the process of inheritance.

2— The distance between the linked genes determines their effective tendency of linkage. Closely related genes show strong linkage while genes widely located show weak or poor linkage.

3— The genes are arranged in linear fashion inside the chromosome.

Kinds of Linkage:

Complete linkage is the phenomenon in which two or more parental characters are inherited together and uniformly appear through two or more generations. In complete linkage genes are closely associated and tend to transmit together. This is due to the fact that there is no break in the male genes combinations in the chromosomes. It is found in male Drosophila.

It is clear from the above cross that there is no breakage of chromosomal segments. Because of this reason in male Drosophila only two types of gametes are formed BV and by which on mating with gamete ‘bv’ produces only parental type progenies like grey long ‘BbVv’ and black vestigial ‘bbvv’ in F2 generation.

It involves the accidental or occasional breakage of chromosomal segments. This arrangement is called crossing over, It was first of all noticed by Morgan in white eyed and miniature winged Drosophila flies.

In cross between grey long (BBW) X black vestigial (bbvv), the F1 hybrid is grey long (BbVv). The F1 hybrid is female and produces gametes of four kinds. Two gametes will show the linked gene and no chromosomal change (non-crossovers) and other two will show chromosomal interchange. Thus, non-cross over gametes formed without chromosomal interchange are in 82% ratio and cross-over gametes formed as a result of crossing over are 18%.

If we cross the F1 hybrid to a black vestigial male (Bb Vv X bbvv ), the F2 will show 82%’ offspring of parental ratio i.e., 41% grey long and 41% black vestigial and 18% offspring of new cross-over ratio, 9% grey vestigial and 9% black long.

Theories of Linkage:

(i) Theory of differential multiplication:

This theory was proposed by Bateson (1930) to explain the phenomenon of linkage. According to this theory, the set of gametes possessing parental combinations multiply more rapidly than the set having non- parental combinations after the segregation of characters during gamete formation. This results in the formation of a greater number of gametes with parental combinations.

This theory has no cytological basis hence has been condemned by other cytologists. We know from our knowledge of gametogenesis, that after segregation, only a single division comes before the gametes are formed. Moreover the theoretical results do not agree with the statistics obtained.

(ii) Chromosomal theory of linkage:

This theory was proposed by Castle and Morgan. They claimed that genes situated in the same chromosomes are inherited together i.e., they are linked while those located in different chromosomes are inherited freely or independently i.e., they are not linked. The extent of linkage is correlated with the distance between the genes in the chromosomes— closer the genes, stronger the linkage & vice- versa.

The linkage of genes, according to this theory, is linear. An important distinguishing feature is that genes are located in the chromosomes longitudinally in linear fashion. The chromosome theory of linkage is well supported by other cytologists and is widely accepted.


Chromosomes and DNA MCQs | Chromosomes & their Behaviour

1. Sutton and Boveri used… movement to explain Mendel’s laws.

2. According to chromosomal theory of linkage of Morgan and Castle

(a) strength of linkage between two successive genes is inversely proportional to distance between two genes

(b) genes lie in a linear order in the chromosomes

(c) linked genes are arranged in cis or trans manner cis AB arrangement a brings coupling (parental b. combinations) whiletrans-arrangement Ab brings a recombinations (repulsions)

3. Consider the following statements regarding linkage. I. The linked genes are located on the same chromosome. II. Crossing over between linked genes is rare. III. Linked genes are always inherited together. IV. Linkage affects the percentage of homozygosity following hybridisation. Of these statements are correct.

4. Variability may originate during meiosis due to

(b) chromosomal aberrations

5. Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead

(a) the segregation of a pair of factors they carried

(b) the segregation of the characters

(c) recombination of the factors they carried

6. The coupling and repulsion theory of Bateson and Punnett later on modified in linkage and crossing over by Morgan. Two completely linked genes shows a dihybrid ratio of

7. Criss-cross inheritance means

(a) X-chromosome from female will pass to female of next generation

(b) X-chromosome from a male will pass to a female of next generation

(c) X-chromosome from male will pass to a male of next generation

8. Crossing over occurs at four stranded stage. This was proved by the observation that

(a) usually two gametes resulting frommeiosis arerecombinants

(b) all the four gametes resulting from meiosis are recombinants

(c) chiasmata are seen only at four stranded stage

(d) chiasmata are seen only at two stranded stage

9. Sex-linked inheritance was discovered in 1910 by

10. The first attempt to show linkage in plants was done in

11. The genetic maps are used as a starting point in the sequencing of whole genomes as in the case of

(b) human gene sequencing project

(c) human genome sequencing project

12. Sutton gave chromosomal theory of inheritance he united the knowledge of chromosomal segregation with

13. The crossing over between homologous chromosomes never exceeds beyond

14. Linkage and cytological maps for the same chromosome

(a) are both based on mutant phenotypes and recombination data

(b) may have different sequences of genes

(c) have both the same sequences of genes and intergenic distances

(d) have the same sequences of genes but different intergenic distances

15. A linkage group is explained as

(a) different groups of genes located on the same chromosomes

(b) that all the linked genes of a chromosome pair

(c) that all the linked genes of a chromosome

16. According to the chromosomal theory of inheritance

(a) chromosomes are decondensed in telophase

(b) genetic codes are found on chromosomes

(c) chromosomes carry alleles that are segregated as chromosomes and are pulled apart during the cell division


Crossing Over is Random

Many factors affect crossing over, and the position on the chromosome where crossing over will occur is unpredictable. Crossing over is a random event. While the location of the break points on the DNA sequence of the chromosomes are fairly random, the recombination frequency is relatively constant between homologous chromosomes. (For a given chromosome, N number of cross overs will occur, but where they will occur is random.)

The probability of crossing over between genes on a chromosome is dependent on the distances between the genes. This shouldn't surprise you because the greater the distance between two genes, the greater the chance a break will occur.

Genes that are located on the same chromosome and that tend to be inherited together are called linked genes because the DNA sequence containing the genes is passed along as a unit during meiosis unless they are separated by crossing over. The closer together that genes are located on a particular chromosome, the higher the probability that they will be inherited as a unit, since crossing over between two linked genes is less frequent the closer together the two genes are (genes with complete linkage are close enough together on a chromosome that they never recombine and are always inherited as a unit).

Because of this, linked genes do not follow the expected inheritance patterns predicted by Mendel's Theory of Independent Assortment when observed across several generations of crosses. For two heterozygous genes that are unlinked and undergoing independent assortment, you expect to see parental and recombinant gametes in a ratio of 1:1:1:1 (if you don’t remember why, please review Tutorial 29).

When two genes are linked on a chromosome,crossing over between the two genes will be less common than having no crossing over, so fewer recombinant chromosomes will be produced. Under this circumstance, a ratio that deviates from the usual 1:1:1:1 will be observed, indicating that the genes are linked. The number of parental genotypes in the gametes will be higher and the number of recombinant genotypes will be lower.


Chromosomal Basis of Inheritance Questions and Answers

Q1. Term chromosome was coined by

Q2. Chromosomes were first seen by

Q3. Chromosomes found in the salivary gland of drosophila is

Q4. Giant chromosome with a number of chromonemeta is

  1. Lampbrush chromosome
  2. Hetrochromosome
  3. Supernumerary chromosome
  4. Polytene chromosome

Q5. Lampbrush chromosomes occur in

Q6. Chromosome ends are called

Q7. Chromatid is

  1. One-half of chromosome
  2. Haploid chromosome
  3. Complete chromosome
  4. Duplicate chromosome

Q8. Centromere is that part of chromosome where

  1. Nucleoli are formed
  2. Crossing over takes place
  3. Chromatids are attached
  4. Nicking occurs

Q9. A chromosome with sub-terminal centromere is

Q10. A chromosome with centromere near the middle is

Q11. Puffs or balbiani rings in salivary gland chromosome are site of

Q12. Chromosome theory of inheritance was proposed by

Q13. More than 200 chromosomes occur in

Q14. Drosophila has four homologous pairs of chromosome. What is the number of linkage group in this animal?

Q15. Gene for colour blindness in man is located on

  1. X-chromosome only
  2. Y-chromosome only
  3. Either X-chromosome or Y-chromosome
  4. Both X and Y chromosome

Q16. A colour blind daughter may be born if the

  1. Father is normal and mother is colour blind
  2. Father is colour blind and mother is normal
  3. Father is normal and mother is carrier
  4. Father is colour blind and mother is carrier

Q17. A somatic cell in human male contains

  1. No gene on sex chromosome
  2. Only one sex linked gene for each character
  3. Two genes for every sex linked character
  4. Genes on only on sex chromosomes

Q18. A normal woman is married to a colour blind man. The children are expected to be

  1. All normal
  2. 50% sons are colour blind
  3. All daughters are normal but carrier whereas all sons are normal phenotypically as well as genotypically
  4. 50% daughters are colour blind

Q19. Which of the following dieses is sex linked?

Q20. The genes for the eye colour and size of the wing in drosophila are located on the same chromosome. They can be separated by

  1. Non-disjunction
  2. Crossing over
  3. Hybridisation
  4. Not be separated at any stage

Q21. A colour blind son is born to a normal parents. It shows that

  1. The father was hetrozygous for colour blindness
  2. The mother was hetrozygous for colour blindness
  3. The mother was genotypically homozygous
  4. Both parents carried a recessive gene for the disorder

Q22. Chromosomal constitution in human female can be best written as

Q23. The sex linked characters are those

  1. Which are related to sexual physiology
  2. The genes of which are present on the sex chromosomes
  3. Which appear either in male and female
  4. Which are controlled by sex hormones

Q24. Chromosomes other than sex chromosomes are called

Q25. If the crossing over had occurred at two strand stage in neurospora the ascospores would be arranged in

Q26. Complete linkage is found in

Q27. In most of the higher unisexual animals there is one chromosomal pair which is not identical in two sexes , these are called

  1. Non homologous chromosomes
  2. None identical chromosomes
  3. Non compatible chromosomes
  4. Sex chromosomes

Q28. The to diverse disciplines of cytology and genetics were correlated by

Q29. Linkage group is defined as

  1. All the linked genes of a chromosomal pair
  2. Different groups of genes present on different chromosomes
  3. All the genes located on the same chromosome
  4. None of the above

Q30. Linkage in drosophila was reported by

Q31. Phenomenon which works opposite to the linkage is

Q32. When two genes are situated very close to one another at a chromosome

  1. The percentage of crossing over between them is very high
  2. Hardly any cross overs are produced
  3. No crossing over can take place
  4. Only double Cross overs can occur between them

Q33. Greater is the distance between the two genes on chromosome

  1. Greater is the linkage strength
  2. Lesser is the linkage strength
  3. Linkage strength remains unchanged
  4. There is no relationship between the two

Q34. Crossing over occurs at

  1. Single strand stage of chromosomes during prophase
  2. 2 strand stage during zygotene
  3. Four strand stage of Pachytene
  4. Metaphase II of meiosis

Q35. The term coupling and repulsion signify the same phenomenon which is termed

Q36. The linked characters would always inherit together till they are

  1. Delinked due to segregation
  2. Mask by dominance
  3. Mutated
  4. Separated due to crossing over

Q37. White eyed and long wind male drosophila was crossed to a vestigial winged red eyed female. The two characters are linked in this animal when a female F1 was just crossed the F2 generation produced

  1. Mostly white eyed with long wings
  2. Mostly red eyed with vestigial wings
  3. Mostly white eyed with long wings and red eyed with vestigial wings
  4. All white eyed and vestigial winged

Q38. A haemophilic man marries a normal women, what is the probability of the disease in the grandchildren in case the daughter marries a normal man

  1. All grandsons would be haemophilic and granddaughters normal
  2. 50% of grandsons are hemophilic other grandchildren are normal
  3. Only granddaughters would be haemophilic
  4. All grandchildren would be haemopgilic

Q39. Colour blind man marries abnormal woman whose father was colorblind what percentage of children is expected to be colour blind

Q40. Mendel did not get any linkage in his experiments on pea , one of the reasons was that

  1. He did not keep an exact record
  2. There is no linkage in pea
  3. He did not have means to detect linkage
  4. All the 7 characters selected by him were present on different chromosomes or showed 50% crossovers

Q41. The genes of different traits located on different loci on the same chromosome are

Q42. In four o’clock plant normal leaves A and veriegated leaves B occur in different plants, if B male is crossed with A female the hybrid has normal lives but when B female is crossed with A male the hybrid has veriegated leaves , it is a case of

  1. Mutation
  2. Cytoplasmic inheritance
  3. Complementary genes
  4. Supplementary genes

Q43. Crossing over occurs between

  1. Two non sister chromatids of a homologous pair of chromosomes
  2. Two chromatids of any chromosomes
  3. Two chromatids of same chromosome
  4. All the foregoing

Q44. Neurospora crassa is widely used in genetic studies because of all the following except one

  1. It is a haploid plant and mutations can be easily detected
  2. It can be easily cultured
  3. Life cycle short
  4. Sports are not affected by mutagenesis

Q45. The frequency of crossing over would be higher if

  1. Two genes are located closely
  2. Two genes are far apart on a chromosome
  3. Two genes are not located on the same chromosome
  4. None of the above

Q46. Mendelian recombinations are due to

  1. Mutation
  2. Linkage
  3. Crossing over
  4. Independent assortment of characters

Q47. The blue green algae and bacteria contain

  1. One linkage group
  2. Two linkage groups
  3. Three linkage groups
  4. None of the above

Q48. Which one of the following character in man is controlled by a recessive gene?


KPK Board 12th Class Biology Ch 22 Inheritance Short Questions Answers

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Define the following terms heredity inheritance and variation genetics?

Define the following terms heredity inheritance and variation genetics?

The ambulance of offspring to their parents is called heredity it is responsible for continuity of life forms.

inheritance the transfer of characteristics from parents to offspring is known as inheritance.

variations in the difference in the members of the same species are called variations in shape height habits.

Then it is a branch of Biology that deals with the study of inheritance variations and the factors controlling them.

Define gene and genome?

Define gene and genome?

the term gene was for the first time used by Johnson gene is the basic unit of biological information and their informations are in form of chemical quotes on DNA genes are segment of nucleic acid consisting of a specific sequence and number of amino acids

Function genes determine the expression of certain characteristics of the genome the complete set of genetic material in a chromosome set of individuals is called its genome function it controls the characteristics of growth and development from zygote to adult.

What are alleles?

What are alleles?

the alternative form of a gene is called allele it is also called alilo moreph

formation alleles are formed as a result of mutation significant alleles cause variations.

What is mutation?

What is mutation?

the sudden heritable change in the genetic material or an organism is called mutation mutagens by mutation causing agent is called mutagen mutant the organism in which mutation the sudden heritable change in the genetic material or an organism is called mutation mutagens by mutation causing agent is called mutagen mutant the organism in which mutation occur is called mutant.

Discuss mendelian genetics?

Discuss mendelian genetics?

gregor John Mendel contributed a lot in the field of genetics that is why he is known as the father of genetics research on garden pea open a new avenue in the field of genetics experiments and result is started the pattern of Inheritance of various characteristics in garden pea and record the result in short he laid the foundation of genetics.

Which is the reason for Mendel's success?

Which is the reason for Mendel's success?

mental success was mainly due to following two reasons his approach was mathematical he studied a single character at a time and record the results choice his choice of experimentally material material also let him to success Garden pea following advantage its life cycle is short it can be cultivated in unlimited areas it produces a large number of varieties it produces a large number of seeds


GSEB Class 12 Biology Principles of Inheritance and Variation Text Book Questions and Answers

Question 1.
Mention the advantages of selecting pea plant for the experiment by Mendel.
Answer:
Mendel selected pea plant because:-

  • It possesses a large number of varieties.
  • Pure varieties of this plant were available.
  • The plant is small and the flower is large enough to hand it manually.
  • Flowers are bisexual and produce fertile hybrids.
  • Each plant produces a large number of seeds.
  • Plants are self-pollinated and can be cross-pollinated.
  • The plant is annual with a growth period of few months.
  • Chances of contamination are very less.

Question 2.
Differentiate between the following:
a. Dominance and Recessive
b. Homozygous and Heterozygous
c. Monohybrid and Dihybrid.
Answer:
a. In Mendel’s experiment of the law of dominance, the expressed character is called dominant character or dominance, and suppressed character is called recessive character or recessiveness.

b. The individual whose chromosome carries similar genes for a character is called homozygous and the individual whose chromosome carries dissimilar genes for a character is called heterozygous.

c. A cross involving two plants differing in one character pair is called a monohybrid cross and that in two characters is called a dihybrid cross.

Question 3.
A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
Answer:
Number of gametes = 2 n
n = Number of loci, n = 4
So, 2 4 = 2×2×2×2 = 16 types of gametes.

Question 4.
Explain the Law of Dominance using a monohybrid cross.
Answer:
This law states that when a pair of contrasting factors for a character are present together, one dominates over the other. This law is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the 3 : 1 ratio of F2 generation.

Question 5.
Define and design a test-cross.
Answer:
The crossing of an F1 hybrid with its recessive parent is called a test cross.

Question 6.
Using a Punnett Square, work out the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
Answer:

Question 7.
When a cross is made between a tall plant with the yellow seed (TtYy) and a tall plant with the green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be
a. tall and green.
b. dwarf and green.
Answer:
Tall and green = 3
Dwarf and green = 1

Question 8.
Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dihybrid cross?
Answer:
The parental phenotypes appear much in excess than expected. It is due to linkage and the absence of independent assortment.

Question 9.
Briefly mention the contribution of T.H. Morgan in genetics.
Answer:
TH Morgan is a Geneticist who got Nobel Prize.

  • He found fruit fly (Drosophila Melanogaster) to be an experimental material as it was easy to rear and multiply.
  • The established presence of genes over the chromosomes.
  • Principle of linkage and crossing over.
  • Discovered sex linkage and crossing over.
  • He observed mutations.
  • The developed technique of chromosome mapping,
  • Wrote the book “The theory of Gene”.

Question 10.
What is pedigree analysis? Suggest how such an analysis, can be useful.
Answer:
Analysing the inheritance of the character through several previous generations in a family is called pedigree analysis. This is a very useful method to study human genetics. Human beings have a long generation time and produce extremely small number of offspring which make human genetic studies difficult. In pedigree analysis inheritance of a particular trait is studied in several generations of a family. The ancestral history of an individual is called pedigree. This data is used to produce a family tree. In this method of pedigree analysis the studies of trait as they have appeared in a given family line for several past generations can be done.

Question 11.
How is sex determined in human beings?
Answer:
In human beings, out of 23 pairs of chromosomes present, 22 pairs are same in both sexes. These chromosomes are called autosomes (the chromo­somes determine the body characters other than sex). In human female, there are two X chromosomes whereas in male one X chromosome and one Y chromosome. During spermatogenesis two types of sperms are produced (heterogametic male) i.e., 50 percent of the sperm produced carry X chromo­some and 50 percent carry Y chromosome.

Females are homogametic i.e., produce only one type of egg with X chromosome. Sex of the baby is deter­mined by the type of sperm that fertilises the egg. If the egg is fertilised by a sperm that carries X chromosome, the resulting baby will be a female and if the egg is fertilised by the sperm that carries Y chromosome, the resulting baby will be a male. Thus it is evident that in each pregnancy there is always 50 percent probability of either a male or a female child. Unfortunately, the society blames the female for producing female children and have been ostracised and ill-treated because of this false notion.

Question 12.
A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offspring.
Answer:
Father I A i
Mother I B i

The possible other genotypes I A I B i.e., AB, I A i i.e., A and I B i i.e., B group.

Question 13.
Explain the following terms with examples.
a. Co-dominance
b. Incomplete dominance
Answer:
(a) Co-dominance: Phenomenon of 2 different alleles of same gene lacking dominance – recessive relationship and expressing their effect simultaneously in the heterozygote.
Eg: 1 A and 1 B alleles in the AB blood group of humans.

(b) Incomplete dominance: Phenomenon of neither of 2 alleles of a gene being dominant over each other so that when both of them are present together, a new phenotype is formed which is somewhat intermediate between both of their independent expression. Eg: Flower colour is Snapdragon and a 4𔃺 clock plant.

Question 14.
What is point mutation? Give one example.
Answer:
The changes occurring in the structure of a gene are called point mutations or gene mutations, e.g. Sickle cell anemia.

Question 15.
Who had proposed the chromosomal theory of inheritance?
Answer:
Sutton and Boveri

Question 16.
Mention any two autosomal genetic disorders with their symptoms.
Answer:
i. Down’s syndrome: It is an autosomal abnormality first discovered by Langdon Down in 1866. The karyotype of the affected person shows 47 chromosomes instead of 46. It is due to the presence of an extra autosome, which is found along with the 21st pair of autosomes i.e., chromosome 21 is represented by three copies. So this condition is called 21 trisomies. The genetic make-up of the person is 45 A + XX or 45 A + XY.

ii. Edward’s syndrome: It is due to the autosomal trisomy of chromosome 18. The babies born with this syndrome have several skeletal defects, a short deformed sternum, cardiac problems, abnormal kidneys, etc. The incidence of this syndrome is about 1 per 15,000 births. Maternal age is one of the factors that influence its incidence.

GSEB Class 12 Biology Principles of Inheritance and Variation Additional Important Questions and Answers

Question 1.
Match the following.

A B
a. Mendel Chromosome theory
b. Karl Landsteiner Genetics
c. Sutton and Boveri Polytene chromosome
d. Morgan Incomplete dominance
e. Bateson Blood groups

A B
a. Mendel Incomplete dominance
b. Karl Landsteiner Blood groups
c. Sutton and Boveri Chromosome theory
d. Morgan Polytene chromosome
e. Bateson Genetics

Question 2.
a. Name the phenomenon of co-existance of two or more genes in same chromosome,
b. Mention its types.
Answer:
a. Linkage
b. Complete linkage: In this type of linkage, two or more genes or traits are inherited together for a number of generations. This is seen only in male Drosophila.
Incomplete linkage: The linked genes sometimes separate and lead to the formation of new combinations. This is seen in female Drosophila.

Question 3.
What is Genetics?
Answer:
The branch of biology that deals with the inheritance and variation of characters from parents to offspring.

Question 4.
Write the symptoms of
Haemophilia
Cystic fibrosis
Sickle cell anemia
Colour blindness
Thalassemia
Answer:
Haemophilia – Blood does not clot
Cystic fibrosis – An inherited disease of secretory gland-like mutons and sweat glands.
Sickle cell anaemia – RBC becomes sickle-shaped and will carry only less oxygen.
Colour blindness – Cannot distinguish colours.
Thalassemia – Reduced amount of hemoglobin or its absence.

Question 5.
The following are the symbols shown in pedigree analysis. Identify them.

Answer:
a. Mating between relatives
b. Female
c. Male
d. Affected female

Question 6.
Complete the flowchart on the criss-cross inheritance seen in human beings.

Answer:
a. Carrier
b. Normal
c. Normal
d. Carrier
e. Normal
f. Haemophilic

Question 7.
Note the relationship between the first two words and suggest a suitable word for the 4th place.
a. Genetics – Bateson :: Genes –
b. Visible characters of an organism – Phenotype :: Genetic constitution of an organism –
c. Epigenesis – Wolf :: Blending inheritance –
d. Multiple alleles – Blood group :: Polygenic traits –
e. Heredity – Resemblance between parents and offspring :: Variation –
f. Garden pea – Pisum sativum :: Sweet pea –
g. Monohybrid ratio – 3:1 :: Dihybrid ratio –
Answer:
a. Johannsen
b. Genotype
c. Galton
d. Skin colour
e. Difference between parents and offspring
f. Lathyrus odoratus g. 9 : 3 : 3 : 1

Question 8.
Find out the odd one in each group.
a. 3 : 1, 1 : 2 : 1, 9 : 3 : 3 : 1, 9:7
b. Homozygous, Heterozygous, Hemizygous, Azygous
c. Monohybrid, Dihybrid, Polyhybrid, Back cross
d. Tall, Axial, Purple, White
Answer:
a. 9 : 7
b. Azygous
c. Back cross
d. White

Question 9.
What is a monohybrid cross?
Answer:
The cross made between individuals of a species, considering the inheritance of the contrasting pair of a single character.

Question 10.
Name a plant that shows incomplete dominance in respect to the colour of its flowers.
Answer:
Mirabilis jalapa (4 o’clock plant)

Question 11.
A heterozygous tall plant is crossed with a homozygous dominant plant. What is the result in F1 F22 generations? Explain with adequate illustration.
Answer:

F1 generation
1 Homozygous tall plant
1 Heterozygous tall plant
1 : 1 is the ratio of F1 generation.

Question 12.
Name the scientists who rediscovered Mendelism.
Answer:
Correns (Germany), De Vries (Holland), Tshermak (Austria)

Question 13.
What do you understand from the following flow chart?

Answer:
The flow chart illustrates the monohybrid test cross.

Question 14.
What do the following genetic symbols mean?
Aa, AA
Answer:
Aa = Heterozygous
AA = Homozygous dominant

Question 15.
A man with blood group A marries a woman with blood group B. Their child has blood group O.
a. What are the genotypes of their parents?
b. What are the other genotypes and in what ratio would you expect in the offspring from this marriage?
Answer:

b. The other possible genotynes are AB, A and B. The ratio is 1 : 1 : 1 : 1 (AB : A : B : O)

Question 16.
Give reason.
a. Mendel selected garden pea (Pisum sativum) as his experimental plant.
b. Blood group identification is not required while transfusing serum.
Answer:
a. Garden pea (Pisum sativum) shows a large number of contrasting characters. Moreover the hybrids are fertile, easy to cultivate, short growth period and life cycle, availability of pure breeding varieties etc. All these are the reasons for- the selection of garden pea as his experimental material.

b. Serum does not contain any antigen.

Question 17.
‘Give the scientific name and the common name of plant in which incomplete dominance was first discovered.
Answer:
Four O’clock plant – Mirabilis jalapa.

Question 18.
What does the letter Fx represent in heredity?
Answer:
F1 stands for First Filial generation.

Question 19.
Match the related items from B and C with column A.

A- Blood groups B – Antigens C- Antibodies
A B A
B A B
AB AB
O AB

A- Blood groups B – Antigens C- Antibodies
A A B
B B A
AB AB
O AB

Question 20.
Identify the personality.

Answer:
Gregor Johann Mendel, the Father of Genetics.

Question 21.
What is the glycoprotein found on the RBC’s of a person with blood group AB?
Answer:
Glycoprotein A and B are found.

Question 22.
Match the following.

A B
i. Incomplete dominance Punnet
ii. Polygenic traits Blood groups
iii. Multiple alleles Skin colour
iv. Checkerboard Mirabilis jalapa

A B
i. Incomplete dominance Mirabilis jalapa
ii. Polygenic traits Skin colour
iii. Multiple alleles Blood groups
iv. Checkerboard Punnet

Question 23.
Mendel, in his last breath said ‘Meine zeit word schoon kommen’. What is the meaning of it?
Answer:
My time will come soon

Question 24.
Give the scientific names of the following.
4 o’clock plant, Sweet pea, Snapdragon.
Answer:
Mirabilis jalapa, Lathyrus odoratus, Antirrhinum majus

Question 25.
Who discovered blood groups?
Answer:
Karl Landsteiner in 1900

Question 26.
Discuss under what conditions the ratio 9 : 3 : 3 : 1 is modified to 9 : 7 ratio.
Answer:
During complementary gene action, the 9:3:3: 1 is modified to 9 : 7 ratio.

Question 27.
A plant with red flowers was crossed with another plant of the same species with white flowers. The offspring thus obtained were 60 plants with only pink flowers. On selling, these plants produced 60 plants with red flowers, 120 plants with pink flowers, and 60 with white flowers.
a. Name the genetic principle behind this.
b. Give a scientific explanation for this.
c. Name the geneticist who conducted this experiment.
Answer:
a. Balsam plant produces offspring in the ratio of 3 : 1 in crossbreeding.
b. Balsam plant has incomplete dominance. So the cross-breeding produces a new colour, pink.
Pink-colored F1 generation is formed because of polygenic traits.
c. Correns

Question 28.
All test crosses are back cross. ‘But all backcrosses are not test cross’. Justify the statement.
Answer:
The crossing of F1 hybrid with its recessive parent is called a test cross. Back cross is the crossing of an F1 hybrid back with any of the parents. So all test crosses are back cross. But all backcrosses are not test cross.

Question 29.
Copy and complete the checkerboard of the dihybrid cross. Write the genotypic and phenotypic ratios. Gametes are given.

Answer:
Mendel explained the dihybrid cross graphically by Punnet square or checkerboard.
Let R be the gene for round seed shape and r for wrinkled shape. Y for yellow colour and y for green colour. So the genotype of the pure breeding round yellow plant is RRYY and that of wrinkled green is rryy. The F1 hybrids produce 4 kinds of gametes in equal numbers.

F2 Checkerboard

Explanation
During gamete formation of a dihybrid, the gene for round shape assorts out independently of the gene for yellow colour. The gene R may combine with dominant gene Y or the recessive gene y of the other character and enter a gamete. Similarly, the gene r may combine with the dominant gene Y or the recessive gene y and enter a gamete. So the F hybrid plants produce four types of gametes such as RY, Ry, rY and ry. A cross of two such F1 plants produces 16 combinations.

Out of the above 16 combinations, (frac<9><16>) are round yellow i.e., they have both dominant genes R and Y, (frac<3><16>) are round green i.e., they have one dominant and one recessive gene R and y, (frac<1><16>) are wrinkled yellow i.e., they have one dominant gene and one recessive gene r and Y and (frac<1><16>) is wrinkled green i.e., it has both recessive genes r and y. This ratio 9:3:3:1 is called the dihybrid phenotypic ratio. The genotypic ratio is 1:2:1:2:4:2:1:2:1.

In the dihybrid cross, the formation of new combinations such as round green (Ry) and wrinkled yellow (rY) no doubt, is the result of independent assortment. This law states that when more than one pair of characters are involved in a cross, factor pairs assort independent of each other.

Question 30.
A test cross is used to identify whether the plant is homozygous or heterozygous. Justify this statement.
Answer:
To test the purity of the plant, subject the doubted plant for test cross. If the doubted plant is homozygous, it will produce only one type of progeny. If it is heterozygous, it will produce two types of progeny in the ratio of 1:1.

Question 31.
Note the relationship between the first two words and suggest a suitable word for the 4th place.
a. XX – XO mechanism – Grasshopper ::
XX – XY mechanism –
b. Sex limited inheritance – Beard in man ::
Sex influenced inheritance –
c. Phenylketonuria – Phenylalanine hydroxylase::
Alkaptonuria –
d. Down syndrome – Langdon Down::
Klinefelter’s syndrome –
e. Turner’s syndrome – 45 chromosomes::
Down’s syndrome –
f. Ascaris megalocephaly – 2::
Ophioglossum reticulatum………………..
Answer:
a. Man
b. Baldness in man
c. Homogentisic acid oxidase
d. Harry Klinefelter
e. 47 chromosomes
f. 1262

Question 32.
Find the odd one of the following.
a. Alkaptonuria, Phenylketonuria, Albinism, Colour blindness
b. Deletion, Duplication, Inversion, Conversion
c. Haploidy, Diploidy, Polyploidy, Polydactyly
d. Nullisomic, Monosomic, Polysomic, Polygenic
e. Klinefelter’s syndrome, Down’s syndrome, Turner’s syndrome, Acquired Immune Deficiency syndrome.
Answer:
a. Colour blindness
b. Conversion
c. Polydactyly
d. Polygenic
e. Acquired Immune Deficiency Syndrome

Question 33.
Give reason.
a. Drosophila is the ideal material for genetic study.
b. Haemophilia is more common in males.
c. Aneuploidy leads to variation.
d. Sex of the child is determined by the father.
Answer:
a. It breeds very quickly, has a short generation time, produces numerous offspring, only 4 pairs of chromosomes, the chromosomes can be easily distinguished with the help of a microscope.
b. It is an ‘X’ linked disorder. Male possesses only one X chromosome. So it is more common in males.
c. Aneuploidy is a chromosomal mutation that causes variation in chromosome number which in turn leads to variation.
d. Father is heterogametic and produces two types of sperms i.e., half with ‘X’ and a half with A” chromosomes.

Question 34.
Construct a flow chart showing criss-cross inheritance.
Answer:
Haemophilia (Bleeder’s disease) is a hereditary sex-linked blood disease in man. It was discovered by John Cotto in 1803. This disease is caused by the absence of an antihaemophilic factor. This disease is also called bleeder’s disease because even minor injuries can cause death due to excessive bleeding. This disease was first reported in the royal families of Europe and Queen Victoria, hence it is also called royal disease.

The genes responsible for haemophilia are located on the ‘X’ chromosome. The ‘Y’ – chromosome carries no gene for this character. Haemophilia is caused by a recessive gene ‘h’. The dominant gene ‘H’ controls normal clotting of blood.

The females have two ‘X’ chromosomes, they may be of the genotype ‘HH’ or ‘Hh’ or ‘hh’. Homozygous dominant females (HH) are normal. Heterozygous females (Hh) are not haemophilic since ‘H’ is dominant over ‘h’. But they will transmit the haemophilia to all their sons. So heterozyous females are called carriers. Homozygous recessive females (hh) will be haemophilic.

A normal woman (HH) marries a haemophilic man (h), all their daughters (Hh) are carriers of disease because the father passes his ‘X’ chromosome only to his daughters. Mothers pass their ‘X’ chromosomes to both sons and daughters. All the songs are normal (H) because the “Y’ chromosome from the father contains no gene for this trait.
It can be noted that haemophilia is transmitted from the father to the grandson through the daughter. So this type of inheritance is said to be a crisscross inheritance or zig-zag inheritance. In females, X-linked recessive genes are expressed only in homozygous conditions (hh). In males, only one ‘X’ chromosome expresses the recessive gene in a single dose ‘h So ‘X’- linked disorders are more common in males than in females.

Question 35.
Observe the following diagrams and identify the different types of chromosomal mutation.

Answer:
A – Deletion
B- Duplication
C – Inversion
D- Translocation

Question 36.
Substitution of a wrong amino acid valine instead of glutamic acid in the 6th position of globin chain of RBC causes disease in man.
a. Name the disease.
b. Draw the amino acid sequence of both normal and diseased Hb.
Answer:
a. Sickle cell anemia
b. This is a genetic defect characterized by sickle-shaped RBC with abnormal Hb. It is due to a gene mutation that causes the substitution of a wrong amino acid valine instead of glutamic acid in the 6th position of the globin chain (beta chain) of RBC. At low oxygen concentration, they interact with each other to form fiber-like structures that distort the RBC membrane and cause the cell to form a sickle shape and is unable to take oxygen. Hence this defect is also known as the sickle – cell crisis. The sickle-shaped erythrocytes (RBC) are destroyed more rapidly than the normal RBC leading to anemia.

This disease is controlled by a pair of alleles namely, Hb A Hb s . Pedigree analysis revealed that three genotypes (Hb A Hb A , Hb A Hb s , Hb s Hb s ) and two phenotypes (normal and affected) individuals result from homozygous genotypes Hb A Hb A and Hb A Hb s respectively. The third genotype Hb A Hbs exhibit sickle cell trait. Though the Hb A Hb s individuals are unaffected, they carry one normal gene and one defective gene and can transmit the same to 50 percent of their offspring.
The aminoacid sequence of normal haemoglobin is valine, histidine, leucine, threonine, proline, glutamic acid ………………
The aminoacid sequence of normal haemoglobin is valine, histidine, leucine, threonine, proline, valine …………………….

Question 37.
Who proposed mutation theory of heredity?
Answer:
Hugo de Vries

Question 38.
Which one of the following is male Drosophila?

Answer:
A – male
B – Female

Question 39.
A man suffering from hemophilia marries a carrier woman. Work out the chances, of their progeny suffering from the disease. Use a flow chart/Punnet square.
Answer:

50% of the progeny will be haemophilic and 50% of progeny will be normal.

Question 40.
Match the following.

A B
a. Mutation theory Garrod
b. Chromosome map John Cotto
c. Haemophilia Hugo de Vries
d. Inborn errors of metabolism Sturtevent

A B
a. Mutation theory Hugo de Vries
b. Chromosome map Sturtevent
c. Haemophilia John Cotto
d. Inborn errors of metabolism Garrod

Question 41.
Some so called doctors claim that their medicines provide 100% guarantee for getting a son or a daughter according to the wish of the parent. How do you react to such claims on the basis of your knowledge of genetics? Give a suitable explanation.
Answer:
The claim is not correct, because sex of the foetus is determined by the sex chromosomes. Human females are homogametic and produce only one type of eggs whereas the males are heterogametic and produce two types of sperms. The sex of the foetus depends upon the type of sperm that fertilises the egg.

Question 42.
What will be the sex of a child which develops from 44 + XX zygote?
Answer:
Female

Question 43.
What is haemophilia due to? What happens in this disorder?
Answer:
Haemophilia is due to a defective recessive allele located on the ‘X’ chromosome. In this disorder, a single protein which is involved in the clotting of blood is affected. As a result, the individual bleeds even from a simple cut, which may become fatal.

Question 44.
Drosophila is known as the ‘pea’ of animal kingdom. Justify this statement.
Answer:
Drosophila is an ideal material for the study of genetics in the animal kingdom like pea (Garden pea- Pisum sativum) in the plant kingdom.

Question 45.
Some genetic disorders such as haemophilia, colour blindness etc. transmit from father to grandson through daughter. Name the type of inheritance.
Answer:
Cris- cross inheritance (or) zig- zag inheritance

Question 46.
Give one word for the following.
a. Agents that cause mutation.
b. Diagrammatic representation of karyotype
c. Seat of genes
d. Loss of chromosome segment
e. Fixed position of a gene
f. Exchange of segments between non-sister chromatids
g. Failure of separation of chromosome during meiosis.
h. A chromosome pair is lost from the diploid set.
Answer:
a. Mutagen
b. Idiogram
c. Chromosome
d. Deletion
e. Locus
f. Crossing over
g. Non-disjunction
h. Nullisomic

Question 47.
Match the related items from B and C with column A.

Answer:

Question 48.
Give the chromosome numbers of the following animals and plants.

Animals Plants
i. Ascaris megalocephala a. Pisum sativum
ii. Drosophila melanogaster b. Allium cep a
iii. Apis (honey bee) c. Oryza sativa
iv. Homosapiens d. Ophioglossum reticulatum
v. Equus (horse) e. Saccharum officinarum
vi. Culex f. Solanum tuberosum

Animals Plants
i. 2 a. 14
ii. 8 b. 16
iii. 32 c. 24
iv. 46 d. 1262
v. 64 e. 80
vi. 6 f. 48

Question 49.
Complete the flow chart.

Answer:

In many insects and roundworms, the females have two ‘X’ chromosomes (XX) and the males have only one ‘X’ chromosome (XO). The male has one chromosome less than the female.

Eg: In grasshopper females have 22 autosomes and two X chromosomes (XX). The males have 22 autosomes and only one ‘X’ chromosome (XO). The female produces only one type of egg with X chromosome. Male produces two types of sperms. One type with 11 A and one X chromosome (11 A + X) and the other type with 11 A only (11A + O). The fusion of X sperm with X egg produces a female (XX). The fusion of O sperm with X egg produces a male (XO).

Question 50.
Correct the amino acid sequence of sickle cell haemoglobin.

Answer:

Question 51.
Down’s syndrome may occur in both sexes. Comment.
Answer:
Because Down’s syndrome is an autosomal abnormality in which chromosome number 21 is represented by 3 copies (21 trisomy)

Question 52.
Chromosome sets of four individuals are given below 45A + XXY, 45A + XO, 45A + XX, 45A + XY.
a. How many body chromosomes and sex chromosomes are present in normal males and females?
b. Identify and write the names of chromosomal abnormalities in the above-listed chromosome sets.
c. Down’s syndrome is seen in both sexes. Comment.
Answer:
a. 22 pairs of autosomes and one pair of allosomes.
b. Klinefelter’s syndrome, Turner’s syndrome, Down’s syndrome (female), Down’s syndrome (male)
c. Down’s syndrome is trisomy in 21st chromosome. Hence autosomal abnormality doesn’t affect sex.

Question 1.
Mendel’s which law of inheritance is universally accepted without any exception? State the law.
Answer:
Mendel’s I law i.e., the law of dominance. The law states that when a pair of contrasting factors for a character are present together, one will dominate over the other.

Question 2.
AaBb was crossed with aabb. What would be the phenotypic ratio of the progeny? Mention the term to denote this kind of cross.
Answer:

Kind of cross: Test cross between a heterozygous dihybrid and double recessive parents.

Question 3.
The following table shows the genotypes for ABO blood grouping and their phenotypes. Fill in the gaps left in the table.

Answer:

Question 4.
Describe sex determination in certain Birds.
Answer:


Test: Inheritance of Two Genes

Complete linkage is observed in

Complete linkage is defined as the state in which two loci are so close together that alleles of these loci are virtually never separated by crossing over. In the case of male Drosophila there is a complete absence of recombinant types due to the absence of crossing over. This means that all of the genes that start out on a single chromosome, will end up on that same chromosome in their original configuration.
So, the correct answer is 'Male Drosophila'.

Which one of the following conditions of a zygotic cell would lead to the birth of a normal human female child?

One X and one Y chromosome

The sex determining mechanism in case of humans is XY type. Sperms are of two types, one containing X -chromosome and another containing Y- chromosome.

ZZ/ZW type of sex determination is seen in

XX-XO type or Protenor type : Mc clung in male squash bug (Anasa) observed 10 pairs of chromosomes and an unpaired chromosome. Their females have eleven pairs of chromosomes (22). Thus all the eggs carry a set of eleven chromosomes but the sperm are of the two types: fifty percent with eleven chromosomes and the other fifty percent with ten chromosomes. The accessory chromosome was X-chromosomes. Fertilization of an egg by a sperm carrying eleven chromosomes results in a female, while its fertilization by a sperm with ten chromosomes produces male. It is said to be evolved by the loss of Y-chromosome. e.g., Grasshopper and plant kingdom in Dioscorea sinuta and Vallisneria spiralis.

Crossing over is the exchange of genetic material between

Non-sister chromatids of homologous chromosomes

Sister chromatids of homologous chromosomes

Genes which are completely linked

Chromatids of non-homologous chromosomes

Each chromosome has 2 chromatids called sister chromatids. Human has 2 pairs of each chromosome called homologues chromosomes where one comes from mother and other from father. Crossing takes place during pachytene stage between non sister ( one chromatids from each homologues chromosomes) chromatids of homologues chromosomes.