MPSC 2022 Prelims Paper-I · Question 43 of 100

Correct — option (2), Red. Eye colour in Drosophila is the classical example of a sex-linked character. The gene sits on the X chromosome and has no counterpart on the Y, red is the wild-type allele and is dominant, and white is the mutant allele and is recessive. The fly's sex is determined as in humans, the female carrying two X chromosomes and the male one X and one Y, so a female has two copies of this gene and a male has only one. Write the cross out and the answer falls straight from it. The white-eyed mother must carry the recessive allele on both of her X chromosomes, since a single dominant red allele would have made her red-eyed; her genotype is therefore white-white on the two X chromosomes, and every egg she produces carries an X bearing the white allele. The red-eyed father has only one X, and it must bear the red allele because that single copy is what determines his eye colour; his sperm carry either that X or a Y. The daughters of the cross are the offspring that receive an X from each parent — the white-bearing X from the mother and the red-bearing X from the father. Every daughter is therefore heterozygous, carrying one red allele and one white allele, and because red is dominant every one of them has red eyes. That is why the question can ask about all the females at once and expect a single colour: there is only one possible genotype among the daughters of this cross. The sons, by contrast, receive their single X from the mother, and since that X carries the white allele with no second X to mask it, every son is white-eyed. The whole pattern is the one called criss-cross inheritance, in which the father's character passes to his daughters and the mother's character to her sons, and it was worked out by Thomas Hunt Morgan and his students on this very fly. It is also the reason a sex-linked character can be recognised experimentally: the reciprocal cross, a red-eyed female with a white-eyed male, gives a completely different result, with all the F1 flies of both sexes red-eyed, whereas for a character carried on an ordinary chromosome the two reciprocal crosses would give the same outcome. Option (2) is therefore the answer.

• (1)White — White is the eye colour of the sons of this cross, not of the daughters, and this option is the answer of a candidate who worked the cross out correctly but read the stem carelessly. Every son takes his single X chromosome from his mother; that X carries the white allele; there is no second X and the Y carries no copy of this gene, so nothing can mask the recessive allele and every male in the F1 generation is white-eyed. The stem, however, asks only about the female individuals, and a daughter receives a second X from her father which carries the dominant red allele. White would also be the answer if a candidate had the dominance the wrong way round, believing white to be dominant over red; the surest way to fix the direction is to remember that red is the wild type, the colour of the fly as it is found in nature, and that the white-eyed fly was the mutant Morgan noticed in his culture bottles precisely because it was unusual.
• (3)Mixture of Red & White — This option offers blending, and it fails on two counts. First, the daughters of this cross are not a mixture of types: every one of them has the same genotype, one X from the mother carrying white and one X from the father carrying red, so there is no variety among the females for a mixture to describe. Second, a heterozygote for this gene does not show an intermediate or patchy colour; red is completely dominant, so a fly with one red allele and one white allele is as red-eyed as a fly with two. Blending of the two parental characters in the F1 generation would indicate incomplete dominance, of the kind seen when red and white four o'clock flowers give pink offspring, and codominance would give both characters expressed side by side, as the AB blood group does. Drosophila eye colour in this cross is a straightforward case of complete dominance, and the F1 daughters are uniformly red.
• (4)Mixture of Red & Blue — Blue is not one of the eye colours in this system at all, and this option can be eliminated before any genetics is attempted. The two alleles named in the stem are red and white, and no cross between them can produce a colour that neither parent carries the information for. The option is placed in the set to absorb the candidate who is guessing, and it is a reminder of a general habit worth having in genetics questions: begin by listing the characters actually present in the parents, because an option naming a character that appears nowhere in the cross is almost always noise. It is worth adding that Drosophila does have a long list of eye-colour mutants with names such as sepia, vermilion and brown, which is why the fly became the workhorse of classical genetics, but none of them is produced by crossing a white-eyed fly with a red-eyed one.

Sex linkage is the inheritance of a gene that lies on a sex chromosome, and its signature is that the pattern of transmission differs between the sexes. In Drosophila, as in humans, the female carries two X chromosomes and the male one X and a Y, and the Y carries almost none of the genes present on the X. A female therefore has two copies of every X-borne gene and can be homozygous or heterozygous for it; a male has a single copy and is neither, so whichever allele he carries is expressed, dominant or recessive. Three consequences follow, and they are what examiners test. The first is criss-cross inheritance: a recessive X-linked character in the mother appears in all her sons, while the father's X-linked character passes to all his daughters and to none of his sons, since a son receives the Y from his father. The second is that the two reciprocal crosses give different results, which is the experimental test that distinguishes a sex-linked gene from one on an ordinary chromosome. The third is that a recessive X-linked condition appears far more often in males than in females, because a male needs only one copy of the allele while a female needs two — the reason haemophilia and red-green colour blindness are predominantly male conditions in humans. Thomas Hunt Morgan established all of this on Drosophila melanogaster in the second decade of the twentieth century, beginning with a single white-eyed male that appeared in a bottle of otherwise red-eyed flies.

MPSC's biology section returns to Mendelian and post-Mendelian genetics in almost every paper, and sex linkage is the part of it that most rewards writing the cross out rather than recalling a result. The cross in this question takes about thirty seconds on paper — two lines of parental genotypes, one line of gametes, one line of offspring by sex — and thirty seconds is a good investment against an option set that contains the right answer for the wrong sex. The single most valuable habit in this family of questions is to answer for the sex the stem names: examiners routinely ask about the females when the memorable result concerns the males, and the reverse. A second habit is to check which allele is the wild type, since the mutant is normally the recessive one and the wild type the dominant. Drosophila is the standard vehicle for these questions because of its place in the history of the subject, but the same reasoning is asked in human terms as well, through haemophilia and colour blindness pedigrees, and a candidate who can run the fly cross can run those without new learning.

• Eye colour in Drosophila is carried on the X chromosome with no counterpart on the Y; the red allele is the wild type and is dominant, and the white allele is the mutant and is recessive.
• In the cross of a white-eyed female with a red-eyed male, every daughter receives a white-bearing X from her mother and a red-bearing X from her father, so all the F1 females are heterozygous and red-eyed.
• Every son of the same cross receives his only X from his mother and a Y from his father, so the recessive white allele is unmasked and all the F1 males are white-eyed.
• The pattern in which the mother's character passes to the sons and the father's to the daughters is called criss-cross inheritance, and reciprocal crosses giving unlike results is the experimental test for sex linkage.
• Thomas Hunt Morgan established sex linkage using Drosophila melanogaster, working from a white-eyed male that appeared among red-eyed flies in his laboratory cultures.

• Answering for the wrong sex, which is the commonest way this question is lost since the sons of this cross are white-eyed while the daughters are red-eyed
• Reversing the dominance and treating white as dominant, when red is the wild type and the white-eyed fly was the mutant Morgan first observed
• Expecting a heterozygote to show a blend of the parental colours, which would indicate incomplete dominance rather than the complete dominance operating here
• Assuming that reciprocal crosses must give the same result, which is true for a gene on an ordinary chromosome but is precisely what fails for a sex-linked gene

Genetics questions in MPSC papers come in three recognisable shapes. The first is a cross to be worked, as here, where two parental phenotypes are given and the phenotype or ratio of a generation is wanted; these are always quicker on paper than in the head. The second is definitional — what is meant by criss-cross inheritance, incomplete dominance, a test cross, a back cross — and the third is historical, naming the scientist or the organism behind a discovery, with Mendel's peas, Morgan's fruit flies and the maize work of the cytogeneticists recurring. Because all three draw on the same small body of material, the productive preparation is to hold the standard crosses ready-worked, including this one and its reciprocal, so that examination time is spent reading the stem carefully rather than deriving results from first principles.

No directly related past PYQ was found.

• practice — not a real PYQ

  A red-eyed female Drosophila homozygous for the wild-type allele is crossed with a white-eyed male. What will be the eye colour of the F1 males ?

  • (a)All white-eyed
  • (b)All red-eyed
  • (c)Half red-eyed and half white-eyed
  • (d)All with a blend of red and white

  Answer(b) All red-eyed — this is the reciprocal of the cross in the question, and it gives a different result, which is the mark of a sex-linked gene. Each son takes his single X chromosome from his mother, and here the mother carries the dominant red allele on both of her X chromosomes, so every son is red-eyed. The daughters, receiving the father's white-bearing X as well, are heterozygous and also red-eyed, so the entire F1 generation is red-eyed in this direction of the cross.

• practice — not a real PYQ

  Red-green colour blindness in human beings appears far more frequently in males than in females. The reason is that

  • (a)the gene lies on the Y chromosome and passes from father to son
  • (b)the gene is dominant in males and recessive in females
  • (c)the gene is recessive and lies on the X chromosome, of which a male has only one copy
  • (d)males are exposed to the condition by environment rather than by inheritance

  Answer(c) The gene is recessive and lies on the X chromosome, of which a male has only one copy — a male has no second X to carry a normal allele, so a single recessive allele is expressed, whereas a female must inherit the allele from both parents before the condition appears. This is the same asymmetry that makes every son of the white-eyed Drosophila mother white-eyed, and it is why haemophilia and colour blindness are described as sex-linked recessive conditions.