What will be the eye colour of all female individuals in F1 generation when a white eyed female Drosophila is crossed with a red eyed male Drosophila ?

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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.

Why the other options are wrong — (a) 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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2022 Q43 — Science & Technology PYQ with Answer

State PCS 2022 Q43 — Science & Technology PYQ with Answer