Question

A cross of a wild type red-eyed female Drosophila with a violet-eyed male produces all red-eyed offspring. If the gene is sex linked, what should the reciprocal cross(violet-eyed female x red-eyed male) produce? (Assume that the red allele is dominant to the violet allele).
A)All violet-eyed flies
B)3 red-eyed flies to 1 violet-eye
C)1:1 ratio of red and violet eyes in both males and female
D)Red-eyed females and violet-eyed males

Answer 1

Hint: It is said that genes that are borne by either sex chromosome are sex-linked. As Y chromosomes are inherited only from males, they are the only ones to inherit Y-linked traits. Since all inherit X chromosomes, men and women will get X-linked ones.

Complete answer:
Both red-eyed offspring are developed by a cross of the wild red-eyed female Drosophila type with a violet-eyed male. Assuming that the red allele is superior over the violet allele, the reciprocal cross (violet-eyed female x red-eyed male) would produce in both males and females a 1:1 combination of red and violet eyes if the gene is sex-linked.

Three kinds of inheritance can be divided into sex traits: sex-limited, sex-linked, and sex-influenced. Sex-limited characteristics are characteristics that are evident inside one sex only. For instance, barred colouring in chickens normally is visible only in the roosters. Traits such as colour blindness can be called sex-linked traits. They are said to be related because these characteristics are produced by more males (XY) than females (XX). This is because there is a second X gene in the females to combat the recessive phenotype. Thus, the phenotype in the male is more likely to be noticeable. Autosomal characteristics that are affected by sex are sex-influenced traits. If a male has one recessive allele, the characteristic will be seen, but the female will require two recessive ones to display the same characteristic. Baldness is one such mutation.

Hence, the correct answer is option (C)

Note: White eyes is one of the first sex-linked genes found in Drosophila melanogaster flies. Fur colour in domestic cats: on the X chromosome is the gene that causes orange pigment; hence, a Calico or tortoiseshell cat is almost invariably female, with both black (or gray) and orange pigment.