Question

a new student of genetics suspects that a particular recessive trait in fruit flies (dumpy wings: d) is sex linked. A single mating between a female fly having dumpy wings and a male fly with normal wings (D) produces 3 dumpy- winged females and 2 normal males. On the basis of these data, is the trait sex-linked or autosomal? what were the genotypes of the parents? explain how the conclusions can be reached on the basis of so few data. male fruit flies are XY and females are XX.

Answer 1

Answer on Question # 64130 - Biology - Genetics

Let's suppose that the trait is sex-linked. Then,

$\mathbf{X}^{\mathrm{d}}$ - allele for dumpy wings

$\mathbf{X}^{\mathrm{D}}$ - allele for normal wings

The female's genotype is $\mathbf{X}^{\mathrm{d}}\mathbf{X}^{\mathrm{d}}$ , while the male's genotype is $\mathbf{X}^{\mathrm{D}}\mathbf{Y}$

It results in the following cross:

P: $\mathrm{X}^{\mathrm{d}}\mathrm{X}^{\mathrm{d}}$ $\mathrm{X}^{\mathrm{D}}\mathrm{Y}$

G: Xd XD Y

F: $\mathrm{X}^{\mathrm{D}}\mathrm{X}^{\mathrm{d}}$ $\mathrm{X}^{\mathrm{d}}\mathrm{Y}$

normal female male with dumpy wings

Thus, in case of sex-linked inheritance, the observed phenotypes are completely different from the expected ones. It means that there is an autosomal inheritance for this trait.

Then, the cross would be:

D- allele for normal wings

d - allele for dumpy wings

The female's genotype is dd, and the male's one is Dd

P: dd Dd

G: d D d

F: Dd dd

normal dumpy wings

As X and Y-chromosomes are inherited independently from autosomes, the flies with normal and dumpy wings can have any sex.

Thus, 3 dumpy-winged females and 2 normal males, produced in the cross, can be explained by the scheme represented above. Thus, the inheritance is autosomal.

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