Genetics: Q1) A dominant allele H reduces the number of body bristles that Droso

Question

Genetics:

Q1) A dominant allele H reduces the number of body bristles that Drosophila flies have, giving rise to a “hairless” phenotype. In the homozygous condition, H is lethal. An independently assorting dominant allele S has no effect on bristle number except in the presence of H, in which case a single dose of S suppresses the hairless phenotype, thus restoring the hairy phenotype. However, S also is lethal in the homozygous (S/S) condition.

A) What ratio of hairy to hairless flies would you find in the live progeny of a cross between two hairy flies both carrying H in the suppressed condition?

B) When the hairless progeny are backcrossed with a parental hairy fly, what phenotypic ratio would you expect to find among their live progeny?

Explanation / Answer

A)

Lets denote recessive allele of H and S by h and s.

As SS and HH in homozygous condition is lethal, so

Genotype of hairy fly is HhSs

   Gametes produced are: HS, Hs, hS, hs

   A cross between HhSs X HhSs according to punnet square will yield

HS

Hs

hS

hs

HS

HHSS

HHSs

HhSS

HhSs

Hs

HHsS

HHss

HhsS

Hhss

hS

hHSS

hHSs

hhSS

hhSs

hs

hHsS

hHss

hhsS

hhss

Genotypes and phenotypes produced

From above data, it is quite evident that a cross between two hairy flies both carrying H in the suppressed condition will produce ration of hairy to hairless flies = 7 : 2

B)

Genotype of hairless fly from F1 generation = Hhss

Gametes produced are : Hs, Hs, hs, hs

  A cross between HhSs X Hhss according to punnet square will yield

HS

Hs

hS

hs

Hs

HHsS

HHss

HhsS

Hhss

Hs

HHsS

HHss

HhsS

Hhss

hs

hHsS

hHss

hhsS

hhss

hs

hHsS

hHss

hhsS

hhss

Genotypes and phenotypes produced

From above data, it is quite evident that a cross between two hairless flies with hairy flies (parental) will produce ration of hairy to hairless flies = 8 : 4

HS

Hs

hS

hs

HS

HHSS

HHSs

HhSS

HhSs

Hs

HHsS

HHss

HhsS

Hhss

hS

hHSS

hHSs

hhSS

hhSs

hs

hHsS

hHss

hhsS

hhss

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