Question

In Drosophila, an autosomal gene determines the shape of the hair, with B giving straight and b giving bent hairs. On another autosome, there is a gene of which a dominant allele I inhibits hair formation so that the fly is hairless (i has no known phenotypic effect). a. If a straight-haired fly from a pure line is crossed with a fly from a pure-breeding hairless line known to be an inhibited bent genotype, what will the genotypes and phenotypes of the
F1

and the

F2

be? b. What cross would give the ratio 4 hairless : 3 straight : 1 bent?

Answer 1

Answer:

a.  F1 genotype : IiBb  F1 phenotype : Hairless

    F2 genotype and phenotype :

   I_B_ (hairless), iiB_ (straight), iibb (bent)

b. IiBb X iiBb

Explanation:

a. Fly 1 : pure breeding straight haired : iiBB

Fly 2 : pure breeding hairless with bent genotype : IIbb

Fly 1 X Fly 2 : iiBB X IIbb = IiBb (F1)

All the offspring will be hairless since there is one dominant I allele which will inhibit hair production regardless of the type of allele of other gene.

Two F1 flies will be crossed to obtain F2 flies. IiBb X IiBb :

I_B_ : hairless        = 9  

I_bb : hairless        = 3

iiB_  : straight hair = 3

iibb  : bent hair      = 1

Hairless : straight : bent will be in 12 : 3 : 1 ratio.

b. The offspring of this cross :

hairless : I_ _ _

straight : iiB_

bent      : iibb

There is a bent offspring which has both the recessive alleles of both the genes. Hence it must have obtained the two alleles from the two parents. So their genotype is _i_b and _i_b.

Half of the offspring are hairless as given by the ratio. This is only possible if the cross is between Ii and ii fly:

    I   i

i  Ii    ii

i  Ii    ii

So the genotype is Ii_b and ii_b.

The remaining hairy flies are in 3:1 ratio of straight to bent. This is only possible if the cross is between Bb and Bb fly:

    B    b

B  BB Bb

b  Bb  bb

So the final genotype of the flies is IiBb and iiBb.