Question

2.In Drosophila, yellow body is due to an X-linked gene that is recessive to the gene for gray body.

(a) A homozygous gray female is crossed with a yellow male. The F1 are intercrossed to produce F2. Give the genotypes and phenotypes, along with the expected proportions, of the F1 and F 2 progeny.

(b) A yellow female is crossed with a gray male. The F1 are inter- crossed to produce the F2. Give the genotypes and phenotypes, along with the expected proportions, of the F1 and F2 progeny.

(c) A yellow female is crossed with a gray male. The F1 females are backcrossed with gray males. Give the genotypes and phenotypes, along with the expected proportions, of the F2 progeny.

3. How come only female cats show the calico phenotype?

please answer the highlighted question

Answer 1

Answer 2.

As yellow gene is recessive to gray gene, we will write yellow gene as X^g and gray gene as X^G.

a) In this condition,

Female genotype = X^GX^G (homozygous gray female)

Male genotype = X^gY (yellow male)

Hence the gametes will be

The cross for F₁ generation will occur as -

Hence, there will be 1/2 gray males (X^GY) and 1/2 gray females (X^GX^g).

Proportion for F₁ = Gray males : Gray females = 1 : 1

In F₁ generation,

Male genotype = X^GY

Female genotype = X^GX^g

The gametes from the F₁ generation will be

The inter-cross of F₁ generation will occur as follows :

Hence, there will be 1/4 gray males (X^GY), 1/4 yellow males (X^gY)  and 1/2 gray females (X^GX^G and X^GX^g).

Proportion for F₂ = Gray males : Yellow males : Gray females = 1 : 1 : 2

b) In this condition,

Female genotype = X^gX^g (yellow female)

Male genotype = X^GY (gray male)

Hence the gametes will be

The cross for F₁ generation will occur as -

Hence, there will be 1/2 yellow males (X^gY) and 1/2 gray females (X^GX^g) .

Proportion for F₁ = Yellow males : Gray females = 1 : 1

In F₁ generation,

Male genotype = X^gY

Female genotype = X^GX^g

The gametes from the F₁ generation will be

The inter- cross of F₁ generation will occur as follows -

Hence, there will be 1/4 gray males (X^GY), 1/4 yellow males (X^gY), 1/4 gray females (X^GX^g) and 1/4 yellow females (X^gX^g).

Proportion for F₂ = Gray males : Yellow males : Gray females : Yellow females = 1 : 1 : 1 : 1.

c) In this condition,

Female genotype = X^gX^g (yellow female)

Male genotype = X^GY (gray male)

Hence the gametes will be

The cross for F₁ generation will occur as follows -

Hence there will be 1/2 yellow males (X^gY) and 1/2 gray females (X^GX^g).

Proportion for F₁ = Yellow males : Gray females = 1 : 1.

For back crossing,

Male genotype = X^GY

Female genotype = X^GX^g

The gamete for F₁ generation will be -

The back-cross will occur as follows -

Hence, there will be 1/4 gray males (X^GY), 1/4 yellow males (X^gY), 1/4 gray females (X^GX^g) and 1/4 yellow females (X^gX^g).

Proportion for F₂ = Gray males : Yellow males : Gray females : Yellow females = 1 : 1 : 1 : 1.

Answer 3.

In cats, the gene controlling fur or coat colour is X-linked. One allele is X^B i.e. orange fur and another is X^b i.e. black fur in which there is no clear dominance.

However, a heterozygous female cat having genotype X^BX^b, instead of showing orange or black fur, shows patchwork of orange and black which is known as tortoise shell.

The explanation for this appearance can be understood as follows-

• In males, the possible genotypes for coat colour can be X^BY (for orange fur) or X^bY (for black fur). As only one allele exists for the gene, so it will be expressed. Males cannot be heterozygous as they have only one X chromosome and this gene is X-linked.

• In females, there are three possible genotypes - X^BX^B , X^bX^b and X^BX^b. The X^BX^B will produce orange fur, X^bX^b will produce black fur, and X^BX^b will produce patchwork of orange and black. In female cats, there are two copy of X-chromosomes, one of which supercoils in itself during embryonic development and forms a structure called Barr body.

In case of heterozygous female cats, in some cells(blastomeres) the X-chromosome forming Barr body is X^B while in other cells(blastomeres) the X-chromosome forming Barr body is X^b.

The cells emerging from blastomeres of left type will produce skin having black colour and the cells emerging from blastomeres of right type will produce skin having orange colour. In this way there will be a patchwork of orange and black on a heterozygous cat's fur or coat. This is known as   the calico phenotype.

From the above explanation, it is clear that only female cats show the calico phenotype.