Question

In birds the male has two identical Z sex chromosomes (males are ZZ), whereas the female has one Z as well as a nearly blank chromosome, designated W (females are ZW). In parakeets two genes control feather color. The presence of the dominant Y allele at a Z-linked gene results in the production of a yellow pigment. The dominant B allele at an autosomal gene controls melanin production. When dominant alleles are present for both genes, the feathers are green. If a dominant allele is only present for the Y gene, the feathers are yellow. If a dominant allele is only present for the B gene, the feathers are blue. If dominant alleles are not present at either gene, the feathers have no color (albino). A true-breeding green male parakeet is mated with an albino female, and one of their male progeny is backcrossed to his albino female parent. What proportion of their progeny should be albino females?

Answer 1

Let the alleles be -
ZY - dominant Y allele at the Z-linked gene
Zy - recessive y allele at the Z-linked gene
B - dominant B allele at the autosomal gene
b - recessive b allele at the autosomal gene

As given in the question,

• When dominant alleles are present for both genes, the feathers are green - B_ZY_ (the dash indicates there can be either a dominant or recessive allele at that position as the presence of only one dominant allele for both genes is enough to express green colour)
• If a dominant allele is present only for the Y gene, which means the other B gene is homozygous recessive, the feathers are yellow - bbZY_
• If a dominant allele is present only for the B gene, which means that the other Y gene is homozygous recessive, the feathers are blue - B_ZyZy (male) and B_ZyW (female - as female birds have one Z and one W sex chromosome and the W chromosome does not carry the Y gene as it is a nearly blank chromosome. So, presence of only one allele at Z chromosome will express the corresponding trait)
• If dominant alleles are not present at both genes, which means both are homozygous recessive, the feathers have no colour that is, they are albino - bbZyZy (male) and bbZyW (female)
  

Now, it is given that a true breeding male green parakeet is crossed with an albino female. True-breeding means the male is homozygous for both the genes. Hence, the genotype of the male is - BBZYZY and the genotype of the female is - bbZyW
The gametes from the male will be of the type - BZY and from the female will be - bZy and bW
As two gametes having Z chromosome has to come together to produce a male bird (ZZ), all the male progeny of this cross will have the genotype - BbZYZy (by combining the gametes BZY and bZy)
Finally, one of this male progeny is backcrossed with the albino female parent. The cross is - BbZYZy  x bbZyW
Probable gametes from the male parent are - BZY , BZy , bZY , bZy
Probable gametes from the female parent are - bZy , bW , bZy , bW
If we draw a Punnett square for this cross -

	bZy	bW	bZy	bW
BZY	BbZYZy	BbZYW	BbZYZy	BbZYW
BZy	BbZyZy	BbZyW	BbZyZy	BbZyW
bZY	bbZYZy	bbZYW	bbZYZy	bbZYW
bZy	bbZyZy	bbZyW	bbZyZy	bbZyW

Out of the 16 genotypes, 2 are albino females (bbZyW)
Hence, the proportion of the progeny which are albino females is - 2/16 = 1/8.