In Campbell's Biology Chapter 14-15, Mendelian Genetics & The Chromosomal Basis of Inheritance, question 12 reads:

12) In birds, sex is determined by a ZW chromosome scheme. Males are ZZ and females are ZW. A recessive lethal allele that causes death of the embryo is sometimes present on the Z chromosome in pigeons. What would be the sex ratio in the offspring of a cross between a male that is heterozygous for the lethal allele and a normal female?

A) 2:1 male to female

B) 1:2 male to female

C) 1:1 male to female

D) 4:3 male to female

E) 3:1 male to female

The correct answer is A. In my Punnett square, there were two male and two female so shouldn't the answer be C?


Let $Z$ be the chromosome carrying the non-lethal dominant allele, and $Z_l$ the chromosome carrying the lethal recessive allele.

The Punnett square goes:

enter image description here

$Z_lW$ is not viable, so there are two possible male genotypes ($ZZ$ and $ZZ_l$) and only one possible female ($ZW$). So the correct answer is A. indeed.


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