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

Question

Quizplus · Accepted Answer

In this scenario, the male pigeon (ZZ) is heterozygous for the lethal allele, meaning one of his Z chromosomes carries the lethal allele. When he mates with a normal female (ZW), who does not carry the lethal allele, the offspring could inherit the following combinations: Z (normal) from the father and Z from the mother resulting in a normal male (ZZ), Z (with lethal allele) from the father and Z from the mother resulting in a male that dies as an embryo, and Z (either normal or with lethal allele) from the father and W from the mother resulting in a normal female (ZW). Since the Z chromosome with the lethal allele when passed to a male offspring results in death, only half of the male offspring survive, leading to a 2:1 male to female ratio among the surviving offspring.

In Birds, Sex Is Determined by a ZW Chromosome Scheme