In birds, sex is determined by ZW chromosome inheritance. Males are ZZ and females are ZW. Within pigeon populations there is a recessive, Z-linked allele that is lethal in embryos. What is the expected sex ratio in the offspring of a cross between a male that is heterozygous for the lethal allele and a wildtype female?

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In this cross, half of the male offspring (ZZ) will inherit the lethal allele and die, while all female offspring (ZW) will survive because they can only inherit the normal allele from their father. This results in a 2:1 male to female ratio among the surviving offspring.

In Birds, Sex Is Determined by ZW Chromosome Inheritance