In poultry, what determines the sex of the offspring?

In poultry, the sex of the offspring is determined by the female parent, specifically through the type of sex chromosomes she contributes. Chickens have a unique sex-determination system where the female possesses ZW sex chromosomes, while the male has ZZ chromosomes.

When a hen (female) lays an egg, she can contribute either a Z or a W chromosome. The sperm from the rooster (male) will provide either another Z chromosome, resulting in a male (ZZ) chick, or a W chromosome, leading to a female (ZW) chick. Thus, it is primarily the female's genetic contribution that determines the sex of the offspring.

While both parents are involved in the breeding process, it is the combination from the female's ZW chromosomes that ultimately dictates the sex outcome of the resulting chicks. Environmental factors and the male parent do not play a role in the chromosomal determination of the chicks' sex in poultry.