How is sex determined in chickens and what does it mean?

|If you want to know how to tell the difference between males and females in chickens, please see our related questions, below. This question addresses how sex is determined in chickens genetically.

In birds, the sex of the embryo is determined by the egg, not the sperm (as it is in humans). Avian sex differentiation is not determined by XY chromosomes like it is in humans and most mammals (or even the XO system used by some insects). In those systems the sex is determined by the sperm, by what the male contributes to the offspring. By contrast, birds have a ZW system of sex determination (shared with butterflies, moths, fish and reptiles). In that system, sex is determined by the female within her ova, and the sex of offspring is not affected at all by the sperm which the male contributes. Some people think it is this difference which makes parthenogenesis possible for birds. Parthenogenesis occurs when unfertilized eggs see embryonic development.

The truth is that these parthenogenetic eggs are extremely unlikely to hatch, and in most studies, avian parthenogenesis is reported to occur more frequently with turkeys than chickens. (One book I have seen reports just the opposite.) Most parthenogenetic development only involves a few organized cells, and it closely resembles normal embryonic death within the first three days of development in fertilized eggs. Parthenogenesis ceases when the eggs are incubated, although it is theoretically possible for one to hatch. Reports suggest that offspring could be males that could then reproduce sexually. Some suggest that parthenogenesis is more common when males are absent from the parent flock, and that parthenogenesis is nature's way of providing for what is missing.

And now for a bit of related trivia: in Ireland in the 12th century, the barnacle goose was believed to be parthenogenetic, to reproduce without mating. Therefore, priests and religious people could still eat this goose during fast days (days in which normally one could not eat meat without sin). The reason is that the barnacle goose wasn't considered flesh, as it wasn't "a product of the flesh." It was thought barnacle geese reproduced out of driftwood, probably because they were migratory and never seen in the summer while they were reproducing. In 1215 Pope Innocent III outlawed the practice of eating the reputedly parthenogentic barnacle goose on fast days. However, eating the barnacle goose (and later, just any type of goose) is still traditional in England for the religious holiday of Christmas.

Who would have thought that weird, geeky chicken knowledge would lead you to the traditional English Christmas goose?