Sex determination in mammals and most other animals is via the XY chromosome system. Simply put, the X and Y chromosomes determine the sex of the offspring. A female has two X chromosomes, one from the father and one from the mother, while a male has one X chromosome and one Y chromosome, the X from the mother and the Y from the father. In humans and most other animals, the default reproductive organs are female. Male organs develop only in the presence of a male determining gene/protein. This male gene is carried on the Y chromosome. The sperm determines sex in the XY system.
Recently while reading “The Gene” by Siddhartha Mukherjee, I was surprised to learn that in birds and some reptiles, insects, fish and crustaceans there is a different system for determining gender. I had no idea and had to do some research.
In these other animals sex is determined by two completely different chromosomes, the Z and W chromosomes. The ZW chromosomes were derived from a different pair of ancestral chromosomes than the XY chromosomes. Both the XY and ZW ancestral chromosomes were ordinary chromosome in reptilian times. However, once the male-determining gene “landed” on the Y chromosome, the Y started shedding genes held in common with the X chromosome and the Y chromosome shriveled to a fraction of its former size.
The sex-determining gene in birds is on the Z chromosome and the entire system is reversed. A male bird has two Z chromosomes while the female has one Z and one W chromosome. A bird’s sex is determined, not by the simple presence of the sex-determining gene, instead, it is regulated by the amount or “dose” of the sex-determining gene. A female with ZW chromosomes has one dose of the sex gene while the male with ZZ chromosomes has two doses of the sex gene. Two doses are required to develop male reproductive organs. The ovum (egg) in this case determines sex.
Why this reversal of systems in birds and some select other animals? From my reading I discovered several convoluted suggestions, none of which seems totally satisfactory to me. Research on the ZW gender-determining system is ongoing and more information and answers should continue to appear in the literature.
The bird picture is just a generic picture – a pretty white-crowned sparrow (Zonotrichia leucophrys) photographed in our yard (Modoc County CA) last summer.