3.1 Sex Determination
The DNA is a building plan for a living organism. This plan is finalized when the egg is fertilized and the DNA from the mother and the father join together to form the DNA of the offspring. The sex of the offspring is also fixed at this point. The building plan will include details of what a male bird will look like and similarly what a female bird would look like. We have discussed that each feature of an organism is controlled by a single or a group of genes. The genes that determine the sex in birds are grouped together on a segment of the DNA called the X chromosome. Male birds carry two copies of the X chromosome and are denoted by XX. Female birds carry only one copy of the X chromosome paired with a much shorter chromosome called the Y chromosome. Female birds are therefore denoted by XY.
When the DNAs of the parent birds come together, the father will contribute one of its two X chromosomes. The mother may contribute either an X chromosome or a Y chromosome. If the mother gives an X chromosome the baby ends up with two X chromosomes (XX) and becomes a male. If the mother contributes a Y chromosome, the baby ends up with one X chromosome and one Y chromosome (XY) and turns out to be a female.