Sex-Linked Traits in Bird Genetics
Understanding the ZZ/ZW sex chromosome system is important for people who breed birds, whether the interest is in chicken genetics, parrot genetics, or some other type of bird. The way sex-linked traits are inherited is opposite to the way they are inherited by humans and other mammals.
For example, in cockatiels, budgerigars (parakeets), lovebirds, and other small parrots, the lutino color mutation is a sex-linked recessive trait. Lutino birds lack the dark pigment melanin, which is responsible for black, gray, and blue coloration in birds. As a result, lutino birds appear to have significant yellow coloration, which would ordinarily be covered up by melanin.
The lutino gene is located on the Z chromosome. Since lutino females have only one Z chromosome, they will pass this chromosome down to all their sons (remember male birds are ZZ), but not to their daughters (female birds are ZW and get the Z chromosome from their fathers).
A male bird will be lutino only if his father has the gene and his mother has the mutation as well. With a non-lutino mother, a male that inherits lutino from his father will be a heterozygous carrier, but will not have a lutino phenotype. A lutino-colored male must be homozygous, since the trait is recessive. In this situation all his daughters will be lutino-colored and all his sons will be carriers.
For a non-lutino carrier male (heterozygous), each daughter has a 50% chance of being lutino, and each son has a 50% chance of being a carrier.