Sex Determination in Bird Development
Most people have heard that the sex of a human is determined by the X and Y chromosomes. This method of sex determination is not universal across the animal kingdom. Other species determine sex in a variety of ways. Of those that use sex chromosomes, there are variations other than the mammalian XX/XY scheme. Birds use an alternative method of chromosomal sex determination: the ZZ/ZW system.
The ZZ/ZW System
In the XX/XY system, individuals with two X chromosomes (XX) are female, and those with one of each (XY) are male. Females are homozygous and males are heterozygous.
The ZZ/ZW system found in bird genetics is the exact reverse. Males have two Z chromosomes (ZZ) and females have one of each (ZW). The difference between male and female birds, genetically speaking, is that males are homozygous and females are heterozygous. Except for this difference, inheritance of both autosomal chromosomes and sex chromosomes is exactly the same.
Under the ZZ/ZW scheme, the sperm always carries a Z chromosome, while the ovum has a 50/50 chance of carrying either a Z or W chromosome. Therefore, sex is determined by the ovum. The letters Z and W were chosen for the chromosomes in this system to differentiate them from the XY system chromosomes.
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.
Convergent Evolution in Sex Chromosomes
Evolution favors a 50/50 sex ratio in most species, but there is more than one way to achieve it. In a case of convergent evolution, mammals and birds evolved the same kind of system, using two sex chromosomes—but they clearly evolved separately, because they are reversed. The mammal and bird sex determination systems demonstrate the ability of natural selection to arrive at similar and efficient solutions to problems.