A sex-linked gene is any gene located on the “X” chromosome. The avian X chromosome is believed to contain about 10% of the genome or haploid genetic makeup. Sex-linked genes do not have an allelic counterpart on the “Y” chromosome; therefore, the female will carry only one allele for sex linked traits. This one-allelic condition is termed hemizygous in contrast to the homozygous and heterozygous possibilities in the male. The single sex-linked allele that is passed onto the female offspring from the male parent will be expressed by the female offspring. Again, only one sex-linked allele need be present in the female to be expressed.
When the egg is fertilized, the embryo receives 1/2 of it’s genetic makeup from the female and the other 1/2 from the male. Therefore, when calculating the probable outcome of a cross, only one sex chromosome from the male is combined with one sex chromosome from the female during fertilization. Since each bird has two chromosomes that make up the sex chromosomal pair, there will be four possible outcomes for any cross involving a single sex linked mutation.
To explain sex-linked genes further, I will show two different crosses. The first cross involves a male with the Cinnamon phenotype ( that is, a visual Cinnamon plumage color). In order for the male to be a Cinnamon phenotype the Cinnamon gene must be present on both sex chromosomes (Xc,Xc) - the homozygous recessive condition. If you have a Cinnamon male (Xc,Xc) and cross it to a Cinnamon female (Xc,Y), all of the offspring will be Cinnamon. See Figure 8 below.
If the Cinnamon gene occurs on just one of the male’s sex chromosomes (Xc,X+) he will not display the Cinnamon trait. In this case, the male is heterozygous (that is, contains both the dominant (+) and recessive (c) alleles). The phenotype for the heterozygous Cinnamon will be a Gray. In Figure 9 below a Gray Male heterozygous for Cinnamon (Xc,X+) is crossed to a Gray Female (X+,Y) hemizygous for the Gray gene. You will notice that not only does this produce four different types of offspring, but could also allow early sex determination since only half of the female offspring of this particular cross can display the Cinnamon trait.