Chickens, like most higher level animals, have two parents. Each parent has a complete set of 39 paired chromosomes. 38 of those pairs are matched in length and the genes they hold. These chromosomes, and the genes on them are referred to as autosomal in that gender makes no difference for inheritance. The other pair are the sex chromosomes, and there is a gender difference. For birds these two chromosomes are named Z & W (for mammals they are X & Y).
The Z chromosome is longer than the W chromosome, and therefore holds genes that do not have a matching counterpart on the shorter W chromosome. Male birds have two Z chromosomes, females have one Z and one W. The genes on Z that are not on W are referred to as sex-linked genes.
Each parent provides it offspring with one chromosome for each pair. The father provides Z to all his offspring. The mother, however, provides Z to her sons and W to her daughters.
A gene is the DNA at a specific location on a specific chromosome. Each location can contain one of two or more variations. These variations are referred to as alleles. If the same variation, or allele, is on both pairs of chromosomes, dominance is irrelevant–neither takes precedence over the other. This state of having identical alleles is referred to as being homozygous for that gene.
When the alleles do not match, dominance determines which of the two alleles present controls the trait or traits governed by that gene. This is a heterozygous state.
Of all the alleles for a gene there is an order of precedence, or dominance. An allele can be dominant, where it expresses and completely suppresses any indication that another allele is present. Likewise, an allele can be recessive, and only express when no allele that has a higher dominance is present. And there is also incomplete dominance, where a heterozygous pair of alleles cause an intermediate expression between the expression of homozygous dominant alleles and that of homozygous recessive alleles.
A classic example of an incompletely dominant gene is the blue gene. A bird who is homozygous for not-blue, bl+/bl+, is not blue–black pigment is not diluted. A bird heterozygous for blue, Bl/bl+ is blue, all black pigment is diluted (essentially bleached). And a bird with two copies of blue, Bl/Bl is further diluted to splash.
Now let’s go back to the sex chromosomes. Since a female has only one copy of the sex-linked genes, and all her sons will inherit that copy from her, but her daughters will inherit their copy of those genes only from their father, it is possible to manipulate the appearance of the offspring so that males and females have a different appearance.
By pairing a female who has a dominant allele with a male who has two copies of a recessive allele, the sons will inherit the dominant allele from their mother and the recessive allele from their father. Since they have a dominant allele, they will show the dominant trait. The daughters will not inherit that gene from their mother, but only a recessive allele from their father, and thus will show the recessive trait.