For genes on autosomes, we all have two copies—one from each parent. The two copies may be the same, or they may be different. Different versions of the same gene are called “alleles” (uh-LEELZ). Genes code for proteins, and proteins make traits.* Importantly, it’s the two alleles working together that affect what we see—also called a “phenotype.”
Female pigeons (ZW) have just one Z chromosome, and therefore just one allele for each of the genes located there. One gene on the Z chromosome affects feather color; three different alleles make feathers blue, ash-red, or brown. In a female bird (ZW), her single color allele determines her feather color. But in males (ZZ), two alleles work together to determine feather color according to their dominance. That is, 'ash-red' is dominant to 'blue', which is dominant to 'brown'.
Having two copies of a gene can be important when one copy is “broken” or defective. A functional second copy can often work well enough on its own, acting as a sort of back-up to prevent problems. With sex-linked genes, male mammals (and female birds) have no back-up copy. In people, a number of genetic disorders are sex-linked, including Duchenne muscular dystrophy and hemophilia. These and other sex-inked disorders are much more common in boys than in girls.
Red/green colorblindness is also caused by a defective gene on the X-chromosome. You need at least one working copy of the gene to be able to see red and green. Since boys have just one X-chromosome, which they receive from their mother, inheriting one defective copy of the gene will render them colorblind. Girls have two X-chromosomes; to be colorblind they must inherit two defective copies, one from each parent. Consequently, red-green colorblindness is much more frequent in boys (1 in 12) than in girls (1 in 250).
*Some genes code for functional RNAs, which also influence our traits.
The differences in sex chromosomes between males and females leads to specific inheritance patterns for sex-linked genes. (Above) Female pigeons inherit their color allele from their father. Males inherit one allele from each parent. In humans (below), the pattern is reversed.