Anna C. In each pregnancy, if the mother is a carrier and the father has the disease, the expected outcomes are: Principles of genetics. But in females, where the Z chromosome does not recombine, the two alleles always pass to offspring together. These and other sex-inked disorders are much more common in boys than in girls. Male birds make sperm, which always have a Z chromosome. Elsevier Saunders; If a man has an X-linked recessive disorder and his mate does not carry the allele for it, all of their girls will be carriers. Female carriers can have a normal X chromosome that is abnormally inactivated. Only one of the Y chromosome genes, the SRY gene, is responsible for male anatomical traits. Carriers can pass abnormal genes to their children. Chimpanzees are our closest living relatives. While sex linkage is not the same as genetic linkage , sex-linked genes can be genetically linked see bottom of page. She passed the harmful allele for this X-linked trait on to one of her four sons and at least two of her five daughters. These are sex-linked traits.