Polygenic inheritance occurs when one trait is governed by two or more sets of alleles. A hybrid cross for skin color provides a range of intermediates. Includes cleft lip, clubfoot, hypertension, diabetes, schizophrenia, allergies and cancers. The ABO system of human blood type is a multiple allele system. Sickle-cell disease is a blood disorder controlled by incompletely dominant alleles.
Traits controlled by alleles on sex chromosomes are sex-linked.
Since Y chromosome is smaller, most sex-linked genes are on X chromosome. Males receive X-linked traits from mother, source of male's only X chromosome. If female shows a recessive sex-linked trait, her father must have it and her mother is carrier. 13.12 and Fig. X-linked recessive disease involves a mutant gene that fails to produce protein dystrophin.