A carrier is a heterozygous individual who has no apparent abnormality but can pass on an allele for a recessively inherited genetic disorder. Autosomal dominant and autosomal recessive alleles have different patterns of inheritance. Heterozygotes are affected. Most affected children have normal parents since heterozygotes have a normal phenotype. Two affected parents always produce an affected child.
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.