ABSTRACT
When a mutation of only one gene leads to an abnormal phenotype, the pattern of inheritance is said to be monogenic. All genes, except those on the X chromosome in the male, are found in two copies in every person; each copy is an allele of the gene. When an individual has one mutant allele and one normal allele the individual is referred to as a heterozygote. If both alleles are mutants then the individual is homozygous for the mutation. Because males have only one X chromosome, they are hemizygotes for each of the genes on this chromosome.
