Types of genes (alleles) are transmitted from parents to offspring and result in calculable patterns of expression. The mechanisms of expression are expressed in the terms dominant and recessive. For example, if two alleles are paired and one expresses its phenotype and the other does not, then the first allele is dominant and the second is recessive. By Mendel's reasoning, a recessive allele expresses itself only if no dominant allele is present to mask its expression. Since in most cases alleles are paired, two recessive alleles must be present in order for the recessive characteristic to express itself.

Examination of pedigrees reveals five basic patterns of transmission that follow Mendel's laws. They are autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, and Y-linked. In general, recessive conditions occur because a functional protein is not produced. This happens commonly with enzymes, causing inborn...