Inheritance Patterns

Members of each chromosome pair (except for X and Y) carry the same set of genes, so that a diploid cell carries two copies of (almost) every gene, one on the maternally derived chromosome, and one on the paternally derived chromosome. These two copies may be precisely identical, meaning the two genes have precisely the same sequence of nucleotides, or their sequences may be slightly different. These sequence differences may have no effect at all on the phenotype, or they may lead to different forms of the same trait, such as brown versus blue eye color, or smooth versus wrinkled pea texture. The two different forms of the gene are called alleles, and so we speak of the brown eye color allele or the wrinkled pea texture allele.

While a single organism can possess no more than two different alleles for a single gene, many different alleles for a particular gene can exist in a population. For instance, there are three alleles for the ABO blood group gene, namely A, B, and O.