Homozygous: A Genetics Definition

A Punnett square for seed shape
An example of homozygous alleles for seed shape. Courtesy Evelyn Bailey

Homozygous refers to having identical alleles for a single trait. An allele represents one particular form of a gene. Alleles can exist in different forms and diploid organisms typically have two alleles for a given trait. These alleles are inherited from parents during sexual reproduction. Upon fertilization, alleles are randomly united as homologous chromosomes pair up. A human cell, for example, contains 23 pairs of chromosomes for a total of 46 chromosomes. One chromosome in each pair is donated from the mother and the other from the father. The alleles on these chromosomes determine traits or characteristics in organisms.

Homozygous alleles may be dominant or recessive. A homozygous dominant allele combination contains two dominant alleles and expresses the dominant phenotype (expressed physical trait). A homozygous recessive allele combination contains two recessive alleles and expresses the recessive phenotype.

Example: The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). The round seed shape is dominant and the wrinkled seed shape is recessive. A homozygous plant contains either of the following alleles for seed shape: (RR) or (rr). The (RR) genotype is homozygous dominant and the (rr) genotype is homozygous recessive for seed shape.

In the image above, a monohybrid cross is performed between plants that are heterozygous for round seed shape. The predicted inheritance pattern of the offspring results in a 1:2:1 ratio of the genotype. About 1/4 will be homozygous dominant for round seed shape (RR), 1/2 will be heterozygous for round seed shape (Rr), and 1/4 will have the homozygous recessive wrinkled seed shape (rr). The phenotypic ratio in this cross is 3:1. About 3/4 of the offspring will have round seeds and 1/4 will have wrinkled seeds.

Homozygous vs. Heterozygous

A monohybrid cross between a parent that is homozygous dominant and a parent that is homozygous recessive for a particular trait produces offspring that are all heterozygous for that trait. These individuals have two different alleles for that trait. While individuals that are homozygous for a trait express one phenotype, heterozygous individuals may express different phenotypes. In genetic dominance cases in which complete dominance is expressed, the phenotype of the heterozygous dominant allele completely masks the recessive allele phenotype. If the heterozygous individual expresses incomplete dominance, one allele will not completely mask the other resulting in a phenotype that is a mixture of both the dominant and recessive phenotypes. If the heterozygous offspring express co-dominance, both alleles will be expressed completely and both phenotypes will be observed independently.

Homozygous Mutations

Occasionally, organisms can experience changes in DNA sequences of their chromosomes. These changes are called mutations. Should identical gene mutations occur on both alleles of homologous chromosomes, the mutation is considered a homozygous mutation. Should the mutation occur on only one allele, it is called a heterozygous mutation. Homozygous gene mutations are known as recessive mutations. In order for the mutation to be expressed in the phenotype, both alleles must contain abnormal versions of the gene.