A Genetics Definition of Heterozygous

Fresh English peas with the gene for a smooth, round seed shape

Matthew O'Shea / Getty Images

In diploid organisms, heterozygous refers to an individual having two different alleles for a specific trait.

An allele is a version of a gene or specific DNA sequence on a chromosome. Alleles are inherited through sexual reproduction as the resulting offspring inherit half of their chromosomes from the mother and half from the father.

The cells in diploid organisms contain sets of homologous chromosomes, which are paired chromosomes that have the same genes at the same positions along each chromosome pair. Although homologous chromosomes have the same genes, they may have different alleles for those genes. Alleles determine how particular traits are expressed or observed.

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). A heterozygous plant would contain the following alleles for seed shape: (Rr).

Heterozygous Inheritance

The three types of heterozygous inheritance are complete dominance, incomplete dominance, and codominance.

  • Complete Dominance: Diploid organisms have two alleles for each trait, and those alleles are different in heterozygous individuals. In complete dominance inheritance, one allele is dominant and the other is recessive. The dominant trait is observed and the recessive trait is masked. Using the previous example, round seed shape (R) is dominant and wrinkled seed shape (r) is recessive. A plant with round seeds would have either of the following genotypes: (RR) or (Rr). A plant with wrinkled seeds would have the following genotype: (rr). The heterozygous genotype (Rr) has the dominant round seed shape as its recessive allele (r) is masked in the phenotype.
  • Incomplete dominance: One of the heterozygous alleles does not completely mask the other. Instead, a different phenotype is seen that is a combination of the phenotypes of the two alleles. An example of this is the pink flower color in snapdragons. The allele that produces red flower color (R) is not completely expressed over the allele that produces white flower color (r). The result in the heterozygous genotype (Rr) is a phenotype that is a mixture of red and white, or pink.
  • Codominance: Both of the heterozygous alleles are fully expressed in the phenotype. An example of codominance is AB blood type inheritance. The A and B alleles are expressed fully and equally in the phenotype and are said to be codominant.

Heterozygous vs. Homozygous

An individual that is homozygous for a trait has alleles that are similar.

Unlike heterozygous individuals with different alleles, homozygotes produce only homozygous offspring. These offspring may be either homozygous dominant (RR) or homozygous recessive (rr) for a trait. They may not have both dominant and recessive alleles.

In contrast, both heterozygous and homozygous offspring may be derived from a heterozygote (Rr). The heterozygous offspring have both dominant and recessive alleles that may express complete dominance, incomplete dominance, or codominance.

Heterozygous Mutations

Sometimes, mutations can occur on chromosomes that change the DNA sequence. These mutations are typically the result of either errors that happen during meiosis or by exposure to mutagens.

In diploid organisms, a mutation that occurs on only one allele for a gene is called a heterozygous mutation. Identical mutations that occur on both alleles of the same gene are called homozygous mutations. Compound heterozygous mutations occur as a result of different mutations that happen on both alleles for the same gene.

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Bailey, Regina. "A Genetics Definition of Heterozygous." ThoughtCo, Aug. 29, 2020, thoughtco.com/heterozygous-definition-373468. Bailey, Regina. (2020, August 29). A Genetics Definition of Heterozygous. Retrieved from https://www.thoughtco.com/heterozygous-definition-373468 Bailey, Regina. "A Genetics Definition of Heterozygous." ThoughtCo. https://www.thoughtco.com/heterozygous-definition-373468 (accessed March 22, 2023).