Heterozygous and homozygous

Homozygous and heterozygous are terms used to describe pairs of alleles. Individuals who carry two identical alleles (RR or rr) are known as homozygous. While individual organisms that carry different alleles (Rr) are known as heterozygotes. Read on to learn more about it.



DefinitionHomozygous is a genetic condition in which an individual inherits the same alleles of a gene from both parents.Heterozygous is a genetic condition in which an individual inherits different alleles of a gene from both parents.
Genotype representationHomozygous genotypes are represented as AA or aa for homozygous dominant or homozygous recessive conditions, respectively.Heterozygous genotypes are represented by Aa genotypes.
PhenotypesTwo different phenotypes are possible with homozygous dominant or recessive conditions.The phenotype is mainly due to the dominant allele in the heterozygous condition.
GametesHomozygous genotypes result in only one type of gamete.Heterozygous genotypes result in two different types of gametes.
TraitsHomozygous genotypes produce the same traits in different generations.Heterozygous genotypes produce different traits over different generations.
Hybrid VigorThe homozygous condition does not show hybrid vigor.The heterozygous condition shows hybrid vigor.
TypesHomozygous-dominant and homozygous-recessive are two types of homozygous conditions.The heterozygous condition can be expressed in three different ways; co-domain, incomplete domain, and complete domain.
Also calledOrganisms or cells with the homozygous condition are called homozygous.Organisms or cells with the heterozygous condition are called heterozygous.
Observed inHomozygous genotypes are seen in animals that reproduce asexually.Heterozygous genotypes are mainly observed in animals that reproduce sexually.
DiseasesCommon diseases associated with the homozygous condition include fibrosis, sickle cell anemia, and phenylketonuria.Common diseases associated with the heterozygous condition include Huntington’s disease, Marfan syndrome, and familial hypercholesterolemia.

Definition of homozygous

Homo means “equal,” and zygote refers to “having zygotes of a specific type,” so we can combine it by saying that when both alleles present on the homologous chromosomes of a given gene are the same , they are called homozygous.

When two genes share easily detectable sequence similarities (nucleotide sequence in DNA or amino acid sequence) in the proteins they encode, they are homologous. If two homologous genes occur in the same species, they are said to be paralogs and their protein products are paralogs.

An organism is said to be homozygous at a specific locus when it carries two indistinguishable (identical) copies of the gene that affect a trait present on the two reciprocal homologous chromosomes. (For example, the genotype is RR or rr when R and r refer to the different possible alleles of the same gene.) Such a cell or such an organism is called homozygous.

Definition of heterozygous

Hetero means “different” and zygote means “to have zygotes of a specific type.” So we can explain it by saying that when both alleles present on the homologous chromosomes for a given gene are different .

For example, one chromosome in a homologous pair contains brown hair (R) and another chromosome contains black hair (R). So the resulting genotype is Rr.

Examples of homozygous genotypes

Eye color

  • The dominant genetic trait for eye color is brown, which is represented by the BB genotype.
  • Therefore, in homozygous individuals, the eye color genotype will be BB or bb.
  • All other eye colors ranging from blue to green to gray are recessive traits that are only expressed when the genotype is homozygous-recessive, bb.
  • If the genotype is homozygous, the eye color genes at the loci of the two chromosomes should be identical.


  • Freckles are small spots on the skin caused by melanin pigment.
  • Freckle production is also a dominant trait and is controlled by the MC1R gene.
  • Freckled individuals have a homozygous dominant genotype that results in the expression of the dominant allele.
  • However, if the genotype is homozygous-recessive, freckles do not appear in individuals.

Examples of heterozygous genotypes

sickle cell anemia

  • Sickle cell trait is a recessive trait that causes blood cells to form incorrectly.
  • Thus, in individuals with a heterozygous genotype, the dominant trait is expressed, preventing sickle cell anemia.
  • In sickle cell anemia, red blood cells change their structure to sickle-shaped, making it small and insufficient to carry enough oxygen.
  • Therefore, the heterozygous genotype of the gene responsible for sickle cell anemia provides an advantage to these individuals.

Curly hair

  • The dominant trait of the hair type is curly; therefore, only people with homozygous recessive alleles have straight hair.
  • This gene encodes the protein that makes hair curly.
  • The heterozygous genotype causes the hair in these individuals to be wavy, which is between curly and straight hair.
  • This phenomenon is also called incomplete dominance, where the expressed phenotype is between dominant and recessive.
  • In complete dominance, curly hair is seen in individuals with the heterozygous condition.

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