Homozygous and Heterozygous: Definition, Example in Humans and Key Differences

Definition in biology.

Most eukaryotes are diploid organisms; they have two sets of each chromosome. Each gene allele occupies the same locus (position) in a pair of chromosomes called the homologous pair of chromosomes.

They are called homologs because of their similar structure and encode the same genes.

This is true for all chromosomes, except for the sex chromosomes in men, since the Y chromosome is structurally different from the X chromosome and does not have the same genes as the X chromosome.

Therefore, males only carry a set of alleles for the genes located in the sex chromosomes.

A change in the allele results in a different phenotypic expression of the gene product. The similarity or dissimilarity of these alleles determines the zygosity of that particular gene, whether the gene is heterozygous or homozygous.

The concept of zygosity is based on the laws of the genetic inheritance of Gregor Mendel.


Understand homozygosity

When both loci of a gene in homologous chromosomes have identical alleles, the organism is said to be homozygous for that particular gene. It can also be called homozygous for the gene in question.

Pure-bred organisms are always homozygous for the desired traits. Also, in genetic experiments, organisms with unknown zygosity are crossed with organisms of the same species that are homozygous for that trait. Such a deterministic crossing is called a cross-test.

According to the inheritance pattern of the genes, homozygosity also shows a dominant and a recessive allele. In a homozygous dominant case, the organism has two copies (both alleles) of the dominant allele.

If the dominant allele for a gene is represented by “Q,” and the recessive allele is represented by “q,” a homozygous organism dominant for the gene will have a “QQ” genotype.

For example, in the case of eye color in humans, the allele for the color brown (B) is dominant over the allele for the color blue (b). Therefore, an individual, who is homozygous dominant for eye color, will have brown eyes with the “BB” genotype.

Similarly, in a homozygous-recessive case, the organism possesses two copies of the recessive allele, and its genotype would be “qq” for a particular gene.

For example, in the case of the recessive blood disorder of hemophilia (h), the condition occurs only when the individual is homozygous recessive; therefore, the affected individual will have the genotype “hh.”

Understanding heterozygosity

When both loci of a gene, in the homologous chromosomes, have different and different alleles, it is said that the organism is heterozygous for that particular gene, and it can also be called a heterozygote.

As both alleles are different, the organism will have both the recessive and dominant alleles; therefore, the genotype will be “Qq.” Since the individual has both allele forms, the phenotype produced is not based on the simple domain but is a little more complex.

In the case of a given trait that follows a simple dominance, the phenotype shown is the dominant phenotype despite the heterozygous nature of the alleles (the dominant allele cancels the recessive allele).

This zygosity state also shows more complex forms of domination, including domination and incomplete dominance.

In the case of co-dominance, the phenotypic traits of both alleles can be observed in the individual, for example, in human blood groups. In the incomplete domain, the heterozygous state shows a phenotype intermediate between the homozygous dominant and homozygous-recessive phenotype.

For example, the 4 o’clock plant produces a pink flower under heterozygous conditions that are an intermediate between the red flower of the homozygous dominant state and the white flower of the homozygous-recessive state.

Example in humans

These concepts of zygosity are best explained through the example of human blood types. The gene that determines the human blood type shows a presence of three alleles, namely, A, B, and O, where alleles A and B are dominant, and the O allele is recessive.

Therefore, the homozygous genotypes would be AA, OO, and BB. AA and BB are dominant homozygotes and show blood types A and B, respectively, while OO is homozygous recessive and shows blood type O.

The possible heterozygous genotypes are AO, BO, and AB. In this case, AO and BO follow a simple domain, and therefore, they show blood types A and B, respectively.

In the case of AB, the genotype is heterozygous, and since both alleles are dominant, a co-dominant inheritance pattern is observed that results in the AB blood type.

The heterozygous and homozygous terms apply only in the case of the genetic inheritance of the alleles of a gene and, therefore, can not be used as a broad term to describe the complete genome of the individual organism.

This would be highly inaccurate since each gene has a different number of alleles, and these alleles follow different inheritance patterns.

Key differences

Homozygous individual:

  • It is pure for one trait and reproduces true; it gives rise to similar homozygous individuals.
  • The two alleles of a character are identical; for example, TT. tt.
  • The homozygous individuals may carry dominant or recessive alleles, but not both.
  • It produces a type of gametes.
  • It does not show extra vigor.

Heterozygous individual:

  • The heterozygous individual is rarely pure and produces offspring with different genotypes, for example, TT. Tt and tt in selfing of individuals Tt.
  • Carries different alleles, ex. Tt.
  • A heterozygous individual has dominant and recessive alleles.
  • It produces two types of gametes.
  • The individual may show an additional vigor called hybrid vigor or heterosis.