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13 Difference Between Heterozygous and Homozygous

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Homozygous Definition

  • Homozygous is a state in which a particular gene is present as two alleles that are the same, either both are recessive or both are dominant.
  • In a homozygote individual, the recessive traits are phenotypically expressed because there is no dominant allele to block the expression of the gene, assuming it follows Mendelian rules.
  • There are many diseases in humans that are caused by recessive alleles. In these cases, the disease is only expressed in the homozygous form; an example is steroid-resistant nephrotic syndrome (SRNS) which is caused by a recessive mutation in the NPHS2 gene in about ¼ of cases.
  • Example: If we use the letter A to represent a dominant allele and a to represent a recessive allele, then the genotype of a homozygous individual could be either AA or aa.

Heterozygous Definition

  • Heterozygous is the state in which each of the two alleles of a gene is different; one is a dominant allele while the other is a recessive allele.
  • In Mendelian genetics, a dominant allele is always expressed in both the homozygous and heterozygous condition, but a recessive allele is not expressed if present as an allele in the heterozygous state.
  • The sickle-cell trait can occur in the heterozygous condition; in this form, the allele for sickle-cell is an advantage because it protects people from malaria, but at the same time does not make them ill with sickle-cell.
  • Example: If A is used to indicate dominance, and a is used to indicate recessiveness, then the genotype of a heterozygote would be given as Aa.
  • The heterozygous condition means that alleles can be hidden and carried but may not be phenotypically expressed depending on the genotype of the offspring.

What is the difference between heterozygous and homozygous individuals?

Characters Homozygous Heterozygous
Definition Homozygous is the presence of two copies of the same allele which codes for a particular trait. Heterozygous is the presence of two different copies of alleles of a particular gene.
Alleles Homozygous individuals have either dominant or recessive allele pairs (Ex: TT or tt). Heterozygous individuals have both dominant and recessive alleles (Ex: Tt).
Gametes Homozygous individuals produce a single type of gametes. Heterozygous individuals produce both types of gametes with dominant and recessive alleles.
Self-breeding Self-breeding of homozygous individuals produces the same trait over generations. Self-breeding of heterozygous individuals produces a combination of different traits.
Types Homozygous-dominant and homozygous-recessive are the two types of homozygous alleles. Complete dominance, incomplete dominance, and codominance are the three types of heterozygous allleles.
Vigour Homozygous individuals do not show an extra vigour. Heterozygous individuals can show extra vigour called hybrid vigour.
Genotype representation Homozygous genotypes are represented as AA or aa for homozygous-dominant or homozygous-recessive conditions, respectively. Heterozygous genotypes are represented by Aa genotypes.
Phenotypes Two different phenotypes are possible with dominant or recessive homozygous conditions. The phenotype is mostly due to the dominant allele in the heterozygous condition.
Traits Homozygous genotypes produce the same traits over different generations. Heterozygous genotypes produce different traits over different generations.
Other terms Organisms or cells with the homozygous condition are termed as homozygotes. Organisms or cells with the heterozygous condition are termed as heterozygotes.
Observed in Homozygous genotypes are observed in animals reproducing by asexual means. Heterozygous genotypes are mostly seen in animals reproducing by sexual means.
Diseases Common 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’s syndrome, and familial hypercholesterolemia.
ExamplesThe human blood type O, which is determined by the genotype OO, the dominant eye color brown, which is determined by the genotype BB, and the recessive eye color hazel, which is determined by the genotype bb are the examples of homozygous. The human blood type AB, which is determined by the genotype, AB, snapdragon flower color, and the codominance of the hair color in roan horses are the examples of heterozygous.

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