Homozygous vs. Heterozygous

The difference between homozygous and heterozygous can be narrated as, a diploid cell is homozygous  for a gene when similar alleles of the gene are present on the gene locus on homologous chromosomes. A diploid cell is termed as heterozygous for a gene when both the copies of alleles are dissimilar present at gene locus.

A homozygous genotype is represented by two letters both letters capital or both small, i.e. either PP or pp. Capital letters for a dominant trait for small letters for the recessive trait.  The homozygous individual possesses either  a dominant  or recessive trait but not both at a time.  A heterozygous trait is represented by one capital and one small letter, i.e. Pp.  P is for dominant
allele while p is for recessive allele. A heterozygous will possess the phenotypic trait specified by a dominant allele.

A homozygous will produce only one type of gamete while a heterozygous produces two types of gametes which are transmitted to the next generation.

A homozygous individual is pure for a trait and contains both alleles similar for that trait on gene locus. While a heterozygous individual is not pure for a trait and most often possess the dominant trait. A heterozygote individual shows a greater relative strength than homozygous dominant or homozygous recessive genotype. This is termed as heterozygote advantage. This extra vigour
is not shown by homozygous genotype.

Individuals with heterozygous genotype can combat environmental stress in a better way related to that phenotype. They have chances of evolution with good traits thatcan help their next generations to adapt more to their environment. This feature of having chances of good evolution is not possessed by homozygous individuals.

Comparison Chart

Basis Homozygous Heterozygous
Key difference Both the alleles of a gene are identical in a diploid organism.  Both the alleles of a gene are
different in a diploid organism.
Denoted by denoted by either both capital or both small letters, i.e. PP or Pp. Denoted by one capital and one small letter, i.e. Pp.
Gametes produced Only one type of  gametes are produced. Two types of gametes are produced.
Purity Pure for a trait. Not pure for a trait.
Types of alleles produced Either dominant or recessive alleles of a gene are present but not
both at a time.
Both dominant and recessive alleles are present at a time.
Extra Vigor Extra vigour is not shown. They show an extra vigour
called heterozygote advantage.
Evolution Fewer chances of good
More chances of good evolution.

What is homozygous?

An organism is said to be homozygous for a trait when boththe gene versions (alleles) are identical for this specific gene. So this termdescribes the genotype of a diploid organism. Both dominant or both recessivealleles of a gene are present, I.e.genotype is either PP or pp. Thus the individual is attributed as homozygous dominant or homozygous recessive. Homozygous individuals produce only one type of gametes transferred to the next generation.

Example of homozygous trait

The example of a homozygous trait in the human body can be given as in terms of blood group.  Blood group genotype AA or BB or OO are homozygous genotypes which represent the phenotypes of blood group A, blood group B and blood group O respectively.

What is heterozygous?

A diploid organism is said to be heterozygous for atrait  when both the gene versions(alleles) are not identical for that specific gene. This term describes the genotype of an individual when one dominant and one recessive allele is present,i.e. genotype of Pp. Heterozygous individuals produce two types of gametes which are transferred to the next generation. Thus chances of-of newgenetic combinations in the next generation are increased. Thus offspring of heterozygote individuals have greater ability to cope with the environmental stresses. Heterozygote individuals also show an extra vigour.

Example of heterozygous

Example of the heterozygoustrait in the human body can be given is of ABO blood group system. Blood groups AB, AO and BO, are heterozygous genotypes for which phenotypes are blood groups AB, A and B respectively.

The examples of dominant and recessive traits in the humans

  • Earlobe attachment
  • Rolling of tongue
  • Cleft chin
  • Dimples
  • Natural curly hair
  • Freckles
  • Allergies
  • Colour blindness


If the chromosome contains a
single copy of an allele and another one is missing in a diploid organism the
chromosome is termed as hemizygous.


When both alleles are mutant (complete depletion of a gene or complete loss of function) in a diploid organism, the chromosome is termed as nullizygous.

Key Differences between Homozygous and Heterozygous

  1. For homozygous genotype, both the alleles present on a gene locus are similar while for heterozygous genotype, both alleles are different.
  2. In homozygous genotype, both dominant or both recessive alleles are present while in heterozygous genotype one dominat and one recessive allele present.
  3. Heterozygous genotype shows an extra vigour which is not shown by homozygous.
  4. Homozygous character is pure for a trait while heterozygous is not pure for a trait.


In the above article,we see the clear difference between homozygous and heterozygous traits and organisms. Homozygous and heterozygous are two important concepts of genetics and it is important to know the difference between them.

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