Homozygous, heterozygous and hemizygous describes the genotype for a single gene in a diploid organism. What does it mean? In humans, since we are diploid organisms we have two copies of every chromosome and they are called homologous chromosomes – one copy comes from mother and one copy comes from father. So that means we have two copies for every given gene because we have two copies of every chromosome. The locus of these genes is same for homologous chromosomes. For example, locus of gene X would be same on maternal as well as on paternal chromosome. So based on these two copies of every gene or alleles (alleles = different version of the same gene) we have different genotypes.
At this point it is important that we know the difference between gene and alleles to understand this concept. In case you want to know more about it visit our blog on Gene and Alleles.
Let’s understand homozygous and heterozygous first. Let’s say we are talking about eye pigment gene and let’s assume that we have two alleles for this gene – one allele codes for brown color eye (B) and the other allele codes for blue color (b). On the homologous pairs of chromosome we can either have same alleles (both either brown or blue) or different alleles (brown and blue). Only these possible combinations can be there. So based on whether we have got the same alleles or different alleles we describe the terms homozygous and heterozygous.
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Homozygous: Homo = Same
Homozygous means both alleles present on the homologous chromosomes for a given gene are same. For example, either both alleles code for brown color, so the genotype BB or both alleles code for blue color, so the genotype is bb.
Figure 1. Homozygous
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Heterozygous: Hetero = Different
Heterozygous means both alleles present on the homologous chromosomes for a given gene are different. For example, one chromosome in homologous pair contains brown color allele (B) and the other chromosome contains blue color allele (b). So the genotype is Bb.
Figure 2. Heterozygous
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Hemizygous: Hemi = Half
In simple terms hemizyogous means hemi means half. That means only half is present which means only one allele is present instead of two in a diploid organism. This condition is present in males for sex linked genes on X chromosome. Because males are XY, they have only one copy of X chromosome. So for all the genes present on X chromosomes for male, they have only one allele. So we can say only half of the alleles are present as compare to females with genotype XX.
Figure 3. Hemizygous
Hemizygous condition can also occur in autosomal chromosome under abnormal condition such as deletion or loss of chromosome where one of the autosomal allele is missing. So for that particular gene on autosomal chromosome it is called hemizygous.
Figure 4. Summary
I hope this post helps J
To understand this topic more, watch the video on it 🙂