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Homozygous, Heterozygous and Hemizygous – What is the Difference?

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.

  • 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

  • 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

  • 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 🙂

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Homologous and Homozygous – What is the Difference?

Homologous and Homozygous – What is the Difference?

Homologous or homozygous? Let’s understand the difference in this post. We will start with homologous and move to homozygous because that would make it easy to understand.

  • Homologous –

    this term we use many times with reference to chromosome, isn’t it? Homologous chromosomes right! What this means is, in humans; as we are diploid organisms we have total of 46 chromosomes or 23 pairs. Half of these chromosomes come from mother (23 chromosomes) and half of these chromosomes come from father (23 chromosomes). That is how we have total of 46 chromosomes or 23 pairs of chromosomes.

Therefore, for every given pair, one chromosome is maternal and one chromosome is paternal (figure 1). Both these chromosomes in a pair would have the same genes which codes for the same traits. Even the location of all these genes would be same on both the chromosomes (because basically they are the same chromosome but coming from two parents).

Figure 1. Chromosome in pair having the same genes at same locations

Let’s say for example, we are talking about genes which codes for eye color and hair color. Figure 1 shows the position of both these genes on chromosome number 1. Whether it is a maternal chromosome 1 or paternal chromosome 1, these two genes are located on the same place for these chromosomes. This means for every given pair of chromosome, all the genes are same and their location is also same. That is why we call them homologous chromosomes.

Homologous Chromosome = Chromosomes in pair having the same genes (codes for same traits) at same locations (where ever it is present on maternal chromosome, it will be present in the same location on paternal chromosome).

  • Homozygous –

    as just discussed above, we are diploid organisms. This means we have two copies of every chromosome and thus we have two copies of every gene (figure 2). In one of our previous post we had seen that copies of the same gene are called alleles .

In short, alleles are different form of the same gene which might differ slightly in sequence giving us slightly different phenotype such as gene for eye pigmentation can give brown, green, blue etc. pigmentation.

Figure 2. Chromosome pair Homozygous for eye and hair color trait

Figure 2 shows gene for eye color. If both the chromosome 1, maternal as well as paternal codes for the same color eye pigment (blue) that means they have the same allele. In this case they are called homozygous for eye color trait. Let’s say for hair pigmentation both the chromosome 1 has allele for brown hair then they are called homozygous for hair color trait.

Homozygous = when on chromosome pair for a given trait both the alleles are same.

It is also possible that a given chromosome pair has two different alleles for the same gene, right! In that case they are called heterozygous for a given trait.

Remember, we use the term homozygous or heterozygous for individual gene. We cannot use it for the complete chromosome because a chromosome would have many different genes present and for each of them they might be either homozygous or heterozygous. Whereas homologous is the term we use with reference to chromosome in a pair.

I hope this post was helpful 🙂

To understand more details, watch a video on this topic here.