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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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F+, F-, Hfr and F’ Cells – What is the Difference?

F+, F-, Hfr and F’ Cells – What is the Difference?

We talk about F+, F–, Hfr and F’ Cells when we talk about bacterial conjugation which is one of the modes of horizontal gene transfer in bacteria. And it is important that we understand what these cell types are and what is the difference.

  • F+ Cells:

These are the bacterial cells which contains the F plasmid. They are designated F+ simply because they have F plasmid. We know plasmid is an extrachromosomal DNA that can replicate independently. It is called F plasmid because it has F factor which is Fertility factor. This fertility factor contains the genes require for the transfer or conjugation.

F+ Cells = Cells containing F plasmid (F plasmid = Plasmid containing F factor) 

Figure 1. F+ Cells

  •  F– Cells:

F– cells are the cells without F plasmid. These cells act as recipient cells because they don’t have F plasmid and thus they cannot donate the genetic material. They are designated as F– simply because they do not have F plasmid.

F– Cells = Cell lacking F plasmid

Figure 2.  F– Cells

  • Hfr Cells: High frequency of recombination

We just talked about F plasmid. This F plasmid is an episome which means it can either exist as an independent unit or it can integrate itself into chromosomal DNA. This integration is possible because F plasmid also contains the insertion sequence and via homologous recombination it can integrate itself. So when this integration happens, the resultant cells or the derivative of F+ cells is called Hfr cells.

Why are they called Hfr? – When these Hfr cells cross with F– cells; along with F plasmid some part of donor chromosomal DNA also gets transferred in the recipient cells because F plasmid is integrated into donor chromosomal DNA. Therefore the recipient cells now have got three different types of DNA – its own chromosomal DNA, some part of F plasmid along with some part of donor’s chromosomal DNA. So because crossing with Hfr cells results in high degree of recombination in recipient cells such cells are called Hfr cells.

Hfr Cells = Derivatives of F+ cells where F plasmid has integrated into host chromosomal DNA

Figure 3. Hfr Cells

  • F’ Cells:

We just discussed that F plasmid is an episome so once it gets integrated into host chromosomal DNA, it can also disintegration itself back from the host chromosome. Sometime this disintegration is not accurate and while disintegrating from host chromosome; F plasmid might pick up some genes next to its disintegration sites. So such cells where the F plasmid has disintegrated and have got some genes of host chromosome are called F’ cells. They are the derivatives of Hfr cells.

F’ Cells = Derivatives of Hfr cells where F plasmid has disintegrated from host chromosome and picks up some host genes next to F plasmid integration sites

Figure 4. F’ Cells

Figure 5. Summary 

I hope this post helps 🙂

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