Difference Between Homozygous and Hemizygous

The key difference between homozygous and hemizygous is that homozygous is the condition in which both alleles of a diploid organism are the same while hemizygous is the condition in which only one allele is present.

Genes exist as alleles or copies. Generally, a gene has two alleles. An allele is a version of a gene which results in a distinguishable phenotypic effect. Alleles can be dominant or recessive. Dominant alleles are expressed fully in the phenotype while recessive alleles do not show the phenotype when they are with dominant alleles. Homozygous, heterozygous, hemizygous and nullizygous are different zygosity levels. Homozygous is the state of having similar alleles, either two dominant alleles or two recessive alleles. Hemizygous is that state of having only one copy or allele of a gene.

CONTENTS

1. Overview and Key Difference
2. What is Homozygous 
3. What is Hemizygous
4. Similarities Between Homozygous and Hemizygous
5. Side by Side Comparison – Homozygous vs Hemizygous in Tabular Form
6. Summary

What is Homozygous?

Homozygous means that an organism has two copies of the same allele of a gene. In other words, two alleles of a gene are the same in a homozygous individual. If it carries two copies of the dominant allele, it is known as homozygous dominant. Homozygous dominant shows the dominant phenotype. Similarly, if it carries two copies of the recessive allele, it is known as homozygous recessive. Therefore, homozygous recessive show the recessive phenotype.

For example, R is the dominant allele that represents round seeds, while r is the recessive allele that represents wrinkled seed. Therefore, homozygous dominant is the state of RR while homozygous recessive is the rr state. Both RR and rr are homozygous conditions.

What is Hemizygous?

Hemizygous is the state of having only one allele for a gene. Therefore, these genes don’t possess the customary two copies of the gene. Males have XY sex chromosome pair. Therefore, all the genes in the X chromosome are hemizygous because they exist as one copy in the X chromosome.

Males are hemizygous for genes in the X chromosome. Therefore, hemizygosity is often used to describe X linked genes in males. Moreover, in somatic cells, cancer cell lines are hemizygous for certain alleles or chromosome segments.

What are the Similarities Between Homozygous and Hemizygous?

• Homozygous and hemizygous are two zygosity types.
• Both types have at least one copy of a gene.
• Males have both homozygous and hemizygous genes.

What is the Difference Between Homozygous and Hemizygous?

Homozygous is the type of zygosity in which both alleles are the same for a gene. On the other hand, hemizygous is the type of zygosity in which only one allele is present for a gene. So, this is the key difference between homozygous and hemizygous. Furthermore, there are two alleles in homozygous genes while only one allele is present in hemizygous genes.

Moreover, homozygous dominant or homozygous recessive are two types of homozygosity, while hemizygosity can have either dominant allele or recessive allele. Therefore, homozygous can be depicted as X^AX^A or X^aX^a while hemizygous X^aY or X^AY.

Below tabulation summarizes the difference between homozygous and hemizygous.

Summary – Homozygous vs Hemizygous

Homozygous genes have two similar alleles. If a gene has two dominant alleles, we call it homozygous dominant. In contrast, if a gene has two recessive alleles, we call it homozygous recessive. Hemizygous genes have only one copy instead of the customary two copies. Hemizygosity is often used to describe the X linked genes in males since males have only one copy of the X chromosome. Therefore, males have hemizygous X linked genes. Thus, this is the summary of the difference between homozygous and hemizygous.

Reference:

1. “Sex Chromosomes & X-Linked Inheritance (Article) | Khan Academy”. Khan Academy, 2020, Available here.
2. “Zygosity”. En.Wikipedia.Org, 2020, Available here.

Image Courtesy:

1. “Homozygous cross tree method” By Tim DeJulio – Own work (Public Domain) via Commons Wikimedia