Difference Between Homozygous and Heterozygous

Main Difference – Homozygous vs Heterozygous

Homozygous and heterozygous are two terms used to describe allele pairs. Diploid organisms consist of two copies of each gene. Each copy can be dominant or recessive. The dominant allele is expressed to produce the phenotype while the recessive allele remains silent. Both dominant and recessive allele are located on the same locus of the homologous chromosomes. The main difference between homozygous and heterozygous is that homozygous individuals carry two identical alleles whereas heterozygous individuals carry both dominant and recessive allele. The self-breeding between homozygous individuals produces offspring with the same trait over generations. But, self-breeding between heterozygous individuals produces all possible traits that belong to that particular gene.

Key Areas Covered

1. What is Homozygous
      – Definition, Characteristics, Examples
2. What is Heterozygous
      – Definition, Characteristics, Examples
3. What are the Similarities Between Homozygous and Heterozygous
      – Outline of Common Features
4. What is the Difference Between Homozygous and Heterozygous
      – Comparison of Key Differences

Key Terms: Alleles, Codominance, Complete Dominance, Dominant, Genotype, Heterozygous, Homozygous, Incomplete Dominance, Recessive, Trait

What is Homozygous

Homozygous is the presence of two copies of the same allele which codes for a particular trait in diploid organisms. Homozygous traits are inherited by true breeding organisms. A particular trait can have two different allele combinations. They are homozygous-dominant and homozygous-recessive. When an individual carries two dominant alleles in the allele pair, this situation is called homozygous-dominant. Homozygous dominant alleles can be represented as XX. When an individual carries two recessive allele pair, this situation is called homozygous-recessive. Homozygous-recessive alleles can be represented as xx.

Difference Between Homozygous and Heterozygous

Figure 1: Homozygous vs heterozygous

The genes of an individual can be mutated over time. If the same mutation occurs in both alleles of the homozygous pair, the mutation is called homozygous mutation. If the mutation occurs in only one allele in the homozygous pair, the situation is called heterozygous mutation. Homozygous and heterozygous alleles are shown in figure 1.

What is Heterozygous

Heterozygous is the presence of both dominant and recessive alleles in the allele pair, which determines a particular trait of a diploid organism. This situation can be represented as Xx. This means the two alleles in the pair are different from each other. The inheritance of the heterozygous alleles can occur in three ways. They are complete dominance, incomplete dominance, and codominance. In complete dominance, one allele is completely dominant over the other. Therefore, only the dominant allele is expressed and it determines the phenotype of the individual. In incomplete dominance, one allele is not completely dominant over the other. Therefore, a combination of phenotypes can be observed in the offspring. For example, the heterozygous genotype of the snapdragon produces a pink color flower. Here, the dominant flower color is red and the recessive flower color is white.

Main Difference - Homozygous vs Heterozygous

Figure 2: Complete dominance

In codominance, both alleles in the heterozygous pair are expressed. The AB blood group is an example of codominance in which both antigen A and antigen B are independently expressed in the red blood cells. The Punnett square in figure 2 describes the inheritance of flower color in pea plants. It is an example of complete dominance. The Punnett square also shows that self-breeding of two heterozygous individuals produces all possible genotypes for that particular trait.

Similarities Between Homozygous and Heterozygous

Both homozygous and heterozygous are two terms used in describing the genotype of a diploid organism.
Both homozygous and heterozygous alleles can be found at the same locus of homologous chromosomes.

Definition

Homozygous: Homozygous is the presence of two copies of the same allele which codes for a particular trait.

Heterozygous: Heterozygous is the presence of two different copies of alleles of a particular gene.

Alleles

Homozygous: Homozygous individuals have either dominant or recessive allele pairs (Ex: TT or tt).

Heterozygous: Heterozygous individuals have both dominant and recessive alleles (Ex: Tt).

Gametes

Homozygous: Homozygous individuals produce a single type of gametes.

Heterozygous: Heterozygous individuals produce both types of gametes with dominant and recessive alleles.

Self-breeding

Homozygous: Self-breeding of homozygous individuals produces the same trait over generations.

Heterozygous: Self-breeding of heterozygous individuals produces a combination of different traits.

Vigour

Homozygous: Homozygous individuals do not show an extra vigour.

Heterozygous: Heterozygous individuals can show extra vigour called hybrid vigour.

Types

Homozygous: Homozygous-dominant and homozygous-recessive are the two types of homozygous alleles.

Heterozygous: Complete dominance, incomplete dominance, and codominance are the three types of heterozygous allleles.

Examples

Homozygous: The human blood type O, which is determined by the genotype OO, the dominant eye color brown, which is determined by the genotype BB, and the recessive eye color hazel, which is determined by the genotype bb are the examples of homozygous.

Heterozygous: The human blood type AB, which is determined by the genotype, AB, snapdragon flower color, and the codominance of the hair color in roan horses are the examples of heterozygous.

Conclusion

Homozygous and heterozygous are two terms used in describing the genotypes of the individuals. Homozygous is the presence of the same allele pair at the same locus of homologous chromosomes. The allele pair can be either dominant or recessive. Heterozygous is the presence of different allele pairs at the same locus of the homologous chromosomes. That means both dominant and recessive alleles are present in the same individual. Therefore, the main difference between homozygous and heterozygous is the type of alleles present in each condition.

Reference:

1.“ What is Homozygous? – Definition, Traits & Example.” Study.com, Available here .html. Accessed 15 Aug. 2017.
2. Bailey, Regina. “What Does Heterozygous Mean?” ThoughtCo, Available here. Accessed 15 Aug. 2017.

Image Courtesy:

1. “Genotype lg” By National Human Genome Research Institute – National Institutes of Health. National Human Genome Research Institute. “Talking Glossary of Genetic Terms.”Retrieved November 17, 2016, from (Public Domain) via Commons Wikimedia
2 “Punnett square mendel flowers” By Madprime – Own work (CC BY-SA 3.0) via Commons Wikimedia