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Homozygous and Heterozygous are terms that are basically used to describe allele pairs. Homozygous and Heterozygous are the two sets of chromosomes that are possessed by each and every human being. Those individuals carrying a set of two identical alleles, such as (RR or rr) are known as Homozygous. While those individuals carrying different alleles, such as (Rr) are known as Heterozygous. For instance, the gene of hair colour possess two alleles where one of the allele code is responsible for brown (R) and the other code is responsible for black (r). The individuals of homozygous will have the same colour alleles on the homologous pair of chromosomes, i.e. ‘RR’ or ‘rr’ and the individuals of heterozygous will have different alleles on the homologous pair of chromosomes, i.e. ‘Rr’.
| Table of Content |
Key Terms: Homozygous, Heterozygous, Chromosomes, Alleles, Genes, Traits, Dominant Traits, Recessive Traits, Genotype, Allele pair, Allele code, Organism
Homozygous
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Every human being is known to possess the same gene. However, the numbers of genes are varied and as well control our physical health and traits. Each variation is called an Allele, and every individual organism inherits two alleles for each gene – one from its biological father and the other from its biological mother. The term Homozygous is actually a fusion of two words where the word– Homo refers to ‘same’ and zygous refers to ‘having zygotes of a specific kind’.
So, if the alleles of a gene are known to be identical, then the individual is Homozygous for that specific gene. For example – one needs to possess two identical alleles for a particular gene that causes hazel brown eyes.
Some alleles are dominant while some are recessive. However, in a Homozygous, there is no interaction between a dominant and a recessive allele as the individual only holds either two dominant alleles (RR) known as Homozygous dominant or two recessive alleles (rr) known as Homozygous recessive.
Read More: Difference Between Genotype and Phenotype
Heterozygous
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As we already know that every individual organism inherits two alleles for each gene and the only difference is the allele’s versions, i.e. one inherits it from their biological father and the other from their biological mother. The term 'Heterozygous’ is also a fusion of two words where the word – Hetero refers to 'different’ and the word zygous refers to 'having zygotes of a particular specific kind’.
So, if the alleles of a gene are known to be different, then the individual is Heterozygous for that specific gene, i.e. an individual who possesses a Heterozygous will have two different versions of alleles (Rr). For example, in terms of hair colour, a heterozygous individual can possess one allele for brown hair and another allele for red hair. But, the relationship between the two different alleles affects the traits of an individual.
Homozygous and Heterozygous
Read More: Difference between Gene and Allele
Difference between Homozygous and Heterozygous
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The main points of difference between Homozygous and Heterozygous are as follows:
| Homozygous | Heterozygous |
|---|---|
| If an individual is found carrying two same or identical copies of the same allele of each gene, then it is known as the Homozygous Genotype. | If an individual organism is found carrying two different copies of alleles of each gene, either dominant or regressive, then it is known as the Heterozygous Genotype. |
| Homozygous Genotype carries a similar but, only one type of allele – let it be either dominant or recessive for a particular trait. Example- Either RR or rr. | Heterozygous Genotype carries different alleles for a specific trait. It comprises both dominant and recessive alleles. Example- Rr |
| A Homozygous genotype individual can either be Homozygous dominant or Homozygous recessive. | The Heterozygous genotype in an individual can show either complete dominance and co-dominance or incomplete dominance behaviour. |
| Self-fertilization in homologous genotypes results in the repetition of identical or similar traits over the next generations. | Self-fertilization in the Heterozygous genotype results in different combinations of traits over the next generations. |
| For a Homozygous genotype, only one type of gamete is produced. | For a Heterozygous genotype, two types of gamete are produced. |
| Cells or organisms with a Homozygous condition fall under the term known as Homozygote. | Cells or organisms with a Heterozygous condition fall under the term known as Heterozygote. |
| Homozygous Genotypes are observed in organisms that reproduce by asexual reproduction. | Heterozygous Genotypes are observed in organisms that reproduce by sexual reproduction. |
Things to Remember
- Every human being possesses two sets of chromosomes– Homozygous and Heterozygous.
- Homozygous and Heterozygous are terms that are basically used to describe allele pairs.
- Those individuals carrying a set of two identical alleles, such as (RR or rr) are known as Homozygous, whereas those individuals carrying different alleles, such as (Rr) are known as Heterozygous.
- A Homozygous genotype individual can either be homozygous dominant (RR) or homozygous recessive (rr), whereas the Heterozygous genotype in an individual can show either complete dominance and co-dominance or incomplete dominance behaviour.
Read More: Chromosomes and Genes
Sample Questions
Ques. Define Homozygous and Heterozygous. (3 Marks)
Ans. Those individual organisms carrying two identical alleles (RR or rr) are known as Homozygous. And those individual organisms carrying two different alleles (Rr) are known as Heterozygous.
Ques. What are chromosomes? How are they linked with Homozygous and Heterozygous? (3 Marks)
Ans. Chromosomes are thread-like structures found inside the nucleus of a cell. They are generally made up of protein and a single molecule of DNA that serve to carry the genomic information from cell to cell. They are actually linked because Homozygous and Heterozygous are two sets of chromosomes present in every human being.
Ques. Define genes with examples. (2 Marks)
Ans. A gene is essentially a part of the DNA structure and is responsible for all the genetic traits of all individuals. A gene can have many different alleles of its own. For instance- skin, hair or eye colour.
Ques. Define alleles with examples. (3 Marks)
Ans. Alleles are the different variations of a gene and they only determine a single characteristic of an individual. For example, Black, brown or blue eye colour. Homozygous and Heterozygous are terms that are basically used to describe allele pairs which may be Homozygous dominant, Homozygous recessive or Heterozygous.
Ques. How can one determine homozygous and heterozygous? (3 Marks)
Ans. When one individual inherits the same version of the gene from each parent, then the individual has two matching genes and is known as Homozygous. Whereas, when one individual inherits a different version of the gene from each parent, then they do not match and are known as Heterozygous.
Ques. What does the term 'Homozygous’ and 'Heterozygous’ actually refer to? (3 Marks)
Ans. The term 'Homozygous’ is actually a fusion of two words where the word – Homo refers to 'same’ and zygous refers to ' having zygotes of a specific kind’ and the term 'Heterozygous’ – Hetero refers to 'different’ and zygous refers to ' having zygotes of a particular specific kind’.
Ques. Define genotype. What are the types of genotypes? (3 Marks)
Ans. Genotype is defined as an organism’s complete set of genes. In other words, it is also used to refer to the alleles or variant forms of a gene, that is generally carried by an organism.
There are three types of genotype and they are as follows:
- Homozygous Dominant
- Homozygous Recessive
- Heterozygous
Ques. What is the basic difference between Homozygous and Heterozygous? (3 Marks)
Ans. The basic difference between Homozygous and Heterozygous is that the Homozygous genotype carries a similar but, only one type of allele – let it be either dominant or recessive for a particular trait. For example – either RR or rr, whereas, Heterozygous genotype carries different or dissimilar alleles for a specific trait. It comprises both dominant and recessive alleles. For example- Rr.
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