Human beings possess two sets of Chromosomes – Heterozygous and Homozygous. The terms Homozygous and Heterozygous are used to describe Allele pairs. A Homozygous individual carries a set of two identical (RR or rr) Alleles while a Heterozygous individual bears different Alleles (Rr).
For example, let us consider the gene of the hair colour possessing two Alleles, one Allele code is responsible for black (r) while the other Allele code is responsible for black (R). Homozygous individuals will have the same colour Allele on a homologous pair of Chromosomes, either RR or rr, and Heterozygous individuals will hold different Alleles on a homologous pair of Chromosomes like Rr.
Let us discuss what these pairs are and the difference between them.
What are Homozygous and Heterozygous Pair of Chromosomes?
Homozygous
Human beings possess the same gene. However, the number of Genes are varied, and this controls our physical health and traits. Each variation is called an Allele, and every individual inherits two alleles for each gene – one from their biological father and another from their birth mother.
The term ‘Homozygous’ is an amalgamation of two words – Homo which refers to ’same‘, and zygous, which refers to ‘having zygotes of a specified kind‘.
If the Alleles of a gene are identical, then the individual is Homozygous for that specific gene. For example, you could possess two identical or similar Alleles for a particular gene that causes hazel brown eyes.
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Some Alleles are dominant that is expressed more strongly while some Alleles are recessive which get masked by the dominant Alleles. However, in a Homozygous Genotype, the interaction between a dominant and a recessive Allele does not occur as the individual holds either two dominant Alleles known as Homozygous dominant(RR) ot two recessive Alleles known as Homozygous recessive(rr).
Heterozygous
We know that each gene inherits two Alleles and the only difference is the versions of the Alleles. The term ‘Heterozygous’ is an amalgamation of two words – Hetero which refer to ‘different’ and zygous which refers to ‘having zygotes of a particular specified kind‘. This results in two different Alleles present on the homologous Chromosomes.
An individual who possesses a Heterozygous Genotype will have two versions of Alleles (Rr). For example, in terms of hair colour, a Heterozygous individual can possess one Allele for brown hair and one Allele for red hair. The relationship between the two Alleles affects the traits expressed in an individual which also determines the characteristics which a carrier possesses.
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Homozygous vs Heterozygous
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Homozygous |
Heterozygous |
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If a diploid organism carries two same or identical copies of the same Allele of each gene, where the Alleles may be coding for a particular trait, and this is called the Homozygous Genotype. |
If a diploid organism carries two different copies of Alleles of each gene, either dominant or regressive, where the Alleles may be coding for a particular trait, and this is called the Heterozygous Genotype. |
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Homozygous Genotype carries a similar but, only one type of Allele- either dominant or recessive for a particular trait. Example- either RR or rr |
Heterozygous Genotype carries different or dissimilar Alleles for a specific trait. It comprises both dominant and recessive Alleles. Example- Rr |
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A Homozygous individual can either be Homozygous dominant or Homozygous recessive. |
The Heterozygous Alleles in an individual can show either complete dominance and co-dominance or incomplete dominance behaviour. |
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Self-fertilization in homologous Allele results in the repetition of the identical or similar traits over generations |
Self-fertilisation in Heterozygous Allele results in different combinations of traits over the next generations |
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For a Homozygous Allele, only one type of gamete is produced |
For a Heterozygous Allele, two types of gamete are produced |
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Cells or organisms with a Homozygous condition fall under the term known as homozygotes. |
Cells or organisms with a Heterozygous condition fall under the term known as heterozygotes |
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Homozygous Genotypes are observed in organisms that reproduce by asexual means. |
Heterozygous Genotypes are observed in animals that reproduce by sexual means. |
Autozygous and Allozygous Alleles (Alleles Based on origin)
Zygosity in an individual may also refer to the origin of the Alleles in a given Genotype. In the case in which the two alleles at a locus originate from a common ancestor (for example through nonrandom mating (inbreeding)), the Genotype is known as Autozygous Alleles. Such conditions are referred to in genetic studies as being “identical by descent”(abbreviated as IBD). When the two Alleles come from different ancestors (at least to the extent that the ancestry can be traced), the Genotype is referred to as Allozygous Alleles. This phenomenon of identity is referred to as being “identical by state”, (abbreviated as IBS).
Because the Alleles/ Genes for an Autozygous Genotype are received from the same ancestor (source), they are always Homozygous in nature. However, in the case of Allozygous Alleles, Genotypes may be Homozygous or Heterozygous in expression. Heterozygous Genotypes are commonly, but not necessarily, Allozygous because different Alleles may have originated during the course of evolution by means of accumulating mutations sometimes after a common origin. The hemizygous and nullizygous Genotypes lack enough Alleles and hence they do not facilitate comparison studies on the basis of ancestry/ sources. This classification thus proves irrelevant for them.
Monozygotic and Dizygotic Twins
As discussed in the article here, “zygosity” is generally used in the context of a specific genetic locus. However, the word zygosity may also be useful in our understanding of the genetic similarity or dissimilarity between twins. Identical twins are monozygotic. This means that during their course of development, they are formed from the same zygote which has split into two and then developed into two embryos during implantation. Fraternal twins or non-identical twins are dizygotic as they develop from two separate oocytes (egg cells) that are fertilized by two different sperms during the fertilization process. Sesquizygotic twins are halfway between monozygotic and dizygotic and are believed to arise after two sperm fertilize a single oocyte which subsequently splits into two morulae.