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.
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).
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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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.
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.
1. Differentiate Between Homozygous and Heterozygous Pairs of Alleles?
Suppose a diploid organism carries two duplicates of each gene which may comprise identical Alleles. In that case, it is known as Homozygous, whereas if a diploid organism has two copies of each gene, which can be different - either as dominant or regressive Alleles, it is known as Heterozygous.
Homozygous carry a similar set of Alleles for a trait. In contrast, Heterozygous have different sets of Alleles for a feature.
Homozygous results of identical individuals mean that it is pure for a trait and breeds true. For example, RR or rr while a Heterozygous individual results in dissimilar individuals who are rarely pure and produce offspring with a different Genotype. For example, Rr.
A Homozygous individual can carry either a dominant or a recessive Allele while a Heterozygous individual possesses both dominant and recessive Allele.
A Homozygous individual produces one type of gamete while a Heterozygous individual makes two kinds of gametes.
2. State the Different Genes and Disease Caused by a Homozygous Allele?
Several of the known genetic disorders are known to exist due to the inheritance of the Homozygous Genes in the child/ offspring. Some Homozygous Allele related disorders are Nancy Sweeney syndrome, Sickle cell anemia, Tay Sachs disease, etc. In newer findings, scientists have found that several previously unknown Homozygous regions of DNA may be linked with the development of late-onset Alzheimer's, schizophrenia, and autism spectrum disorders (ASDs).
3. How can we define autosomes in genetics?
Autosomes refer to those pairs of Chromosomes found in the nucleus of the cell that has the Genes for somatic traits and is not useful in determining the sex of an individual. In the human cell, there are 23 pairs of autosomes. They code proteins for the normal metabolism of the human body that is not directly associated with the production of sexual characteristics and related molecules (sex hormones, etc.) in the life cycle of the species. The number of autosomes varies from species to species. It is however fixed for a particular species and does not alter (except during genetic disorders such as Down syndrome, Klinefelter syndrome, etc.)
4. How do dominant Genes function in Homozygous and Heterozygous forms?
In a Heterozygous Allele, both Alleles are different in their Genotype. One of them may be dominant while the other carries a recessive trait. Thus, following Mendelian laws of genetics, they will express the dominant features of the dominant Allele present in the Heterozygous pair. In the case of Homozygous pairs of Alleles, the two Alleles carry the same Genotype such that either both of them are dominant Genotypes or both are recessive Genotypes. In such cases, the expression is either a recessive phenotype or a dominant phenotype. One important thing to note is that they are “pure” for a genetic trait and will pass the same (either full dominant or full recessive) Allele to all its progeny.
5. What is the meaning of “hemizygous”? Explain the hemizygous Alleles with the help of an example?
Apart from the Homozygous and Heterozygous Allele, there is another rare in occurrence “Hemizygous” Allele found in nature. In the phenomenon of hemizygosity, the progeny receives only one copy of a pair of Alleles from the parental generation. Thus, sometimes one Allele is said to be missing (total Chromosome number is less than normal), however, there are special hemizygous cases with normal Chromosome numbers. Hemizygosity can occur when there is depletion of one of the Chromosomes or during heterogametic sex. Examples of heterogametic sex include males in the human species. They are said to be hemizygous for the X- Chromosome, as they receive only one copy of it from their parental generation.