Homozygous refers to the state of the genes or genetic condition in which an individual has inherited the similar DNA sequence for a particular gene from both their biological parents and it is generally used with reference to the disease.
For instance, if an individual inherited a mutated allele in the DNA sequence from maternal side and similar gene from the paternal part as well, then the mutation is explained as homozygous for that individual.
Thus, after receiving 2 identical copies of the gene, detrimental effects will express phenotypically for which the genes are coded.
Difference Between Homozygous and Heterozygous
The term “heterozygous” attribute to a pair of alleles so heterozygous is when you have 2 dissimilar alleles that means you have received dissimilar copies from each parent.
In a heterozygous genotype, the allele which is dominant dominates over recessive and hence the dominant feature will be expressed while the recessive will not show but you will be still a carrier. This will be passed it on to your children.
Homozygous is the opposite of the heterozygous where the feature of the similar alleles — either dominant or recessive — is manifested.
There are three possible alleles for A, B, O blood type. However, the contribution from each parent’s gene makes a difference an individual exhibit with phenotypic characteristic.
Sometimes, it may be possible that both the parents pass on the same allele for a gene, making it a homozygous trait.
While in certain conditions, different set of alleles are triggered resulting in heterozygous trait.
A homozygous trait is mainly responsible for phenotypic appearance in the organism.
A pea plant will definitely exhibit long stem, if it receives two alleles associated with tallness.
In contrast, if it gets two “short” alleles, then plant with stunted growth will be evident.
Thus, for a specific characteristic, the genotype may be either homozygous or heterozygous, however, the way it is expressed it is identified as phenotype.
To discuss the genotypic trait, the recognition of allele is done by letters like Aa, Bb, CC etc.
To elaborate specifically, the dominant allele is denoted by the upper case letters like AA, BB etc. while the recessive form with the lower case letter as aa, bb, etc.
Moreover, the mixed type is recognise as combination of both capital and small case letter as Aa, Bb, etc. wherein the dominant gene expresses phenotypically.
Genotypically, recessive allele is indicated with lowercase letter, if both the alleles are recessive, only then it is expressed phenotypically as homozygous character.
However, phenotypic characteristic is not necessarily expressed in mixed trait.
For example, if homozygous genotype for a height of pea plant is tt only then the phenotypic character will exhibit as short pea plant.
Thus, recessive genotype is certainly homozygous. In the similar manner, homozygous genotype of TT will have expressed as tall plant stem phenotypically.
However, in mixed genotype there is dominant gene present which will express phenotypically, suppressing the recessive characteristic. Thus, expressing heterozygous characteristic.
An appropriate example can be of some members of a cattle species which have the presence of black coats: BB or Bb.
The red ones are certainly bb. Most of the traits are noteworthy, however, not all inherited traits are harmless in nature.
A. Eye color
It is evident that brown eye color allele is dominant over the blue eye allele.
You can have brown eyes independent of whether you are homozygous (two alleles for brown eyes) or heterozygous (one for brown and one for blue).
The allele for blue eyes, which is recessive.
To have blue eyes you should have 2 alleles for blue eyes.
Small brown spots on the skin which are made of melanin, is the pigment which imparts your skin and hair.
The MC1R gene controls freckles.
The trait is dominant so if you do do not have freckles, then either you are homozygous for a recessive version that do not cause them.
C. Hair Color
Red color which imparts color to your hair is recessive so a person who is heterozygous for red hair has one allele for a dominant trait, like brown hair, and one allele for red hair and they can pass the red hair allele to their future progeny.
If the kid inherits the same allele from the other parent, they will have red hair and will be homozygous.
Homozygous Genes and Disease
Mutated alleles can lead to various disorders. People who are homozygous are more prone to have disease for the particular mutated gene.
The peril of phenotypic trait is subject to the interaction between dominant and recessive alleles.
For the heterozygous genotypic trait, the normal dominant allele would take over the mutated recessive allele.
In such case, because of incomplete dominance or codominance, the disease may express mildly or not at all.
However, for the homozygous genotype, the mutated recessive allele will express in full blown, since there is no dominant allele to suppress the recessive gene.
There are various genetic conditions associated with the homozygous trait.
A. Cystic Fibrosis
The movement of organelle and other substance within the cell is responsively regulated by the cystic fibrosis transmembrane conductance regulator (CFTR) gene.
In conditions where 2 mutated recessive genes are transmitted to the offspring, the probability of possessing the disease increases.
The mutation leads to dense mucus formation leading to infection of lungs, injury to the pancreas, scarring and cysts in the lungs and problems in digestion.
Individuals who is homozygous for a phenylalanine hydroxylase (PAH) gene mutation gives rise to this PKU.
Under normal conditions, PAH gene causes cells to make an enzyme that cleaves an amino acid called phenylalanine, however in PKU they are unable to make the enzyme which leads to phenylalanine to saturate in the bodily tissues and blood.
It is necessary that a person suffering from PKY should restric phenylalanine in their meals else they have high chance to develop PKU.
Symptoms include skin rashes, neurological problems, musty-smelling breath, skin, or urine, Hyperactivity, psychiatric disorder.
C. Methylenetetrahydrofolate Reductase (MTHFR) Gene Mutation
The MTHFR gene sends positive feedback to produce methylenetetrahydrofolate reductase, an enzyme that is responsible to disintegrate homocysteine.
Mutation of the MTHFR gene will hinder the production of the methylenetetrahydrofolate reductase enzyme. Thus, affecting various organs in the body.
Two notable mutations seen in several studies conducted are:
1. C677T: An extra allele of this same variant, will physiologically raises blood levels of homocysteine and lowers the levels of folate. Its incidence amongst the Caucasian people from North America is 10 -15% while it is 25% for Hispanic group.
2. A1298C: This gene alone is not responsible to phenotypically influence the homocysteine levels in the blood. However, in combination with C677T gene, it has shown the same result as that seen with homozygous C677T gene.
The scientists are still in their infancy to understand the concept behind MTHFR mutation, since it has been associated with various systemic diseases like cardiovascular disease, blood clots, dementia, osteoporosis, multiple sclerosis and pregnancy related complications like pre-eclampsia, pregnancies with neural tube defects and polycystic ovary syndrome.
All in all, the alleles which are mutated can be responsible for various disorders.
People who are homozygous are more prone to have diseases for the particular mutated gene.
To prevent such occurrence, genetic history must be taken into consideration.
Incomplete Dominance : Definition and Examples
- Homozygous deletions scanning in tumor cell lines detects previously unsuspected loci. Int J Cancer . 2003 Aug 20;106(2):216-23.
- Asymptomatic homozygous protein C deficiency. Acta Haematol . 1990;83(3):152-5.
- New homozygous indel in MYORG linked to brain calcification, thyroidopathy and neuropathy. Brain . 2019 Sep 1;142(9):e51.
- Homozygous familial hypercholesterolaemia: light at the end of the tunnel. Eur Heart J . 2018 Apr 7;39(14):1169-1171.
- Managing Patients With Homozygous Familial Hypercholesterolemia. J Am Coll Cardiol . 2017 Aug 29;70(9):1171-1172.
- Multisystem presentation of a homozygous POLG2 variant. Eur J Med Genet . 2020 May;63(5):103899