Homozygous vs. Heterozygous: 10 Differences, Examples

Homozygous Definition

Homozygous is a genetic condition where an individual inherits the same alleles of a gene from both the parents.

• In homozygous chromosomes, both alleles are either dominant or recessive. The dominant trait is represented by two capital letters (XX) and the recessive trait is represented by two lowercase letters (xx).
• Homozygous-dominant chromosomes carry two copies of the allele that codes for the dominant trait whereas the homozygous-recessive chromosomes carry two copies of the allele that codes for the recessive trait.
• In homozygous organisms, the dominant trait is only expressed when both the chromosomes have a dominant gene at a similar locus.
• Organisms reproducing by true breeding mechanisms are always homozygous for all traits that are held constant.
• Methods of asexual reproduction, like parthenogenesis, result in homozygous chromosome formation. This ensures that the phenotypic characteristic in the offsprings is identical to that in the parents.
• Mostly, homozygosity is defined at the specific locus where copies of the gene affecting a trait present on the two reciprocal homologous chromosomes are identical.
• Homozygous cells or organisms are also called homozygotes.
• Genes in homozygous chromosomes share readily detectable sequence similarities in their nucleotide and the proteins they code for.
• Homozygous chromosomes are seen in various genes like the genes for eye color where the dominant trait is brown and is seen as BB in homozygous organisms.
• Homozygosity might result in various diseases in a homozygous-recessive genotype where the recessive genotype is a mutant gene.
• Diseases like cystic fibrosis, sickle cell anemia, phenylketonuria are associated with homozygous genotypes.

Heterozygous Definition

Heterozygous is a genetic condition where an individual inherits different alleles of a gene from the two parents.

• Heterozygosity is observed in a diploid organism where a gene contains two different alleles at a gene locus.
• In heterozygous chromosomes, the two alleles are different, and the heterozygous genotypes are represented by a capital letter, which indicates the dominant allele and a lowercase letter, which indicates the recessive allele, like Bb for eye color.
• In heterozygous chromosomes with genes having traits that are expressed via complete dominance, only the trait coded by the dominant allele is expressed. In complex dominance schemes, however, the expression of genes is more complicated.
• In incomplete dominance, the phenotypic trait observed is somewhere between dominant and recessive phenotypes. Similarly, in codominance, the phenotypes are expressed by individual alleles in different parts of the body.
• The heterozygous genotype has relatively higher fitness than the homozygous-dominant or homozygous-recessive genotypes. This fitness is termed as ‘hybrid vigour’.
• Organisms reproducing by sexual breeding mechanisms are usually heterozygous for the traits that are varied.
• Methods of sexual reproduction result in heterozygous chromosome formation. This ensures that the phenotypic characteristic in the offsprings is different from that in the parents.
• Mostly, heterozygosity is defined at the specific locus where copies of the gene affecting a trait present on the two reciprocal homologous chromosomes are different.
• Heterozygous cells or organisms are also called heterozygotes.
• As with homozygous genotypes, some heterozygous genotypes are also often associated with genetic conditions.
• If the mutated allele is dominant, only the mutated copy is capable of causing the disease. This condition is called dominant-disease.
• If the mutated allele is recessive, then the disease will not appear, and the organism will act as a carrier.
• Diseases like Huntington’s disease, Marfan’s syndrome, and familial hypercholesterolemia are associated with heterozygous genotypes.

Key Differences (Homozygous vs. Heterozygous)

Examples of homozygous genotypes

Eye color

• The dominant genetic trait for eye color is brown, which is represented by BB genotype.
• Thus, in homozygous individuals, the genotype for eye color will be BB or bb.
• All other eye colors ranging from blue to green to grey are recessive traits that are only expressed when the genotype is homozygous-recessive, bb.
• If the genotype is homozygous, the genes for eye color in the loci of the two chromosomes should be identical.

Freckles

• Freckles are tiny spots on the skin caused due to the melanin pigment.
• The production of freckles is also a dominant trait and is controlled by the MC1R gene.
• Individuals with freckles have a homozygous-dominant genotype that results in the expression of the dominant allele.
• However, if the genotype is homozygous-recessive, no freckles appear in the individuals.

Examples of heterozygous genotypes

Sickle-cell anemia

• The trait for sickle-cell anemia is a recessive trait that causes the blood cells to be formed incorrectly.
• Thus, in individuals with heterozygous genotype, the dominant trait is expressed, preventing the sickle-cell anemic condition.
• In sickle-cell anemia, the red blood cells change its structure into sickle-shaped, which makes it small and insufficient to carry enough oxygen.
• Thus, the heterozygous genotype of the gene responsible for sickle-cell anemia provides an advantage to such individuals.

Curly hair

• The dominant trait for hair type is curly; thus, only the people with homozygous-recessive alleles have straight hair.
• This gene codes for the protein that causes the hair to be curly.
• Heterozygous genotype causes the hair in such individuals to be wavy, which is between curly and straight hair.
• This phenomenon is also called incomplete dominance, where the phenotype expressed is between the dominant and recessive ones.
• In complete dominance, the curly hair strait is observed in individuals with the heterozygous condition.

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