Homozygous denotes a genetic state where an individual inherits identical alleles of a gene from both parents.
In homozygous chromosomes, alleles are either both dominant (XX) or both recessive (xx).
Homozygous-dominant chromosomes carry two copies of the dominant trait allele, while homozygous-recessive chromosomes carry two copies of the recessive trait allele.
The expression of the dominant trait in homozygous organisms occurs when both chromosomes carry a dominant gene at the same locus.
True-breeding mechanisms result in organisms being homozygous for constant traits.
Asexual reproduction methods, like parthenogenesis, lead to homozygous chromosome formation, ensuring identical phenotypic characteristics in offspring.
Homozygosity is primarily defined at the locus where copies of the gene on reciprocal homologous chromosomes are identical.
Homozygous cells or organisms are referred to as homozygotes.
Genes in homozygous chromosomes exhibit detectable sequence similarities in nucleotides and coded proteins.
Homozygous chromosomes are observed in various genes, such as those determining eye color (e.g., BB for brown eyes), and may lead to diseases in homozygous-recessive genotypes.
Heterozygous Definition:
Heterozygous refers to a genetic condition where an individual inherits different alleles of a gene from both parents.
Heterozygosity occurs in diploid organisms where a gene contains two different alleles at a locus (e.g., Bb for eye color).
In heterozygous chromosomes with traits expressed through complete dominance, only the dominant allele's trait is expressed.
In complex dominance schemes, gene expression becomes more intricate.
In incomplete dominance, the phenotypic trait observed is intermediate between dominant and recessive phenotypes, while codominance results in distinct phenotypes expressed by individual alleles in different body parts.
The heterozygous genotype often exhibits higher fitness than homozygous-dominant or homozygous-recessive genotypes, termed 'hybrid vigor.'
Sexual breeding mechanisms result in organisms being heterozygous for varied traits.
Sexual reproduction methods contribute to heterozygous chromosome formation, ensuring phenotypic diversity between parents and offspring.
Heterozygosity is mainly defined at the locus where copies of the gene on reciprocal homologous chromosomes are different.
Heterozygous cells or organisms are termed heterozygotes and may be associated with genetic conditions, with the manifestation depending on whether the mutated allele is dominant or recessive. Diseases such as Huntington’s disease, Marfan’s syndrome, and familial hypercholesterolemia are linked to heterozygous genotypes.
Key Differences (Homozygous vs. Heterozygous)
Examples of homozygous genotypes
Eye color
The dominant trait for eye color is brown (BB genotype) in homozygous individuals.
Homozygous-recessive individuals (bb genotype) express recessive traits like blue, green, or grey eyes.
Homozygous genotypes ensure that the genes for eye color on both chromosomes are identical.
Freckles:
Freckles, controlled by the MC1R gene, are a dominant trait.
Homozygous-dominant individuals express the dominant allele and have freckles.
Homozygous-recessive individuals (no freckles) have an identical genotype for the freckle gene.
Examples of Heterozygous Genotypes:
Sickle-Cell Anemia:
Sickle-cell anemia is a recessive trait affecting blood cell formation.
Heterozygous individuals express the dominant trait, preventing the anemic condition.
The advantage lies in avoiding the structural changes that occur in red blood cells in sickle-cell anemia.
Curly Hair:
The dominant trait for hair type is curly, and straight hair is the homozygous-recessive condition.
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