Question

Distinguish between homozygosity and heterozygosity.

Short answer

Answer: The main differences between homozygosity and heterozygosity in genetics are: 1. Homozygosity involves inheriting two identical alleles of a given gene, while heterozygosity involves inheriting two different alleles of a given gene. 2. Homozygous individuals can be either dominant or recessive, whereas heterozygous individuals have one dominant and one recessive allele. 3. In a homozygous individual, the phenotype (physical appearance) is determined by the nature of the alleles (dominant or recessive), while in a heterozygous individual, the phenotype is determined by the dominant allele.

Step-by-step solution

Define Homozygosity

Homozygosity occurs when an individual inherits the same allele for a specific gene from both of their parents. In other words, homozygosity is when both homologous chromosomes carry the same allele of a given gene. These individuals are called homozygous for that particular gene, and the alleles can either be dominant or recessive.

Define Heterozygosity

Heterozygosity occurs when an individual inherits different alleles for a specific gene from each of their parents. These individuals are called heterozygous for that particular gene. Heterozygosity reflects the presence of two different alleles of a given gene on a pair of homologous chromosomes.

Examples

In order to better understand homozygosity and heterozygosity, consider the gene responsible for eye color. Using the simplified example where the gene has only two alleles, the dominant allele B (brown eyes) and the recessive allele b (blue eyes). A homozygous individual would have either two dominant alleles (BB) or two recessive alleles (bb). In the first case, the individual would have brown eyes, while in the second case, the individual would have blue eyes. A heterozygous individual would have one dominant allele and one recessive allele (Bb). In this case, since the dominant allele masks the effect of the recessive allele, the person would have brown eyes.

Differences between Homozygosity and Heterozygosity

1. Homozygosity involves two identical alleles of a given gene, while heterozygosity involves two different alleles of a given gene. 2. Homozygous individuals can be either dominant or recessive, whereas heterozygous individuals have one dominant and one recessive allele. 3. In a homozygous individual, the phenotype (physical appearance) is determined by the nature of the alleles (dominant or recessive), while in a heterozygous individual, the phenotype is determined by the dominant allele.

Key concepts

Alleles

Alleles are different forms of a gene that are found at the same place on a chromosome. Each individual has two alleles for a given gene, one inherited from each parent.

These alleles can influence traits such as eye color, hair color, or blood type. Although each person carries two alleles per gene, the combinations can vary greatly.

• Homozygous: If the two alleles are the same (e.g., BB or bb), an individual is homozygous for that gene.
• Heterozygous: If the two alleles are different (e.g., Bb), an individual is heterozygous for that gene.

Dominant and Recessive

Dominant and recessive alleles determine how certain traits are expressed in individuals.

• **Dominant Allele**: This allele expresses its trait even if just one copy is present. It will mask the effect of a recessive allele if both are present.
• **Recessive Allele**: This allele only expresses its trait when two copies are present, as it is masked by a dominant allele when paired together in a heterozygous individual.

In the context of eye color:

• Two dominant alleles (BB) or one dominant and one recessive allele (Bb) would result in brown eyes, as the dominant allele dictates the trait.
• Only two recessive alleles (bb) would result in a recessive trait expression, leading to blue eyes.

Phenotype

The phenotype is the observable physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences. When discussing alleles, the phenotype usually refers to traits such as height, skin color, or eye color.

The phenotype is notably influenced by the combinations of alleles that an individual possesses:

• **Homozygous Dominant (BB)**: Only the dominant trait is visible in phenotype.
• **Homozygous Recessive (bb)**: Only the recessive trait is observable in phenotype.
• **Heterozygous (Bb)**: The dominant trait is expressed in the phenotype due to the recessive trait being masked.

Phenotypes are the way traits manifest publicly, which is why they are a crucial component of understanding genetics.