What is homozygous dominant?
Homozygous dominant refers to having two identical copies of the dominant allele for a specific gene.
Based on genetics, homozygous dominant is a genotype where an individual inherits two copies of a dominant gene. This means both alleles for a particular trait are the same and are the dominant form. Genes are fundamental units of heredity; they are sections of DNA that code for proteins, which ultimately give us our traits, such as eye color or height. Humans are diploid organisms, meaning we inherit two copies of each gene—one from each parent.
When a gene exists in different forms, these forms are called alleles. A dominant allele is one whose trait is expressed even if only one copy is present. The term "homozygous" comes from "homo," meaning same, and "zygous," relating to a zygote or fertilized egg. So, homozygous means the individual has inherited the same allele from both parents.
Breaking Down the Terms
- Homozygous: An individual has two identical alleles for a specific gene. These alleles can be two dominant alleles or two recessive alleles.
- Dominant: This describes an allele that masks the effect of the recessive allele when both are present. The trait associated with the dominant allele is expressed in the phenotype.
The Homozygous Dominant Genotype
A homozygous dominant genotype is typically represented by two uppercase letters, with each letter representing a copy of the dominant allele.
- Example: If 'A' represents the dominant allele for a trait, the homozygous dominant genotype is written as AA.
Other examples might include:
- BB for a gene where 'B' is the dominant allele.
- RR for a gene where 'R' is the dominant allele.
Phenotype Expression
Individuals with a homozygous dominant genotype (e.g., AA) will express the trait associated with the dominant allele. This is because they have two copies of the dominant allele, and the dominant trait is expressed whenever at least one dominant allele is present.
Comparing Genotypes
Understanding homozygous dominant is clearer when compared to other possible genotypes for a single gene:
| Genotype | Description | Allele Combination | Phenotype |
|---|---|---|---|
| Homozygous Dominant | Two dominant alleles | AA | Dominant Trait |
| Heterozygous | One dominant, one recessive | Aa | Dominant Trait |
| Homozygous Recessive | Two recessive alleles | aa | Recessive Trait |
As shown, both homozygous dominant (AA) and heterozygous (Aa) genotypes result in the dominant trait being expressed in the phenotype. The homozygous recessive genotype (aa) is the only one that expresses the recessive trait.
Practical Insight
Consider a hypothetical plant where the allele for red flowers (R) is dominant over the allele for white flowers (r).
- A plant with the genotype RR is homozygous dominant. It will have red flowers.
- A plant with the genotype Rr is heterozygous. It will also have red flowers because R is dominant.
- A plant with the genotype rr is homozygous recessive. It will have white flowers.
In summary, a homozygous dominant individual has received a dominant allele from both parents, resulting in the expression of the dominant trait.
