Genes determine specific characteristics, such as eye color or blood type, and each gene exists in variant forms. These variant forms are known as alleles, and they are inherited in pairs, with one allele coming from each biological parent. The specific combination of these two inherited alleles for a particular gene is referred to as the genotype. If an organism inherits two copies of the exact same allele for a given gene, what is the specific term used to describe this genotype?
Defining the Homozygous Genotype
The term for a genotype made of two identical alleles is homozygous. This designation comes from the Greek words homo, meaning “same,” and zygous, referring to a fertilized egg. When an individual is homozygous for a gene, it means they received the same version of that gene from both parents.
This state is further categorized based on the nature of the allele involved. If the organism inherits two copies of the allele that is expressed more strongly, it is called homozygous dominant. In genetic notation, a dominant allele is typically represented by an uppercase letter, so a homozygous dominant genotype would be written as two identical capital letters, such as AA or BB.
Conversely, if the organism inherits two copies of the allele that is typically masked, the genotype is referred to as homozygous recessive. This type of allele is represented by a lowercase letter, meaning the notation would be two identical lowercase letters, such as aa or bb. The precise DNA sequence for that specific gene is the same on both chromosomes inherited from the parents.
The Heterozygous Contrast
In contrast to the homozygous state, a genotype composed of two different alleles is called heterozygous, derived from the Greek prefix hetero (“different”). This means the individual inherited one allele variant from each parent. A heterozygous genotype is always represented by one uppercase letter and one lowercase letter, such as Aa or Bb. This combination indicates the presence of both the dominant and the recessive allele for that specific gene. The difference in the allele pair is fundamental to inheritance, as a heterozygous organism can contribute either the dominant or the recessive allele to its gametes.
Genotype Influence on Physical Traits
The functional significance of a genotype lies in its influence on the organism’s physical characteristics, known as the phenotype. This outcome is determined by the relationship between the two alleles, primarily through dominance and recessiveness. A dominant allele is one that will be expressed even if only one copy is present in the genotype.
For example, if a gene controls flower color and the allele for purple flowers (P) is dominant over the allele for white flowers (p), the homozygous dominant genotype (PP) will result in purple flowers. The heterozygous genotype (Pp) will also result in purple flowers because the dominant allele effectively masks the presence of the recessive allele.
The recessive allele can only be expressed when the dominant allele is completely absent. Therefore, an organism must be homozygous recessive (pp) to exhibit the recessive trait, such as white flowers or blue eyes.