What defines a homozygous genotype?

A homozygous genotype is defined by the presence of two identical alleles for a specific trait. This means that both alleles inherited from the parents are the same, whether they are dominant or recessive. For example, if an organism has two alleles for a gene that are both for blue eye color, it is considered homozygous for that trait (e.g., both alleles being denoted as 'bb' for blue eyes). This uniformity can significantly influence the phenotype, typically leading to a consistent expression of that trait.

In contrast, having two different alleles for a trait indicates a heterozygous genotype, which can result in a mix of traits being expressed, depending on the dominance of the alleles involved. Multiple alleles refer to situations where more than two variations of a gene exist in the population, not that an individual organism carries multiple copies of one gene. Lastly, having no alleles for a trait suggests the absence of a genetic basis for that trait, which does not align with the concept of genotypes.