Biology STAAR (State of Texas Assessments of Academic Readiness) Practice Test

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What does it mean if an organism is homozygous?

Its allele pairs are different
It has two identical alleles
It has no alleles present
It is genetically diverse

The correct answer is: It has two identical alleles

When an organism is described as homozygous, it means that it has two identical alleles for a particular gene. This can occur in two forms: homozygous dominant, where both alleles are the dominant form, or homozygous recessive, where both alleles are the recessive form. Having identical alleles can affect traits that are expressed, as both will contribute equally to the phenotype. In contrast, if an organism had different allele pairs, it would be classified as heterozygous, which introduces genetic variation. If an organism had no alleles present at all, it would not be able to inherit or express traits from that particular gene. Additionally, genetic diversity implies the presence of a variety of alleles rather than identical ones. Thus, being homozygous specifically refers to the identity of the alleles rather than their presence or the genetic diversity of the organism.