What describes an organism with two identical alleles for a gene?

An organism with two identical alleles for a gene is described as homozygous. This term comes from the prefix "homo-", meaning "same," and "zygous," which relates to the condition of the alleles. In a homozygous individual, the two alleles at a specific locus on homologous chromosomes are identical, meaning that the organism has inherited the same allele from both parents. This can apply to both dominant and recessive alleles.

Understanding the term is crucial in genetics because homozygosity can impact traits expressed in the organism. For example, if a plant has two identical alleles for flower color, whether red or white, it will consistently produce that flower color in offspring through self-pollination. This contrasts with organisms that are heterozygous for a gene, where the alleles are different and may result in a mix of traits depending on dominance, making a clear distinction between these terms essential for genetic discussions.

In the provided choices, the terms monomer and polymer relate to the structure of molecules, particularly in biology discussing nucleic acids or proteins, rather than to the genetic make-up of an organism. Therefore, they do not apply to the context of alleles and genotype.