What determines whether an organism is homozygous or heterozygous?

The determination of whether an organism is homozygous or heterozygous is based on the number of copies of a specific allele that the organism possesses for a particular gene. A homozygous organism has two identical alleles for a given trait, meaning both inherited versions of the gene are the same (e.g., AA or aa). In contrast, a heterozygous organism has two different alleles for the same gene, indicating that it inherited one allele from each parent that are not identical (e.g., Aa).

This distinction is crucial in genetics because it affects how traits are expressed in an organism. Traits associated with dominant alleles may be expressed differently in homozygous versus heterozygous genotypes. For example, if the dominant allele is present alongside a recessive allele in a heterozygous pairing, the trait associated with the dominant allele will typically be expressed.

The other options (number of chromosomes, size of the genome, and age of the organism) do not provide a basis for distinguishing between homozygosity and heterozygosity, focusing instead on aspects unrelated to allele variation at specific gene loci. Therefore, understanding the number of copies of a specific allele is essential for categorizing the genetic makeup of