Final answer:
An organism with two nonidentical alleles for a gene is known as heterozygous. Homozygous organisms have two identical alleles. Dominant traits are expressed in both homozygous dominant and heterozygous individuals, while recessive traits are expressed only in homozygous recessive individuals.
Step-by-step explanation:
When an organism has inherited two nonidentical alleles, it is said to be heterozygous. An organism can be either homozygous dominant (like AA) or homozygous recessive (like aa). However, when there are two different alleles for a gene, such as Aa, this condition is known as heterozygous. In the context of dominant and recessive alleles, individuals with homozygous recessive (like aa) genotypes will express the recessive trait. On the other hand, both homozygous dominant and heterozygous individuals will display the dominant trait phenotype, as the dominant allele masks the effect of the recessive allele in a heterozygous pair.