Final answer:
The offspring of two heterozygous individuals for a gene with one dominant and one recessive allele can exhibit up to three phenotypes: homozygous dominant, heterozygous, and homozygous recessive.
Step-by-step explanation:
When we consider a gene with 2 alleles, one dominant (A) and one recessive (a), and two heterozygous individuals (Aa) mate, their offspring can display up to three phenotypes. These phenotypes are based on the combinations of alleles the offspring inherit. With a simple Punnett square, we can predict the possible genotypes and thus their associated phenotypes. The genotypes from this cross can be AA, Aa, and aa, resulting in phenotypes: homozygous dominant (displaying the dominant trait), heterozygous (also displaying the dominant trait), and homozygous recessive (displaying the recessive trait).