Final answer:
A diploid individual with two identical alleles is homozygous, while an individual with two different alleles is heterozygous. Homozygosity can be either dominant or recessive, whereas heterozygosity involves one of each allele type.
Step-by-step explanation:
A diploid individual with two identical alleles of a gene is homozygous, while an individual with two different alleles of a gene is heterozygous.
In genetics, alleles are different versions of a gene that reside on homologous chromosomes in diploid organisms. When these alleles are identical, the organism is said to be homozygous for that particular gene. Examples include homozygous dominant (AA) or homozygous recessive (aa), where 'A' denotes a dominant allele and 'a' a recessive allele. Heterozygous organisms, on the other hand, have one dominant and one recessive allele for a gene (Aa).
This genetic composition can influence the organism's phenotype—its observable characteristics. Although homozygous dominant and heterozygous organisms often exhibit the same phenotype, their genotypes are distinct, and this diversity is important for the variation and heritability of traits.