Homozygous refers to having identical alleles for a single trait at corresponding chromosomal loci. Heterozygous, on the other hand, refers to having two different alleles for a single trait.
Example - The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). A homozygous plant would contain the following alleles for seed shape: (RR) or (rr). A heterozygous plant would contain the following alleles for seed shape: (Rr).
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