What do the genetic terms homozygous and heterozygous mean?

In general, homozygous means "the same" and heterozygous means "different." In genetics, we inherit two copies of each gene, one from the mother and a second from the father. We are homozygous when both copies are the same. We are heterozygous when the copies are different (not the same). However, some genes are ancestral or "wild type" while others contain single nucleotide polymorphisms (SNPs) or "mutant." Therefore, there are three possible combinations for each gene.

  • Homozygous "wild type" means you have two copies of the ancestral or "wild type" allele.
  • Heterozygous "variant" means you have one copy of the ancestral/wild type allele and one copy of the SNP or "variant" allele.
  • Homozygous "variant" means you have two copies of the SNP or "variant" allele.
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