The term Zygosity is most often seen in a more complex form of either homozygous or heterozygous. Zygosity refers to the similarity between copies of the same chromosome. Basically, deriving from an early term to understand if twins are identical (same zygote) or fraternal (different zygote). Homozygous thus meaning the same in both and heterozygous being different. Large runs of homozygosity (RoH) generally imply the parents were related and thus have endogamy in their history. It can also reflect a genetic abnormality where multiple copies of the same chromosome exist in the tester.