Genes come in pairs. If we say that a gene pair is homozygous, that means both members of the gene pair are the same. Two identical normal genes or two identical mutant genes. A gene pair is heterozygous when the two genes are different. It could be a mutant gene paired with its normal counterpart. Or it could be a pair containing two different mutant genes, but this scenario has not turned up in boa constrictors yet, as far as I know. It has turned up in other species.

Sometimes people say that heterozygous animals look normal. This is not true. Some hets look normal, like a het albino, which has an albino mutant gene paired with a normal gene. Some hets do not look normal, like a het salmon, which has a salmon mutant gene paired with its normal counterpart. What a het looks like depends on the genes involved.

A double het is an animal that has two gene pairs that are heterozygous. For example, a DH for snow has an albino gene paired with its normal counterpart and an anerythristic gene paired with its normal counterpart. (The normal counterpart of the albino mutant is not the same gene as the normal counterpart of the anerythristic mutant.) Such a boa looks normal. A DH for sunglow has a salmon (AKA hypo) mutant gene paired with its normal counterpart and an albino mutant gene paired with its normal counterpart. Such a snake does not show the effect of the single albino mutant gene but does show the effect of the single salmon mutant gene.

A triple het has three gene pairs that are heterozygous. And so on up. A single individual can have more than three gene pairs be heterozygous. Clear as mud?

Paul Hollander

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