"Heterozygous" or "het" simply means that the two genes in a gene pair are different. They could be a normal and a mutant gene or, in the case of multiple alleles, two different mutant genes. There is no requirement as to what a heterozygous animal looks like in that definition. In other words, a het tiger carpet python has a tiger mutant gene paired with a normal gene. What that het tiger looks like has nothing to do with the fact that it is a het tiger.

The heterozygous animal's appearance does determine whether a mutant gene is considered to be dominant, codominant, or recessive to the normal gene. Here are the ideals:

If a mutant gene is a recessive, a snake with a normal gene paired with the mutant gene looks like a snake with two normal genes.

If a mutant gene is a dominant, a snake with a normal gene paired with the mutant gene looks like a snake with two mutant genes.

If a mutant gene is a codominant, a snake with a normal gene paired with the mutant gene looks like neither the snake with two normal genes nor the snake with two mutant genes.

Those three categories are human inventions. Sometimes nature gets sloppy and doesn't exactly fit a category. Then it's like picking a pair of shoes -- none fits perfectly, and the choice comes down to picking the best fit available.

The best way to be get animals that are known heterozygous is to mate a snake with two mutant genes to a snake with two normal genes. All of the babies are then heterozygous (AKA 100% probability heterozygous). If these babies do not look normal but also can be picked out from snakes with two tiger mutant genes, the best fit may be "codominant", with variable expressivity producing a variety of more or less intermediate appearances in heterozygous animals.

Let's say that a "classic" tiger has two tiger mutant genes and a heterozygous tiger (one tiger mutant gene paired with a normal gene) has a tiger head pattern and some broken striping. And a snake with two normal genes looks normal, without the tiger nead pattern or any striping. Then expectations for the various possible mating are as follows.

"classic" tiger x "classic" tiger -->
all babies "classic" tiger

"classic" tiger x heterozygous tiger -->
1/2 "classic" tiger
1/2 heterozygous tiger

"classic" tiger x normal -->
all babies heterozygous tiger

heterozygous tiger x heterozygous tiger -->
1/4 "classic" tiger
2/4 heterozygous tiger
1/4 normal

heterozygous tiger x normal -->
1/2 heterozygous tiger
1/2 normal

normal x normal -->
all babies normal

In other words, if there are no normal babies, then the most likely explanations are 1) at least one parent has two tiger mutant genes, or 2) it's a small clutch of eggs and the luck of the draw was against you.

Hope this helps.

Paul Hollander

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