This article is about reproduction from a genetics perspective. See elsewhere for information about pregnancy and mating behavior.

Even though only one version from each gene pair gets expressed to create the creature, it still contains both of them. It inherited one from mom and the other from dad.

When it goes on to have children of its own, it will donate one of the pair at random to the child, and its mate will donate one of theirs, meaning that the child has two copies too, but possibly different ones.

Let’s say that mom is Aa, and dad is also Aa. Both parents will express the same characteristic, A, because A is dominant over a.

The child, on the other hand could be any combination of these four. It could inherit A from both parents and become AA. It will have the same characteristics as both its parents – all three of them will express the A form.

Alternatively, it could inherit a and a, which makes it aa. Now it will be different from both its parents! They both had an a version but it didn’t show up, because it was hidden by their A version. But the child inherited nothing but a, and so it will express the a form and be different from both of them.

Lastly, it could inherit A from mom and a from dad, or a from mom and A from dad. In all these cases it will express the dominant A form.

Essentially, there are three ways to make the A form (AA, Aa, aA) and one way to make the a form (aa). And so, if the same couple have multiple children, those children are three times as likely to be A, but some of them might still be a.

To work out the genetics of a given creature, you really need to know its own characteristics and those of both its parents. You can’t just look at the creature and know its genotype. Tracing family trees this way can be quite interesting. Much more interesting than doing it with hypothetical green and yellow peas, like most of us learn in school! And it can matter, too, because sometimes homozygous recessive creatures will show certain disabilities or oddities, and will need caring for differently.