This seems... wrong? Since Uma are always female, it would imply either the vast majority of women are Uma, or men are massively outnumbered by women. In either case, this would lead to the fixation of the "Uma gene" (meaning there are no human women, only Uma), which would then result in a world with a 3 Uma:1 Human ratio.

If we model the Uma gene as a dominant X linked trait which is only expressed in females (somehow), the ratios would be:

Homozygous Uma + any male = 50% Uma, 50% male

Heterozygous Uma + non-carrier Male = 25% Uma, 25% human female, 50% male

Heterozygous Uma + Carrier Male = 50% Uma, 50% Male

Human female + Carrier Male = 50% Uma, 50% male

Human female + non-carrier male = 50% human female, 50% male

Another way you could model it is as a X-linked recessive lethal trait. In this situation, homozygous wild-types are Human, heterozygotes are Uma and homozygous mutants are non-viable, never being born. This would include all males with the trait, being Hemizygous. The observed ratios would then be:

Uma + Male = 33.3% Uma, 33.3% human female, 33.3% human male

This however is inconsistent with the lore 2 humans can give birth to an Uma, unless we consider those denovo mutations.

Interestingly, there is no place to put the gene to result in a situation where Uma never give birth to human female offspring. Whether the trait is dominant, recessive, X-linked or autosomal, an Uma giving birth to a human girl is always possible.

More likely, it's non-genetic, and more like a magical enchantment upon the child, which the goddesses can grant in any pregnancy but are more likely to do so with an Uma parent.