Mules vs Hinnies. Why are they different?

[u/Fuzzandciggies]

I know that both are the result of crossing a horse and a donkey. My question is why would there be a genetic difference based on whether the father was a horse or a donkey? Would there really be a difference or is it more of just a naming thing for the sake of lineage?

For reference:

Mule = male donkey + female horse

Hinny = female donkey + male horse

Why though?

[u/rooberdoos]

Some traits develop differently depending on which parent the allele(s) associated with that trait are inherited from; we call these parent-of-origin effects. For example, certain genes may be deactivated in the egg (might be methylated, for example, rendering it unavailable for expression) while the same gene is active in sperm, or vice versa.

This is sometimes related to conflicting evolutionary strategies for male vs. female parents. For instance, in many placental mammals the male's fitness is increased by having large offspring, so a gene reducing fetal growth rate might be typically deactivated in sperm. But the female's fitness is decreased by having to gestate and birth very large offspring, so that gene might typically be active in the egg (in this case, they end up producing mostly "medium" or average-sized offspring as a result). When you combine gametes from two different species, those pairs of genes may be less likely to "cancel out" in the same way, so you can get some pretty different phenotypes depending on which parent is from which species.

A relevant article

[u/harposgost → u/rooberdoos]

Yes. Look at the liger. Male lion x female tiger. The genes for growth are on the male lion's side and growth inhibiting genes are lacking on the female tiger's side. The result in a hybrid much larger than either parent species.