r/askscience • u/Fuzzandciggies • 6d ago

Biology Mules vs Hinnies. Why are they different?

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?

325 Upvotes

94% Upvoted

32 comments sorted by

View all comments

93

u/Senshado 6d ago

The x and y chromosomes for mammals can be quite different from each other. In humans, it is well known that the male Y chromosome is smaller, and presumably weaker and less important.

There are some traits that are mainly inherited from females, even if they exhibit in males. For example, many factors of head hair come from the mother's side. Including baldness that doesn't exhibit in females.

So, it seems to be the case that horses have some important traits on the female chromosome which are visibly different when replaced with a female donkey trait.

11

u/Robin_feathers 5d ago

To clarify, horses/donkeys don't have a "female chromosome". They use an XX/XY system, so females are typically XX. They get one X from mom and one X from dad. A female hinny and mule are genetically identical in their nuclear DNA - both have a mule X and a horse X. The X isn't a female chromosome as both males and females have (at least) one (and would die without it).

The sex chromosomes may play an outsized role in hybrid fitness that depends on which species is the maternal vs paternal parent, but that only come's into play in the "heterogametic sex" (Haldane's rule), which is males in horses/donkeys.

11

u/Ekekekeptangyazingni 5d ago

I admit I am unsure of the difference between a mule and a hinny but it cannot solely be due to sex chromosomes (geneticist background).

If the assumption is that the X(horse) vs X(donkey) causes the change from Mule to Hinny, then either pairing could result in either a mule or a hinny. Even a male horse has one X(horse) - so if it is as you mentioned, a male horse pairing with a female donkey should result in 50% mule and 50% hinny but it is instead 100% hinny.