Why are hybrid animals different depending on which parent is from which species ?
In: Biology
The data you get from your mother is not the same sort of data you get from your father.
It’s half your DNA…. but not EXACTLY half. The Y chromosome only comes from fathers. Any information on a donkey’s Y chromosome will only be passed down when the father is a donkey. Likewise, sperm have no mitochondria* (it’s the powerhouse of the cell), so you get your mitochondria DNA from only your mother. If there is data in either of these which affect the offspring (there is) then it matters which data they’re getting. Hinnys and mules are simply genetically different.
(*Oh, it’s weirder than that. Only the tail have it, and it’s usually discarded, but there are recorded exceptions. …EVERY damn thing in biology is just riddled with exceptions to the rule. ugh.)
Well, at least partly it’s because hinnies are gestated in donkeys and mules are gestated in horses. Since mules are smaller than horses, a hinny will usually be smaller than a mule to reflect the mother’s size and nutrients it got in utero and through breastfeeding. Plus a hinny is raised by donkeys and a mule is raised by horses, so their behavior will reflect how they were raised, a bit. They do look very similar and it’s hard to tell the difference by just appearance. But there are also certain genetic differences–hinnies have donkey mitochondrial DNA (mtDNA) and mules have horse mtDNA. You only get mtDNA from your mother and there are certain mtDNA differences between the species. Likewise hinnies have horse y-chromosomes and mules have donkey y-chromosomes, so whatever genes are being controlled by y-chromosomes are going to reflect the species it came from.
Sometimes there are differences in terms of genes that are activated in females vs. males. Like in the liger tigon example given by someone else, the liger is huge–much bigger than either lions or tigers. this is because the male lion has a gene that encourages growth in offspring, while the female lion has a gene that suppresses that growth, resulting in the lions we see today. But when you breed a male lion with a female tiger to get a liger, you get the gene from the male to increase growth but tiger females don’t have the gene that suppresses growth meaning you get far more growth in a liger than you get in a lion or a tiger. Likewise the tigon is a lot smaller than a liger, because the female lion passes on genes to suppress growth, without having the gene to increase growth from the male lion.
And finally, there’s something called genomic imprinting: some genes are expressed differently depending upon whether they were inherited from the mother or the father. This isn’t hugely well understood yet, but basically there’s a process whereby a gene’s expression is chemically altered based on whether it came from a mother or a father. The classic example of this is Prader-Willi Syndrome and Angelman Syndrome. Basically, in humans, if a particular spot on chromosome 15 is deleted in the genes inherited from one’s father, then the child will have Prader-Willi Syndrome. If that same spot is deleted in the genes inherited from the mother, the child will have Angelman Syndrome.
For those who don’t know, mules and hinnies are both horse/donkey crossbreeds, but a horse is a mule’s mom and a donkey is a hinny’s mom.
One big difference is that hinnies are smaller than mules. Still ornery, though. I got kicked in the shoulder by a hinny and my arm was out of commission for weeks.
My theory is that hinnies are smaller because the mom is smaller—donkeys are smaller than horses. A smaller mom with a smaller womb will naturally provide less nutrition and create a smaller baby.
I mean, imagine a chihuahua/great dane crossbreed. There is no way that a dog halfway between a chihuahua and a great dane is coming out of that poor little mom. (I’m going to assume that such a cross would have to be done with artificial insemination, but that’s a whole different story.)
An excellent example is big cat hybrids, such the Liger (m tiger x f lion) versus the Tigon (f tiger x m lion) which look very different despite both being Lion-Tiger hybrids. The Liger has stripes over tan skin and is massive. Significantly larger than its tiger (the largest big cat) parent. The Tigon however, isn’t as large and has more pronounced stripes. It also has head spots and a mane. If one managed to see both in the wild, they’d easily be mistaken for different species. And in a very concrete since, they are different species.
A Liger and a Tigon will not be able to produce offspring. However a male and female Tigon can sometimes produce viable offspring together. Same for a male and female Liger. So it’s not hybrid sterility. They are essentially their own distinct animals.
In other kinds of hybrids, both variations might simply be sterile altogether and just be one-offs. Or they may not be able produce viable offspring with either parent species or with others of the same hybridization because of chromosomal issues. But sometimes you just get a brand new species that can successfully reproduce even if they only exists in captivity.
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