Hybrid creatures, sterile?

Most of the time, yes, hybrids are sterile.

Even among close relatives, like a donkey and a horse (both members of the same animal family), if they mate, their offspring will be sterile. It's called a mule.

The sterility is caused because the two sides of the DNA (the one inherited from the father and the one inherited from the mother) have different numbers of chromosomes.

 

I thought creatures with different numbers of chromosomes couldn't interbreed at all?

 

I'm not a geneticist (genetics gives me a headache), but I think the mule is one of the rare exceptions to that rule.

 

If you want to research the underlying science, here's an article on Wikipedia. Warning, your eyes may glaze over.

 

Here is an easier explanation than the wickipedia pade (also are DragonCats sterile, as I think I saw a stray dragoncat on my street the other day!)
a horse and a donkey can have kids. A male horse and a female donkey have a hinny. A female horse and a male donkey have a mule.

But hinnies and mules can't have babies of their own. They are sterile because they can't make sperm or eggs.

They have trouble making sperm or eggs because their chromosomes don't match up well. And, to a lesser extent, because of their chromosome number.

A mule gets 32 horse chromosomes from mom and 31 donkey chromosomes from dad for a total of 63 chromosomes. (A horse has 64 chromosomes and a donkey has 62).

To understand why this is a problem, we need to understand how sperm and eggs are made. And to understand that, we need to go into a bit more detail about chromosomes.

Remember, we have two copies of each of our chromosomes -- one from mom and one from dad. This means we have two copies of chromosome 1, two copies of chromosome 2, etc. However, this isn't entirely true for the mules.

The mule has a set of horse chromosomes from its mom. And a set of donkey ones from its dad.

These chromosomes aren't really matched sets like in a horse, a donkey, or a person. In these cases, a chromosome 1 is very similar to another chromosome 1. It looks pretty much the same and has nearly the same set of A's, G's, T's and C's. For example, two human chromosome 1's differ only every 1000 letters or so.

But a donkey chromosome doesn't necessarily look like a horse one. And the poor mule even has an unmatched horse chromosome just sitting there.

To make a sperm or an egg, cells need to do something called meiosis. The idea behind meiosis is to get one copy of each chromosome into the sperm or egg.

For example, let's focus on chromosome 1. Like I said, we have one from mom and one from dad. At the end of meiosis, the sperm or egg has either mom's or dad's chromosome 1. Not both.

This process requires two things. First, the chromosomes have to look pretty similar, meaning they are about the same size and have the same information. This will have to do with how well they match up during meiosis.

And second, at a later critical stage, there has to be four of each kind of chromosome. Neither of these can happen completely with a mule.

Let's take a closer look at meiosis to see why this is. The first step in meiosis is that all of the chromosomes make copies of themselves. No problem here...a mule cell can pull this off just fine.

So now we have a cell with 63 doubled chromosomes. It is the next step that causes the real problem.

In the next step, all the same chromosomes need to match up in a very particular way. So, the four chromosome 1's all need to line up together. But this can't happen in a mule very well.

Like I said, a donkey and a horse chromosome aren't necessarily similar enough to match up. Add to this the unmatched chromosome and you have a real problem. The chromosomes can't find their partners and this causes the sperm and eggs not to get made.

So this is a big reason for a mule being sterile. But how is the silly thing alive at all?

Well, there are a couple of reasons. First, having an odd number of chromosomes doesn't matter for every day life. A mule's cells can divide and make new cells just fine. Which is important considering a mule went from 1 cell to trillions of them!

Chromosomes sort differently in regular cells than they do in sperm and eggs. Regular cells (called somatic cells) use a process called mitosis.

Mitosis is like the first step of meiosis. The chromosomes all make copies of themselves. But instead of matching up, they just sort into two new cells. So for the mule, each cell ends up with 63 chromosomes. No matching needs to happen. And our lone horse chromosome is fine.

The other reason a mule is alive is that nothing on the extra or missing chromosome causes it any harm. This seems obvious at first except that usually having extra DNA causes severe problems. In people, extra chromosomes usually result in miscarriages. Sometimes though, a child can survive with an extra chromosome.

For example, people with an extra chromosome 21 have Down syndrome. Having all of the extra genes on that extra copy of chromosome 21 cause the symptoms associated with Down syndrome.

So having extra chromosomes often leads to real problems. But the mule is by and large OK.

The extra genes must not be that big a deal for the mule. In other words, the extra genes on the horse chromosome do not cause problems for the every day life of a mule.

So mules are sterile because horse and donkey chromosomes are just too different. But they are alive because horse and donkey chromosomes are similar enough to mate.

 

Just want to point out that the case with mules and hinnies(its really hard to be serious when I'm using the word hinny) is rare. Differences is ploidy number usually has bad consequences. http:// http://www.nature.com/scitable/topicpage/hybrid-incompatibility-and-speciation-820. Its very technical. Basic idea seems to be gene expression is screwed up during crossover, ploidy number, and actual genes.

As a side note sterility is usually seen between closely related species. From my understanding vampirism is a disease. Diseases in themselves can cause sterility though I have never heard of a disease causing sterility in offspring and not the parent.

 

That is a very good point that I had missed.

 

Not to jack your thread Dragoncat, but what would the likelihoood then be of a woman born with an omnivore father and a herbivore mother being able to have children?

P.S. dragoncat I like that you are having your vampires being a species rather than supernatural. I am doing the same in mine.

 

Ok just thought to point it out in case you overlooked that aspect.

 

I think it would go back to whether your races are created by the gods, in which case it may depend on how well the deities in question get along, or whether the races all evolved from a common ancestor and represent different branches on the evolutionary tree. Modern Human DNA contains traces of Neanderthal, and other "cousins" so obviously that interbreeding didn't lead to widespread sterility. As far as other hybrids it would depend on whether you want them to be a race or a one-off freak. And culturally if you knew your children would be sterile would you breed outside your race?

 

No gods, just all from a common ancestor. They are all able to interbreed. In fact it is central to the story. Three of the main characters are hybrid from different races and this fact is used to foreshadow future events.

 

To me it still comes down to would the races interbreed if they knew it would result in sterile offspring. If they're just interbreeding to produce hybrids for a specific purpose(Like we do with mules) then sterility might be a necessary evil. If these characters are the result of loving relationships, I just can't see voluntarily ending your line, and denying your children the chance to procreate being a popular choice.

 

What about the weaknesses of both races? Can your vampelf go out in sunlight? He might not burst into flame (or whatever happens to your vamps), but he might sunburn far more easily than a human or elf would. I have a similar problem with my light-elf/black-elf hybrid character. Black-elves turn to stone in the sunlight, and light-elves don't. I'm not sure what might happen if a hybrid tried. Then again, it's a moot point, since she never really gets to go into the sun... but it would be handy to know anyway. I don't want her to be totally invulnerable to both races' weaknesses because of her mixed ancestry, and I think it's something for authors of other kinds of hybrids to consider as well.

 

I like the idea that a hybrid could be the worst of both worlds/species/races...
Rather than an army of super warriors [ala LOTR's Ukru Hai] you get short bow legged humans who are afraid of the sun...