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 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 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. Well, there are a couple of reasons.
First, having an odd number of chromosomes doesn't matter for every day life. The hinny is sterile with only one recorded exception.
Hinnies are not similar to mules in that they are generally more intelligent than horses, and more cooperative than donkeys. Both are also healthier and less expensive to feed and maintain than horses. This is a trait these hybrids get from their donkey heritage.
The donkey is a notoriously hardy creature that, in the wild, survives on a harsh diet in a desert environment. Hinnies are on average slightly smaller than mules. There is much speculation as to the size variances among the two types of hybrids. Some fanciers believe it is merely physiological , due to the smaller size of the donkey dam versus a large horse mare.
Others claim it is genetic. The view of the American Donkey and Mule Society is, "The genetic inheritance of the hinny is exactly the same as the mule.
Hinnies are smaller because donkeys are, for the most part, smaller than horses, and the growth potential of equine offspring is influenced by the size of the dam's womb. Hinnies do however, like mules, come in many sizes. This is because donkeys come in many sizes, from miniatures as small as 24 inches mm at the withers , to Mammoth donkey jacks and jennies that may be over 15 hands, approximately 60 inches mm at the withers. Thus, a hinny is restricted to being about the size of the largest breed of donkey.
Mules, however, have horse females as mothers, so they can be as large as the size of the largest breed of horse. There are some huge mules, mostly from work horse breeds such as the Belgian. Other than size, there are some minor differences that occur frequently between mules and hinnies. The head of a hinny resembles that of a horse, more so than mule heads do. Hinnies often have shorter ears, although they are still longer than those of horses, and more horse-like manes and tails than mules do.
They often come in horse colors, as the male parent often determines the color of the coat. Therefore, mules usually have donkey coat colors. Certain traits, like the popular gait that some horses and donkeys possess, seem to pass more readily though the male parent. Therefore, many people have tried to produce gaited hybrids by using gaited male horses on female donkeys in hopes of creating gaited hinnies. Like the mule, the hinny is almost always sterile.
Both are also healthier and less expensive to feed and maintain than horses. This is a trait these hybrids get from their donkey heritage. Hinnies are on average slightly smaller than mules. Related Stories. What's more, these offspring go on to have longer, healthier lifespans -- and in turn A team of The results show that breaking up coevolved sets of sex chromosomes could lead to lower
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