Explain why mitotis is normal in cells containing both horse and donkey chromosomes but the mixed set of ....?

Question

10. A mule is the offspring of a horse and a donkey. A donkey sperm contains 31 chromosomes and a horse egg 32 chromosomes, so the zygote contains a total of 63 chromosomes. The zygote develops normally. The combined set of chromosomes is not a problem in mitosis, and the mule combines some of the best characteristics of horses and donkeys. However, a mule is sterile; meiosis cannot occur normally in its testes or ovaries. Explain why mitotis is normal in cells containing both horse and donkey chromosomes but the mixed set of chromosomes interferes with meiosis.

Answer 1

That’s pretty complicated and to be quite honest we still don’t know the whole story. The basic problem is that after the ancestors of donkeys and horses split the chromosomes of BOTH species fused and then those fused chromosomes re-split (Ancestral Fusion). That has led to a situation where the information originally carried on a single chromosome in the ancestral species is now found on 2 different chromosomes in mules and 2 totally unrelated chromosomes in horses.

Normally a developing foal embryo would receive two copies of each piece of genetic information: one from the mother and one from the father. In the case of a mule foal it receives a random number of copies. It will receive three copies of some pieces of information: 2 from the mule parent and one from the horse/donkey parent, or it will receive only a single copy from the horse/donkey parent.

Because of that mismatch of genetic material the embryo has a hard time developing. Some traits it will lack sufficient information for since it has only half the required genetic information, while other traits will be overdeveloped because there are 3 or more genes where there should be only two. All mules will get pregnant if mated, but the embryo develops for just a few days and then dies because it lacks the correct genetic instruction to develop further.

There are rare cases in which mules aren't sterile