Why are people with an odd number of chromosomes able to reproduce if animals with an odd number of chromosomes cannot?


I want to be very clear, I am not saying that there is anything wrong with people with Down syndrome having or raising children. I am confused because I learn in high school biology that animals like mules cannot reproduce because of their odd number of chromosomes so why is this not true for humans with an odd number of chromosomes?

In: 146

Largely speaking it *is* true. Statistically the majority of females and nearly all males with Down syndrome are infertile.

Well actually, it is true in humans as well. No living human has an odd number of chromosomes, such a condition is 100% fatal. Instead, we have individuals (such as those with Downs) that have a partial trisomy, meaning they have a piece of one extra chromosome, not an entire extra chromosome.

These are two different issues you are dealing with here.

In the case of hybrid animals like ligers (lion/tiger), mules (horse/donkey) and geep (goat/sheep), the parent animals have enough similarity that the gametes are able to fertilise and form a viable offspring due to them having a common ancestor and similar genetic makeup. These offspring unfortunately will have an extra chromosome from one parent that the other parent did not have, so during meiosis this single chromosome is often damaged beyond repair, and a second generation will result in non-viable fertilisation events in the majority of cases.

In the case of humans with trisomy 21 or other extra chromosomal anomalies, there is a chance during meiosis that a single, undamaged chromosome of the correct type is produced and sorted into a functional gamete. Given the large number of gametes a human can produce, there is a chance then (however small) that someone with an extra chromosomal condition can have offspring with normal chromosomal makeup (just very unlikely).

There’s actually almost no difference between humans and other animals in this regard. In both cases, an odd number of chromosomes leads to sterility or reduced fertility. Usually, odd numbers of sex chromosomes is better tolerated and allows for the greatest chance of reproduction.

Most people with an odd number of chromosomes are unable to have children.