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?
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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.
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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.
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).
This is a misconception. Animals with mismatched chromosome numbers can reproduce and have fertile offspring. Here’s a great comment explaining it:
>Just to be clear about this since it’s widely misunderstood: chromosome count has absolutely zero to do with whether two organisms are able to produce offspring together.
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>Homologous chromosomes never pair during fertilization or embryonic development, so the number is completely unimportant. What matters for producing offspring is molecular recognition between the gametes that allows for fertilization and gene regulatory interactions between the parent genomes in the zygote/embryo during development. These things are based on the *nucleotide sequences* in the genome, not how many chromosomes it’s made up of.
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>Chromosome pairing doesn’t come into play until meiosis, so it can affect the fertility of offspring, but not whether they’re produced in the first place. Those are two completely different things. The chromosome count alone still doesn’t determine anything, though, because more than two chromosomes can line up, forming a trivalent instead of a bivalent, and functional gametes can still produced even if chromsome numbers inherited from the parents don’t match. This only fails when the *nucleotide sequences* along the chromosomes are too different from one another, so it’s again about the sequence, not the number.
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>Even the sterility is not determined by chromosome count. The hybrid cross between a Przewalski’s horse and a domestic horse, which also have different numbers of chromosomes (66 vs. 64), are **not** sterile, for example. Meanwhile, the hybrid offspring of a Muscovy duck and mallard duck are healthy, but sterile even though the parent species have the same number of chromosomes. And of course a human and a Reeve’s muntjac can’t produce offspring together at all, sterile or otherwise, even though we both have the same number of chromosomes.
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>I think the example of mules is the main source of this misunderstanding, precisely because they’re so well known. It makes people think chromosome count is important, but it’s really not.
Full credit to u/yerfukkinbaws
[https://www.reddit.com/r/biology/comments/tfqvcj/ive_recently_come_across_macroevolutionnet_and_im/i0y3smm?utm_medium=android_app&utm_source=share&context=3](https://www.reddit.com/r/biology/comments/tfqvcj/ive_recently_come_across_macroevolutionnet_and_im/i0y3smm?utm_medium=android_app&utm_source=share&context=3)
You’re thinking of this incorrectly. A mule is a mix between a donkey and a horse, it can’t reproduce because it’s a genetic monstrosity made of two different animals. This is why they can’t bread, because they’re missing important reproductive genes. This would be like if a human had a baby with an gorilla.
Someone with Down syndrome is still a human made of two humans, so most of the genetic material is available for baby making.
As others have said, humans with odd numbers of chromosome, usually cannot reproduce. However there are always exceptions.
There is a something g that is called mosaicism. It is when not all the cells in someone’s body contain a mutation, on and extra/missing chromosome. When this occurs in women it appears that their eggs are more than likely to be unaffected. It’s to do with how the ovaries and eggs form before birth. In men it’s less likely that all the sperm will be unaffected, but in mosaicism, it’s likely that some of the sperm are normal. It may even be that all the sperm are normal, but lower in number (the extra/missing chromosome in the affected cells may prevent sperm forming from an affected origin cell. Hope that makes sense). So for those with mosaicism, there is highly likely to be normal eggs and sperm around.
Next, the three most common chromosome number syndrome are Down syndrome (extra ch21), turners (women with only one X chromosome) and klinefleters syndrome (men with two X chromosomes and one Y chromosome).
For Down syndrome men are rarely fertile. Only three reported cases of men with DS becoming fathers without help.
Women with DS, can be fertile. Sometimes they will produce eggs with normal ch numbers (such as if they are mosaics, and possible occasionally a rare way of getting an extra chromosome when there are two 21 ch stuck together plus and extra single one). There may be other mechanisms that produce eggs. However women with DS will often have problems getting pregnant. One reason for this is earlier menopause.
Next, Turner’s syndrome. These ladies actually experience ovarian failure before puberty onset (basically menopause before going through puberty), so won’t produce eggs. Puberty usually needs to be done with medication and they go oN HRT after that.
Finally Klinefelters. These men usually have testicular dysgenesis/dysplasia (testicles don’t form properly and sperm production doesn’t work, or occurs at a really low level to the point of infertility). The testicles will still work to make hormones, but not sperm. But again there are always cases where some of the few sperm made makes it to the finish line (the egg).
This explanations of some of the reasons people with odd number of chromosome can’t have children has been longer that I planned and not very ELI5 language, but hope it makes some sense and is some help
So we do not exactly know why males are infertile but there have only been 3 cases of fertility in Down’s males.
Females on the other hand are rarely fertile but many can be sub fertile, this means that they get pregnant but most babies end up with horrible disfigurations or are aborted spontaneously.
So, just like other animals, humans too cannot reproduce with odd chromosome numbers. This does not have to do with having 3 chromosomes but the effects of having 3 chromosomes.
Humans are also animals and largely follow the same genetic patterns and rules.
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