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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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
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