Whence the extra chromosomes?

I understand the basics of evolution. Random mutation, survival of advantageous characteristics leading to new species arising. Got it.

Genes mutate, okay. So how do animals end up with extra chromosomes? Horses and donkeys produces infertile offspring, mules, because they have different numbers of chromosomes, I have read. How did that happen?

That’s the question, by what mechanism does the number of chromosomes in a species increase?

Chromosomes are not immutable and it’s not just single neucliotide changes that can happen. They can swap whole sections and sometimes this results in what is essentially a merging of chromosomes. Or a splitting, that happens too. Here’s a page that explains it a lot better than I can.

Pretty good wiki article (though a bit on the technical side.)

ETA: the one on nondisjunction may be even more useful.

And the reason that horses and donkeys are interfertile at all is because the split chromosomes can still both more-or-less match up with the one that they originally split from. That is, you end up with a “pairing” that looks something like this:

|| || || |¦ || ||

It can go the other way, too. Our Chromosome #2 is the typical great ape chromosomes often labeled 2A and 2B, fused together. Because of the way we think the great ape lines split, we assume the basal (ancestral) great ape condition was 24 chromosome pairs, not 23. We are the weirdo-mutant ape!

Plants often double the number of chromosomes, or quadruple them.

many many humans have extra chromosomes.
Having one extra of some of them usually causes severe issues, such as miscarriage and health problems like downs syndrome (downs being three of the 21 st ? well trisomy - 3 - of one specific chromosome ).

But extra of most cause no problem.
And trisomy is defined as having more than two complete chromosomes… so if you have two and half, you have half extra… so thats why the levels of effect vary.

bolding is mine
An extra chromosome is usually lethal in humans. Down Syndrome (Trisomy 21) is the major exception. Persons with this chromosome compliment may be severly impared or may live a long and mostly healthy life.

Patau Syndrome (Trisomy 13) and Edwards Syndrome (Trisomy 18) are the only other human somatic* trisomies which regularly are compatible with life, though persons with these conditions rarely live a full life.

Trisomies of other somatic chromosomes are not compatible with life. Such atypical chromosome complements are a frequent cause of spontaneous abortion.

*An extra sex chromosome (X or Y) is rarely a significant concern. Patients with these karotypes may live a full life with no reason to suspect an atypical chromosome compliment.

Thankyou, very interesting.

I have heard of humans with more than the usual chromosomes, although I didn’t know this happened with non sex chromosomes. I believe that generally caused physical symptoms such as extra height and lower IQ.