I read somewhere that human chromosome 2 is the result of 2 primate chromosomes fusing together somewhere along our evolutionary journey. This is why we have 23 chromosomes while other primates have 24. My question is this: for the first ancestor whose chromosomes fused to have 23 chromosomes instead of 24, how did he reproduce? Don't offspring receive half of their genetic makeup from each parent, so if one parent has 23 chromosomes and the other has 24, how would that work? I know that there have been animals with different chromosome counts being bred together, but the offspring typically have issues and are sterile, so even if the difference in chromosome count was not a problem, wouldn't the offspring between an organism with 23 chromosomes and one with 24 be infertile, thus leading to the end of animals with 23 chromosomes? Would there have to be two organisms of different genders experiencing the same chromosome fusion at the same time in order to avoid this problem? I'm just a bit confused as to how this would work exactly.
My question is this: for the first ancestor whose chromosomes fused to have 23 chromosomes instead of 24, how did he reproduce?
He or she reproduced the same way people today with balanced Robertsonian translactions reproduce. During meiosis I, the separated chromosomes line up with the fused one, so the resultant cell gets either the replicated fused chromosome, or the replicated separate chromosomes. Then meiosis proceeds as usual, with half the gametes having the fused chromosome, and half having the two separate chromsomes. The fused chromosome has just about all the information the two separate ones have, so the gametes with the fused chromosome can go one to make a organism with all the required genetic information.
Strangely enough we actually do have an answer for that.
There are a several examples of individuals who are born with two chromosomes fused together in all their cells.
We even have a few examples where these fused chromosome is in a diploid state, ie all instances of that chromosome are fused in the individual. Such individuals come from families in small isolated villages that are very inbred.
Chromosome number does not prevent fertility but it does reduce it in heterozygous state as it is now harder to get correct chromosome distribution during meiosis and thus fewer functional gametes are made. However in terms of function, as all genes are present in a somatic cell in the correct number, function is usually normal and the individual is alive.
If the population is inbred, it is possible to get an egg and a sperm that carry the same fused chromosome to meet. In which case you get a diploid individual with fused chromosomes. Fertility is now restored. And in an inbred population, a homozygote individual for the fused chromosome can meet another similar individual, and have offspring who are fully fertile.
AS you can imagine, over time there is a selection pressure for the population to separate. The homozygous state (be it fused or unfused chromosome) is more fertile that the heterozygous state.
So to put it short, chromosome fusion does not kill a cell. All gene are still present. However in the heterozygous state, where both fused and unfused chromosomes are present, fertility is dramatically reduced BUT is none zero. Fertility is restored in a homozygous state when an individual is diploid for either fused or unfused chromosomes. In small isolated populations, inbreeding allows for the emergence of diploid individuals which can reproduce with each other