Chromosome 2 and speciation

So some gametes will have 2 normal infused chromosomes. The will be viable and passed on to the following generation. Some gametes will have 1 infused chromosomes. That will also be viable and passed on to the following generation.

I’m just going to clarify one point again. So when the individual with the fused chromosome has offspring the offspring will also have a fused chromosome. They will then at some point meet another individual who also had the fusion, and the offspring ended up with 46, 23 from each parent. This is the way Ken Miller seemed to explain it but just want to double check.

So 3 is viable. That way it would be passed on to the next generation and the the offspring would have a total of 47 (23 from the individual with the fusion, and 24 from the other “normal” individual) chromosomes not 48. Then when two individuals with the fusion mated the offspring had 46.

This would be correct if the first “infused” were “unfused” and the second one were “fused”.

Some of them will, some of them won’t. Depends on which sort of gamete ends up contributing to the offspring.

That’s certainly one possible outcome. They could also get the fused chromosome from one parent or from neither.

It could happen that way. Independent assortment. If you’re asking how the change became fixed in the human population, it just involves the fusion increasing in frequency in the population, homozygotes becoming more and more common, until everyone is like that.

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No. A carrier will produce one of the following:

  1. One fused chromosome (balanced)
  2. One pair of normal chromosomes (balanced)
  3. One fused chromosome with a single normal chromosome (unbalanced)
  4. A single normal chromosome (unbalanced)

Refer back to one of the diagrams.

If a balanced gamete meets a balanced gamete, the zygote will be viable. Otherwise there will be a spontaneous termination.

Not necessarily, because the carrier can produce normal gametes. Only about a sixth of the gametes of a carrier will have a balanced fusion, a sixth will be normal with no fusion, and 2/3s will be non-viable.

Sorry, I meant 2 “unfused” (as in a pair of normal chromosomes) not “infused”. Predictive text changed it.

Okay. So if individual A with the fusion, mates worth an individual B without the fusion, and a gamete with one fused chromosome (balanced) meets the other individual’s normal gamete (balance) there will be offspring missing one chromosome as one parents contributed 24, the other contribute 23. 47 in total. Later 2 individuals both with 47 mate, and the offspring ended up with 46. That’s one way that human chromosome 2 ended up fused and we have 2 less chromosomes than other primates?

‘Missing’ a chromosome suggests they lack that chromosome’s genetic complement, which isn’t the case. Otherwise yes.

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That was strongly garbled. Just think of it this way: it’s like any other allelic variation. If you have blood type alleles A and B (let’s forget O for now), anyone could have genotypes AA, AB, or BB. If we start with everyone AA, and there’s a mutation that produces a B allele, the first person with that allele will have genotype AB. Mating with a “normal” AA person produces half AA offspring and half AB. If the B allele increases in frequency, eventually two AB people will mate, and this gives you 1/4 AA, 1/4 BB, and 1/2 AB offspring. Mating of AA and BB gives you all AB; mating of AB and BB gives half AB and half BB. As allele B becomes more common, perhaps solely by drift, BB individuals also become more common, to the point where everyone is BB and the A allele is lost. It’s exactly the same with fused and unfused chromosomes except for that detail about aneuploid gametes.


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