I'm completely ignorant when it comes to genomes and DNA sequencing etc... so i would like someone to explain to me, when Chromosome 2 became as it is (through fusion) how did that human (or homo erect-us or whatever) reproduce? would it not be sterile? or did the fusion happen gradually?
forgive me if i sound stupid just trying to understand it
how did that human (or homo erect-us or whatever) reproduce
Not necessarily. There are examples of breeding between animals with different numbers of chromosomes creating fertile offspring. And in the case of a fused chromosome versus some other complete mismatch like we'd find between different unrelated species, it is completely unclear that the result would be unable to reproduce.
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The big bar to reproduction between those with sufficiently different genomes is the different proteins and signal pathways that need to exist. A hybrid often has such a jumbled set of proteins and signal elements that, assuming it can even live, reproduction is almost impossible. That's why humans cannot mate with oak trees but could mate with a close cousin species like Neanderthal.
In the case of Chrom-2 the genes for the proteins and signal elements are all there just in a different location than before the fusion. It helps that in the transcription process the transcriptase RNAs are not specifically targeted to a specific chromosome but to the control regions of the gene.
Prior to fusion each chromosome matches up with the matching chromosome from the other parent.
A mutation occurs that fuses 2a and 2b into one longer chromosome, but otherwise does not alter the string of genes on either 2a or 2b.
When mating the fused chromosome can still match up with the two chromosomes from the other parent. This would produce viable offspring.
As mutations occur this becomes less possible so either the fused chromosome genotype dies out or the original dual chromosome genotype dies out or the population splits into two reproductively isolated populations and speciation has occurred.