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I was recently studying gametogenesis (I'm a medical student), and I was thinking about meiosis and fertilization. So on the surface it sounds simple, 23 s-chromosomes from the father and 23 from the mother, and together we have 46 chromosomes and 23 "homologous pairs" of autosomes. I already knew beforehand that each chromosome has 2 copies, but today was the first time that I asked "but why are they similar"? The father and mother should be genetically non-identical, but somehow the maternal and paternal copies of chromosome 1 for example are similar actually, enough to warrant them being called homologs of each other. So why is that? I have some sneaking suspicion it involves evolutionary genetics, but I might be wrong.

Any response is appreciated!

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They are similar because, on average, your genome is 99.9% identical to other humans. So, while there are traceable differences on each chromosome that can be mostly mapped back to each parent, the sequence of the chromosomal pairs is nearly the same.

Each chromosome has a specific length and set of genes. Except in rare cases of intra-chomosomal recombination (and the special case of the X/Y chromosomes), each chromosome in the pair will encode the same genes, which will be nearly identical except for the parent-specific differences, and (rare) mutations that may arise alter which are unique to the progeny.

karyotype from NHGRI

karyotype image from NHGRI

These two questions give more background about homologous chromosomes:

  1. What are homologous chromosomes?
  2. In our 23 chromosome pairs, do the 2 members of the pair have distinct or virtually identical sequences?
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