On what basis is each chromosome given a number? There appears to me to be no visible ordering between each pair. Is there any reason other than differentiation?
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asked Jun 11, 2018 at 20:39
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Chromosomes are generally ordered by size in terms of number of nucleotides. The first chromosome is the biggest. Note that when there are sexual chromosomes (as it is the case in mammals or birds for example), the pair of sexual chromosomes are typically considered last irrespective of their sizes.
Note that in some species the ordering has been decided and further studies have shown that the actual size order does not perfectly match the ordering. This is for example the case in humans, where chromosome 11 is actually bigger than chromosome 10 (thanks @canadianer).
Numbering is mainly just a simple naming system. It is just handy to be able to refer to a chromosome as chromosome 3 rather than having to somehow describe this chromosome for other to understand what we are talking about.
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$\begingroup$ I think they were ordered by size before the number of nucleotides in each was known. Chromosome 11 is actually bigger than chromosomes 10 and 12. $\endgroup$ Commented Jun 11, 2018 at 20:54
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$\begingroup$ Well that depends upon the species and when we got interested in their genome. It is true that for humans the ordering does not perfectly match the size order. $\endgroup$– Remi.bCommented Jun 11, 2018 at 20:55
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2$\begingroup$ Good point, I forgot there were other species out there besides us :) $\endgroup$ Commented Jun 11, 2018 at 20:58