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Chromosome 1 is the designation for the largest human chromosome. Humans have two copies of chromosome 1, as they do with all of the autosomes, which are the non-sex chromosomes. Chromosome 1 spans about 249 million nucleotide base pairs, which are the basic units of information for DNA. It represents about 8% of the total DNA in human cells.

Why is Chromosome 1 called Chromosome 1? Is being the largest human chromosome the only reason?

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    $\begingroup$ If they had started with the smallest, then what if one is found later that was missed because of its small size? If you start from the largest, that's less of a problem. $\endgroup$ – forest Aug 7 '19 at 3:14
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    $\begingroup$ Because biologists aren't C-language coders :-) $\endgroup$ – Carl Witthoft Aug 7 '19 at 13:27
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    $\begingroup$ @forest Or, as apparently happened, if the smallest one was found to be an artifact: until 1956 it was 'accepted as fact' that humans contained 48 chromosomes! See Can't any body count? $\endgroup$ – user1136 Aug 7 '19 at 20:20
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    $\begingroup$ @forest But it would have been much more embarrassing if they'd started with the largest and then found a larger one! (I'm looking at you, zeroth law of thermodynamics.) $\endgroup$ – David Richerby Aug 8 '19 at 17:12
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    $\begingroup$ @user1136 Is there some reason it's written "any body" (this is the actual title of the paper) or do I need to write a follow-up called "Can Anybody Spell?" $\endgroup$ – David Richerby Aug 8 '19 at 17:17
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Chromosomes were first known about from karyograms (that's the word for chromosome pictures like these) sort of like this one (1)(2):

enter image description here

The scientists looking at these chromosomes didn't know much about them at first. They were discovered before anything was understood about genes, but by 1922 it was thought they were the carriers of genes. Without much understanding of the chromosomes, and certainly no understanding of what they carried, scientists needed an easy way to order and compare them. They chose a straight forward option - size - and paired them up and then lined them up biggest to smallest. Because the sex chromosomes didn't fit into this scheme of matching pairs they were left to the end.

Hence Chromosome 1 is Chromosome 1 because it is the largest autosomal chromosome.

Notes:

  1. The actual first karyogram was of a plant, this one is of a human male.
  2. The image above is public domain, obtained from Wikipedia, and originally made by the National Human Genome Research Institute.
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    $\begingroup$ This also works for chromosomes in other species, but sometimes with complications due to interspecific phylogenetics. Chromosome 2 turned out to be two smaller primate chromosomes fused, and these half-chromosomes are now called 2A and 2B. In particular, in non-human primates this has required a renaming of chromosomes, and one that isn't as simple for them as numbering by size. $\endgroup$ – J.G. Aug 7 '19 at 7:58
  • $\begingroup$ @user1136: double checking, I see that it was the first observation of chromosomes that was in 1842 but Karyograms were presumably a bit later. Human chromosomes are not the easiest to see so the science advanced in other organisms earlier $\endgroup$ – Jack Aidley Aug 8 '19 at 6:38
  • $\begingroup$ It seems to me that most of the early 'chromosome counters' were concerned with 'how many' rather than 'taking a roll call'. One of the first to tabulate the chromosomes and to label the largest (in terms of length) 'Chromosome 1' is (Hsu, 1952). To quote from that paper (Fig 14): "Diagrammatic representation of the haploid set of human chromosomes including the sex pair... The autosomes are numbered according to their length, regardless of the position of the centromeres $\endgroup$ – user1136 Aug 12 '19 at 9:40
  • $\begingroup$ There is, of course, a glaring error in Fig. 14 of (Hsu, 1952): his diagram shows 48 chromosomes. (The definitive 'game-changing' paper giving the correct number of 46 is (Tjio & Levan, 1956) $\endgroup$ – user1136 Aug 12 '19 at 9:52
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Size is a difficult one to judge when chromosomes are similar sizes, and it was more difficult before staining of the bands.

Because of this, some chromosome naming systems group chromosomes into different categories of the centromere type (acrocentric, telocentric), so the largest do not always necessarily descend in order of size. Some genomes still use a sub-classification system, such as Feline (A1, A2, B1, B2), though it is a lot easier to treat chromosomes numerically in programs.

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  • $\begingroup$ Welcome to Biology.SE! While interesting, this doesn't really answer the OP question. ——— Please take the tour and then consult the help pages on the standards for How to Answer effectively. Thanks! 😊 $\endgroup$ – tyersome Aug 9 '19 at 16:30

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