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I keep reading that the p arm is shorter than the q arm. But I cannot find an explanation of how much shorter nor an explanation for the difference.

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    $\begingroup$ Which chromosome in which species? $\endgroup$
    – De Novo
    Oct 16, 2018 at 2:21
  • $\begingroup$ Let's say homo sapiens, any chromosomes. $\endgroup$
    – Breck
    Oct 17, 2018 at 3:04
  • $\begingroup$ @Remi.b has answered your question. The point to my comment was that "p" just means short, and "q" just means long. Just like you can't say how much shorter a short person is than a tall person, you can't say how much shorter a p arm is than a q arm. It depends on the chromosome (person). $\endgroup$
    – De Novo
    Oct 17, 2018 at 3:41
  • $\begingroup$ I guess I'm looking for some data on distribution of size differences between the 2 arms. Is the average difference 1%? .00001%? et cetera. $\endgroup$
    – Breck
    Oct 18, 2018 at 4:09

2 Answers 2

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It varies from chromosome to chromosome and from species to species. In some cases, both arms are roughly the same length. In some cases, one arm is so short that it is almost inexistant. For chromosomes with equal length arms, of course the terms p and q do not apply.

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p and q are used to describe the relative size of the arms of a chromosome. According to this wikipedia article p stands for petite (french for small) and q is just the next letter in the alphabet.

So p is just defined as the smallest arm in any chromosome. How much smaller is not relevant for this definition. The names are just jargon which make it easy to refer to an arm.

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