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I am reading the book "Molecular biology of the Cell, 6th edition" and in chapter one page 18 the following figure is included

enter image description here

I am presuming this figure represents all genes within the Bacillus subtilis, which makes me wonder: why are there no families with 20-37 genes? There is a group for 4-19, and one for 38-77. What happened to all sizes in between?

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    $\begingroup$ You would need to read the original paper on which this figure is based. Do you have access to Nature? If not I can send you a reprint, but I don’t think this is a question about biology, but one of arbitrary data selection. $\endgroup$ – David Dec 21 '20 at 20:22
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    $\begingroup$ @David that may be the case, but to the OP that distinction cannot be easily known. So although an answer to this post might in fact point out arbitrary data selectivity as the cause, I think the question is otherwise biological in nature and therefore OK. $\endgroup$ – theforestecologist Dec 21 '20 at 20:32
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Based on the paper (open access) from which this figure was adapted, the 38-77 group consists of 5 gene families with 38, 47, 57, 64 or 77 members. In contrast, there are 284 “families” with 2 members, 91 with 3 members, 42 with 4 members, 25 with 5 members, etc. This is clearer in the original figure:

enter image description here

Thus the larger gene families are outliers and there simply exists no gene families in B. subtilis with 20-37 genes (at least by the methodology of this paper).

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    $\begingroup$ Very useful to see the original image and now that I know that there are also gaps within the groups given by my textbook the single gap between 19 and 38 seems a lot less extraordinary $\endgroup$ – Martin - Reinstate Monica Dec 21 '20 at 21:14

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