10
$\begingroup$

While chromosome 19 only is the 19th largest autosomal chromosome, it contains 1440 protein-coding genes, and thus has the second highest number of protein-coding genes of any human chromosome.

For comparison: If one would naively assume the same density of protein-coding genes as on chr1, which has the highest absolute number of protein coding genes (2109), the anticipation for chr19 would be ~540 protein-coding genes.

The x-axis of this graph highlights the increased density of protein-coding genes on chromosome 19; The x-axis highlights the increased density of genes on Chromosome 19.

$\endgroup$
  • 6
    $\begingroup$ I don't think there is any answer to "why". $\endgroup$ – WYSIWYG Jul 18 '16 at 11:19
  • 1
    $\begingroup$ Within any natural population there are going to be outliers. The distribution graph you show has a long tail, which is to be expected. Look for graphs on the distribution of height, for example, and you'll see that 90% of the values are within a fairly small range, but the 5% on either side stretch out quite far, as there are extreme outliers on both ends. $\endgroup$ – MattDMo Jul 18 '16 at 13:31
  • 2
    $\begingroup$ As extreme outliers in natural populations often are somehow special, and as a n of over 1000 genes would strongly argue against as many independent chance events, I was curious if there is something special about chr 19 or its genes, which could favor such a comparatively high gene density. $\endgroup$ – tsttst Jul 18 '16 at 14:02
  • 1
    $\begingroup$ I think that OP is correct that there is something to explain here (i.e., the standard null models for gene distribution across the genome would not predict such an outlier), but I think that nobody knows the answer. $\endgroup$ – Daniel Weissman Jul 21 '16 at 16:15
  • 1
    $\begingroup$ @WYSIWYG I think "why" in this case translates to "what events led to". $\endgroup$ – James Jul 27 '16 at 4:25
11
+50
$\begingroup$

This Nature paper from 2004, by Jane Grimwood et al. goes at least a long way towards giving an answer to the question of the OP. In short: there were inordinately many duplications, especially during an event 30-40 million years ago, as well as during a much more recent event. These duplications are, uncharacteristically, predominantly intra-chromosomal rather than inter-chromosomal. Also, chromosome 19 contains a lot of immunoglobin-like paralogues: a type of gene for which it is clearly evolutionarily adaptive to undergo rapid duplication followed by random mutation, as they play a role in adapting to potential antigens.

$\endgroup$
3
$\begingroup$

It's interesting that not only the leader 19, but also 16 and 17 follow a similar trend. Perhaps their size could be the best weight/length proportion to ensure a safe replication? Then what would have to be explained would be 18, so far to the left. That could be if 18 is newer, resulting from the split of a larger chromosome or the fusion of two smaller, having yet no time to accumulate a greater number of genes favoured by the advantages of the "genes positional co-evolution" (if such thing exists).

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.