Pharyngula

Life will find a way

Creationists sometimes try to argue that what we consider straightforward, well-demonstrated cytological and genetic events don’t and can’t occur: that you can’t get chromosome rearrangements, or that variations in chromosome number and organization are obstacles to evolution, making discussions of synteny, or the rearrangement of chromosomal material in evolution, an impossibility. These are absurd conclusions, of course—we see evidence of chromosomal variation in people all the time.

For example, A friend sent along (yes, Virginia, there is a secret network of evilutionists busily sharing information with one another) a remarkable case study of a radical chromosome arrangement in a mother and daughter. When you see how these chromosomes are scrambled, you’ll wonder how they ever managed to sort themselves out meiotically to produce viable offspring…but life will find a way.

First, here’s a partial karyotype to show the affected chromosomes 6, 9, 11, and 20. These are four pairs of chromosomes, and in a normal karyotype, each member of the pair should be basically identical to the other; here you can see that they’re of different lengths, and even the pattern of bands shows some variations that are difficult to make out.

i-2bc639a31b5487e85571421cbe09f365-cfcr_karyo.jpg
(a, b) Partial high-resolution GBG-banded karyotypes showing the rearranged chromosomes 6, 9, 11 and 20 of mother (a) and daughter (b).

After a fair amount of work, the investigators sorted out what was wrong. There were 12 breakpoints in these four chromosomes, and the fragments had been scrambled about to form these rearranged chromosomes, color-coded to help sort out who is who.

i-22c6badd690fd54c89b1c72f0271a2c2-cfcr_chrom.jpg
Summary of all breakpoints after completion of all mapping efforts. The chromosome fragments are drawn to scale.

This is amazing stuff. This is what cells do, though: when chromosomes are damaged, there are repair enzymes that struggle to put them back together. These enzymes are not smart or guided in any way, so they just do the best job they can…and sometimes that means they are reassembled so they can function, but it may not be the usual order of things.

Now the mother in this pair had the above arrangement of chromosome bits. The father’s chromosome set was normal (as was that of the woman’s parents), except that he did have a small deletion (again, these kinds of minor variations in chromosomes are common). In meiosis, when the woman produces haploid eggs, one step requires that chromosomes pair up—both orange chromosomes line up together, as do both blue ones, both green ones, and both yellow ones. Then they separate in an orderly fashion to guarantee that each egg receives exactly one orange chromosome, one blue, one green, and one yellow.

Umm, wait a minute…both orange chromosomes? In this woman, the orange chromosome is scattered in fragments across four chromosomes. How could this woman’s cells arrange and sort out their contents in a well-distributed way?

Well, one way is for the chromosomes to contort themselves into strange yoga positions that allow most of the color-coded bits to pair up appropriately. Here, for example, is the most likely solution that allows the majority of the chromosomal homologs to pair up in meiosis. That’s impressive.

i-e7630f5cd4042729aa3c9f86bfaa371b-cfcr_tetravalent.jpg
(b–d) Putative pachytene configurations in meiosis. (b) One
possible pachytene configuration could consist of two tetravalents.

Even more impressive is a truly maximal pairing that allows all of the homologous portions to be in register, a structure called an octavalent that brings all 4 pairs of chromosomes together in a very specific tangle. This is an optimal arrangement for pairing, but is less likely to have occurred simply because getting that many chromosomes into an ideal arrangement is difficult.

i-5150ec534f586e9d8712fc49e26fd007-cfrc_octavalent.jpg
(c) Possible octavalent pachytene figure allowing more regions to
synapse. (d) An alternative octavalent figure allowing even more
regions to synapse.

The real test of whether this can happen, of course, is that the woman successfully reproduced. It was difficult, and she had three miscarriages first—probably a result of occasions when her chromosomes did not sort out properly, and so her egg had an unbalanced arrangement—but on the fourth try she had a healthy daughter. The daughter had some developmental abnormalities, developmental delays, some retardation (although she did graduate from school), and later was diagnosed with diabetes. These problems are not a direct consequence of the chromosome arrangement, however, since the mother had the same arrangement without the developmental problems. Both mother and daughter shared one unusual trait, the prolonged expression of fetal hemoglobin, which could be correlated with one of the breakpoints.

This is an extreme example of a scrambled chromosome; lesser variations, like single translocations or fusions, are far less problematic.


Fauth C, Gribble SM, Porter KM, Codina-Pascual M, Ng BL, Kraus J, Uhrig S, Leifheit J, Haaf T, Fiegler H, Carter NP, Speicher MR (2006) Micro-array analyses decipher exceptional complex familial chromosomal rearrangement.
Hum Genet 119(1-2):145-53.

Comments

  1. #1 Paper Han
    July 14, 2008

    Fascinating! Thank you very much. I have often wondered how descendant species could have different numbers of chromosomes, since it would seemingly be a barrier to reproduction. Now I begin to understand. I knew there must be a solution, since it obviously happened. :-)