Chromosomal Problems Affect Nearly All Human Embryos: Discovery May Explain Low Fertility Rates In Humans:

For the first time, scientists have shown that chromosomal abnormalities are present in more than 90% of IVF embryos, even those produced by young, fertile couples. Ms Evelyne Vanneste, a PhD student in the Centre for Human Genetics and the University Fertility Center, Leuven University, Belgium, told the 25th annual conference of the European Society of Human Reproduction and Embryology on July 1st, that the surprising finding meant that current techniques used in preimplantation genetic screening (PGS), where embryos are screened genetically in order to select the best embryo for transfer, do nothing to improve pregnancy and live birth rates. Indeed, it can lead to potentially viable embryos being discarded, she said.
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"Although in vitro culture conditions are known to have a limited influence on the rate of chromosomal imbalances in IVF/ICSI embryos, it is probable that the chromosome instability observed in vitro also occurs in spontaneous pregnancies since, at most, 30% of human conceptions result in a live birth and more than 50% of spontaneous abortions carry chromosomal aberrations. The high rate of chromosomal abnormalities is almost certainly responsible for the low fecundity of humans compared with other mammals," she added.

The exact proportion of fertilizations which end in spontaneous abortion (or lack of implantation) seems rather sketchy from what I can tell, but it's very high. In The Cooperative Gene Mark Ridley suggests that the very high rates of spontaneous abortion among humans is one reason he is not particularly worried about increased genetic load, a prime concern of W. D. Hamilton. I assume he believes that the proportion of spontaneous abortions would simply increase for individuals who have a high mutational load. Hamilton worried about the fact that many more humans lived to reproduce than would have in the past, so that natural selection was no longer operative. Ridley is suggesting that actually the power of selection may simply be transferred to the gestational stage.

One interesting idea would be to see if different populations have different rates of spontaneous abortion. How one would get a precise measure of this, I don't know.

Labels: Genetics