Confusion over a footnote in "The Selfish Gene"

[mellowmorgan]

I got confused while reading a footnote by Dawkins in his book The Selfish Gene (which I so far thoroughly enjoy), elaborating upon why incestuous relations are highly detrimintal to offspring genetically. Here is the full note:

“A lethal gene is one that kills its possessor. A recessive lethal, like any recessive gene, doesn't exert its effect unless it is in double dose. Recessive lethals get by in the gene pool, because most individuals possessing them have only one copy and therefore never suffer the effects. Any given lethal is rare, because if it ever gets common it meets copies of itself and kills off its carriers. There could nevertheless be lots of different types of lethal, so we could still all be riddled with them. Estimates vary as to how many different ones there are lurking in the human gene pool. Some books reckon as many as two lethals, on average, per person. If a random male mates with a random female, the chances are that his lethals will not match hers and their children will not suffer. But if a brother mates with a sister, or a father with a daughter, things are ominously different. However rare my lethal recessives may be in the population at large, and however rare my sister's lethal recessives may be in the population at large, there is a disquietingly high chance that hers and mine are the same. If you do the sums, it turns out that, for every lethal recessive that I possess, if I mate with my sister one in eight of our offspring will be born dead or will die young. Incidentally, dying in adolescence is even more `lethal', genetically speaking, than dying at birth: a stillborn child doesn't waste so much of the parents’ vital time and energy. But, which ever way you look at it, close incest is not just mildly deleterious. It is potentially catastrophic. Selection for active incest--avoidance could be as strong as any selection pressure that has been measured in nature."

The bold print is where my confusion arose, and probably owes to my poor comprehension of genetics. With an autosomal recessive disorder like albinism (I'm not saying it's necessarily lethal in modern civilisation, but in the wild it is and it's the first thing I could think of), if two siblings carried the alleles heterozygously with only one recessive, then mated, wouldn't it be that 1 in 4 of their children would be albino, not one in eight? I figured this out using a Punnett square. Is this the wrong approach? Please explain this to me, and the proper calculations involved.

Thank you.

[chadn737]

Dawkins has left out some detail it seems. The most straight-forward explanation is that he assuming that the recessive lethal allele is being carried as a heterozygote in one of the parents. After all, if both a brother and sister carry the same recessive allele, then that allele was inherited from one or both of the parents. If only one of the parents possesses the recessive lethal allele, then the probability that the sister possesses that same allele is 50%. This would half the typical 1 in 4 ratio, giving you a 1 in 8 chance that mating with your sister will produce a lethal combination. If he is calculating it some other way, then he has obviously left out crucial details.

Also, as a side note, as you read the Selfish Gene....keep in mind that the central father of the entire idea is W. D. Hamilton. In no small part, also Robert Trivers, George Price, and many others. I find it rather disgraceful how these brilliant minds are often ignored for their huge contributions to evolution, with much of the credit for their work going to Dawkins for having come up with a clever analogy for their ideas.

Edited February 3, 2014 by chadn737

[mellowmorgan]

Oh, I suppose that kind of makes sense. I was assuming both had a lethal recessive allele, because that seemed necessary. When he says, "for every lethal recessive that I possess, if I mate with my sister one in eight of our offspring will be born dead or will die young" it seems like that is the assumption because he is saying one in eight of the offspring WILL have the disorder and die young. None of the offspring would have it though if one of them didn't have the lethal recessive allele. Maybe it would have been more accurate to say, "for every lethal recessive that I possess, if I mate with my sister, there is a 50% chance that one in four of our offspring willl be born dead or will die young."

I would not say Hamilton is the father to the entire idea of the selfish gene. He is the theorist responsible for first expounding upon kin selection and altrusim, both of which are essential to the selfish gene though. Also, Dawkins gives credit to these men several times and quotes them throughout his book, espiecially W. D. Hamilton. To my understanding, he helped popularize his work.

[chadn737]

Oh, I suppose that kind of makes sense. I was assuming both had a lethal recessive allele, because that seemed necessary. When he says, "for every lethal recessive that I possess, if I mate with my sister one in eight of our offspring will be born dead or will die young" it seems like that is the assumption because he is saying one in eight of the offspring WILL have the disorder and die young. None of the offspring would have it though if one of them didn't have the lethal recessive allele. Maybe it would have been more accurate to say, "for every lethal recessive that I possess, if I mate with my sister, there is a 50% chance that one in four of our offspring willl be born dead or will die young."

 

I would not say Hamilton is the father to the entire idea of the selfish gene. He is the theorist responsible for first expounding upon kin selection and altrusim, both of which are essential to the selfish gene though. Also, Dawkins gives credit to these men several times and quotes them throughout his book, espiecially W. D. Hamilton. To my understanding, he helped popularize his work.

 

Yes, the situation he describes is not at all clear. That is the simplest explanation and if he meant something else, then he has left out crucial details.

 

Dawkins does give credit, but unfortunately, his metaphor (the selfish gene) has become so popular that many outside of science credit him with the science. There is a thread on this site on the Selfish Gene that goes for 5 pages without a single mention of Hamilton. That is symptomatic of the problem, that people confuse Dawkins book as the basis.

[CharonY]

That is typical for the perception of science in the public, though. Often scientific ideas are attributed to those researchers that publicize discoveries in a concise/accessible manner. Those that did detailed work on it often do not get the the same credit outside scientific circles.

Edited February 4, 2014 by CharonY