Xref: utzoo sci.research:637 talk.politics.misc:21615 sci.bio:1822 Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!gatech!ncsuvx!ncsugn!emigh From: emigh@ncsugn.ncsu.edu (Ted H. Emigh) Newsgroups: sci.research,talk.politics.misc,sci.bio Subject: Re: Gene Pool Summary: Long detailed explanation of population genetics Message-ID: <5579@ncsugn.ncsu.edu> Date: 7 Feb 89 15:16:13 GMT References: <674@intvax.UUCP> <1252@meccsd.MECC.MN.ORG> <1254@meccsd.MECC.MN.ORG> <10120@ut-emx.UUCP> <23378@beta.lanl.gov> Reply-To: emigh@ncsugn.UUCP (Ted H. Emigh) Distribution: na Organization: Genetics, North Carolina State University, Raleigh, NC Lines: 65 In article <23378@beta.lanl.gov> dd@beta.lanl.gov (Dan Davison) writes: >In article <10120@ut-emx.UUCP>, ethan@ut-emx.UUCP (Ethan Tecumseh Vishniac) writes: >> [...] >> How fast this affects the gene pool adversely depends to some >> extent on the rate at which such mutations appear. [...] >> I'm personally not very worried about our gene pool. [...] > >Quite right. I recall that in my freshman biology course ('73) >we were offered the chance to figure this out. With a "deleterious" >gene present in a population at a specific frequency (less than 20% but >greater than 5%, as I recall), how long would it take for that gene >to go to fixation ("permanently") part of the population. I don't >recall the rest of the details. Those worried about "contamination" >of the gene pool are cordially invited to go to their local library >look it up, and calculate it. > >The upshot was that the gene's frequency never went above 20%. > >Ted or Henry may have more current data, but the last three times >I got interested in the subject and did the calculations I found >that there was basically nothing to worry about. How can I pass up an invitation (unless Dan is talking about some other Ted)? Let's consider the standard "genetic disease." Some trait which is currently deleterious (to some extent) and currently has a fairly low incidence in the population. Examples are: Tay-Sachs (frequency of <10^-3 => or an incidence of <10^-6); Ichthyosis congenita (freq.=10^-3; inc.=10^-6); Xeroderma pigmentosum (freq=2x10^-3; inc.=4x10^-6); or even albinism (freq=3x10-3; inc=10^-5). The frequency is the frequency of the gene in the US Caucasian population, the incidence is the rate at which these RECESSIVE traits appear. If the genetic trait is completely neutral, then it would be spread through the generation through mutation alone. However, it would take an unbelievably long time. A trait that has a mutation rate of 10^-5 would take about 10,000 GENERATIONS to increase in frequency from current frequencies to about 10% (which would lead to an incidence of people with the trait of about 1%). Note the 10,000 generations is about 200,000 years, which is older than Homo sapiens. If the genetic trait had some selective disadvantage, no matter how slight, eventually the population would reach an equilibrium. If the trait were completely recessive, then the equilibrium gene frequency would be sqrt(u/s), where u is the mutation rate and s is the selective disadvantage of the recessive individuals (a number between 0 and 1). If we take a modest selection coefficient of 1/100 and a mutation rate of 10^-5, then the equilibrium gene frequency is .03 and the rate of incidence is .001. You can work out for yourselves the gene frequency for other selection coefficients. A more reasonable selective disadvantage for these traits AFTER MEDICAL SCIENCE HAS ELIMINATED DIFFERENTIAL MORTALITY may be more like 1/10 or higher. That would lead to an equilibrium gene frequency of .01 and a rate of incidence of .0001. It should be pointed out that the selection coefficient is NOT merely whether individuals survive to reproduce. One way of looking at it is to define the selection coefficient as the ratio of the expected family size of a newborn individual with the genetic trait to the expected family size of a newborn individual without the trait. This not only includes survival, but also fertility, etc. I refer you to Steve VanDevender's comments to understand how there is selection against genetic traits even if we can use medical science to exclude mortality. -- Ted H. Emigh, Dept. Genetics and Statistics, NCSU, Raleigh, NC uucp: mcnc!ncsuvx!ncsugn!emigh internet: emigh@ncsugn.ncsu.edu BITNET: emigh%ncsugn@MCNC.UUCP or emigh%ncsugn@ncsuvx.ncsu.edu