2011-11_Heri - EVOLUTION (11:704-486) - HERITABLE VARIATION...

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EVOLUTION (11:704-486) - HERITABLE VARIATION SMOUSE - SPRING 2011 1 H ERITABLE V ARIATION Mendelian Genetics: You’ve had a course in Genetics, and I’m going to assume that you know simple genetics. If you do not, you should read Chapter 9 carefully, and/or review your genetics. By the late 1800s, chromosomes and their meiotic and mitotic mechanics had been discovered; Mendle’s genes had a place to live and Mendel’s laws (1865) were finally ‘discovered’ (again). It was immediately obvious that genes were the stuff of heredity, which is why “like begets like.” Given genes, we no longer needed the inheritance of acquired characteristics . The idea of nervous fluid had been defined in more precise terms that made chromosomal sense. Let’s review Mendelian genetics just briefly. Imagine a gene with two alleles, say smooth and wrinkled for Mendel’s peas, with S (smooth) dominant to s (wrinkled). We have three genotypes, SS, Ss and ss, and we can make crosses, among which are the following: SS x SS SS x Ss SS x ss SS ½ SS : ½ Ss Ss all smooth all smooth all smooth There are other crosses you could make, so let’s do that. How about? Ss x Ss Ss x ss ss x ss The point is that “descent with modification” now makes perfect sense. Genes are carried across generations, and everything behaves nicely. Darwin had the right observations but the wrong mechanism. Mendelian genetics filled the gap, so he had the punch line right. Sometimes you just plain get lucky! Or maybe he and Mendel were both just smart. Of course, some loci are codominant, rather than being dominant/recessive. Take flower color, for example, and imagine a gene coding for RR (red), Rr (pink) or rr (white) flowers. Work on the following three crosses for a little practice (just to refresh your memory): RR x r r Rr x Rr r r x r r
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EVOLUTION (11:704-486) - HERITABLE VARIATION SMOUSE - SPRING 2011 2 And then, there is the business of sex-linkage. This one was historically important, because Morgan used the sex-linked red/white eye color in Drosophila melanogaster to prove that genes were actually on chromosomes. Let’s refresh our memory on those crosses: red eyes white eyes red eyes ( X r X r x X w Y ) parental ( X w X w x X r Y ) fem male fem male Wherever the X-chromosome goes, the eye color allele goes with it. Hardy-Weinberg Equilibrium: It wasn’t clear how genes and alleles would respond to natural selection. It seemed to most folks as though you would always end up with a 3 : 1 ratio, since crossing Ss x Ss would yield 3 : 1 smooth : wrinkled. Thinking wasn’t very clear on this point. Two people, Hardy and Weinberg, independently worked out the connection between allele frequencies and genotype frequencies in populations. Let’s start with the codominant flower color example, because it’s a little easier to follow. We’ll start the population with 500 individuals, in the following numbers (I made these up for convenience): red pink white # (RR) = 20 # (Rr) = 160
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2011-11_Heri - EVOLUTION (11:704-486) - HERITABLE VARIATION...

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