Lecture 4- Mendelian genetics

Lecture 4- Mendelian genetics - Mendelian Genetics: Linkage...

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Mendelian Genetics: Linkage and Continuous Variation To review, Mendel proposed a simple theory of exquisite predictive power that explains the rules governing heredity in terms of chromosomes and genes. The problem is, Mendel still has some explaining to do. First of all, our theory has explained the exception, discrete traits, not the rule, continuous traits. Second, in order for all genes to segregate, they must all be on different chromosomes. But there are only 23 pairs of human homologs. Does that mean there are only 23 genes? Ever notice how baldness runs in families yet only men go bald. There are several traits like this that run in families but predominantly affect males called sex-linked traits. Let's look closely at one example: color blindness. Consider; a normal woman and a normal man who produce some color blind children. What will the ratios be? Colorblindness must be recessive since it turns up in children of unaffected parents (think F1 x F1 cross). Both parents must be heterozygous or you wouldn't see it at all (think test cross). If sex and colorblindness followed the law of independant assortment, you should get the following ratios: 1/2M x 1/4c = 1/8Mc, 3/8MC; 1/8Fc, 3/8FC . But what you actually see is 1/2FC 1/4MC 1/4Mc .
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Among the progeny of non-affected parents, only the males are ever color blind. How can we explain this? Suppose if in the male, the recessive color blindness allele were always on the same chromosome as, and therefore attached to, the dominant maleness allele (i.e. on the Y chromosome). Then it makes sense. Indeed, if you cross a colorblind man with a normal homozygous woman, all children are normal but females will be heterozygous (also called carriers in the context of human genetics). And if a rare, colorblind woman is crossed with a normal man, all male children are colorblind and all female are heterozygous. So here is a case where independent assortment does not apply. The genes are linked because they are on the same chromosome. Now, you might ask, how come the colorblindness allele on the Y chromosome is always recessive? The reason is that the Y is small and doesn't have a gene for colorblindness. How can the Y have a recessive allele for a gene that doesn't exist? The absence of a gene
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Lecture 4- Mendelian genetics - Mendelian Genetics: Linkage...

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