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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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