Bio171-F10-Lec 18 - Biology 171 Friday, October 22, 2010...

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Biology 171 Friday, October 22, 2010 Lecture 18: Complex Patterns of Inheritance Announcements Next week in Discussion: Genetics problems -Exam II – Wed. Oct. 26 -covers lectures 10-19 & Discussions 3-5 Text Reading Lecture 18: 4 th : Chapter 13 (243-252) 3 rd : Chapter 13 (281-289) Lecture 19: 4 th : Chapter 25 (435-440, 443-448, 450-452) 3 rd : Chapter 25 (503-508, 511-515, 518-520) Some Applications for Human Genetics Malaria and Balanced Polymorphisms T.H. Morgan - Autosomes & Sex Chromos. Sex-linked Genes Genetic Recombination & Linkage Maps Other Exceptions to Mendelian Inheritance 1
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Applications of Mendel’s principles of inheritance to humans 29
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Human Pedigree Analysis for Trivial Traits Squares = Males; Circles = Females 30
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Unfortunately, the Mendelian inheritance of some human traits is far from trivial………. . Some alleles present in human populations are seriously deleterious. They may be dominant or recessive. If the deleterious effect is realized prior to sexual maturity (lowered Ftness), selection is expected to purge them from populations. However in some special cases, recessive deleterious alleles may persist at higher levels through a phenomenon called Balanced Polymorphism 31
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Balanced Polymorphism If natural selection eliminates individuals with detrimental phenotypes from a population, then why do harmful mutant alleles persist in a gene pool? A genetic disease can remain prevalent when heterozygotes have some other advantage over individuals who have two copies of the normal allele. When carriers have advantages that allow a detrimental allele to persist in a population, balanced polymorphism is at work. This form of polymorphism often entails heterozygosity for an inherited illness that protects against an infectious disease. www.pbs.org/wgbh/evolution/educators/course/session7/explain_b_pop1.html 32
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Evolutionary impact of Malaria on human hosts Summary page on evolution of sickle cell anemia in areas of endemic malaria: http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html 33
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For parents who each carry* the sickle cell trait, the chance that their child will also have the trait -- and be resistant to malaria -- is 50 percent. There is a 25 percent chance that the child will have neither sickle cell anemia nor the trait which enables immunity to malaria. Finally, the chances that their child will have two copies of the gene, and therefore sickle cell anemia, is also 25 percent. This situation is a stark example of genetic compromise, or an evolutionary "trade-off.” *are heterozygous: carry one sickle-cell allele but do not develop the disease phenotype. http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html SCA - An Example of a Balanced Polymorphism 34
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Other Putative Human Balanced Polymorphisms PKU Phenylketonuria is an inborn error of metabolism in which a missing enzyme causes the amino acid phenylalanine to build up. Women who are PKU carriers have a much lower than average incidence of miscarriage. One
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Bio171-F10-Lec 18 - Biology 171 Friday, October 22, 2010...

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