popgen - Population Genetics: From previous lecture you...

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Population Genetics: From previous lecture you know that Sickle Cell anemia and Thalassemia, both genetic diseases that prevent normal globin production occurs more often in some populations than others. For both of these conditions, which are recessive lethals, heterozygotes are more reproductive in areas of the world where malaria is a problem than either homozygous class. What we are going to do in the next two lectures is to take a look at this situation in a more formal way. This branch of genetics is usually called POPULATION GENETICS . To begin, let's narrow the field a great deal as to the type of genes and situations we will consider to simplify the concepts. 1) we will look only at one gene at a time; 2) we will only consider the cases where there are only two alleles A vs a, or B' vs B; 3) we will only look at Mendelian situations, that is genes in diploid organisms that reproduce sexually; .not in bacteria, viruses etc. A "population" is a group of potentially interbreeding individuals; it may be a local population such as the mice in one barn or it could be as large as an entire species. Gene frequency vs genotypic frequency. Genotypic frequencies are the actual frequency of each genotype; example; a population has 0.81 AA; 0.18 Aa: 0.01 aa (this is the same as 81% AA, 18% As and 1% aa) These are actual frequencies of each genotype. Standard usage in population genetics uses the term gene frequency for what is actually allele frequency. For example if f(A) = 0.9 and f(a) is 0.1 these are referred to as the gene frequencies. By convention f(A)= p and f(a) = q Since we are only looking at cases with two alleles p + q + 1 Further, many of the gene frequencies we deal with may not be genes at all. They may be frequencies of chromosomal aberrations, RFLPs or
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PCR products of an unknown gene; anything that produces a detectable polymorphism (difference) in the DNA or gene products in individuals in the population will be included when we talk about gene frequencies. Lets calculate gene frequencies for a couple of examples, starting with an easy case where we can differentiate all the genotypes. The MN blood types of 100 people are determined to be: 50 I M I M : 20 I M I N : 30I N I N What are the frequencies of the M and N alleles in this population of 100 people? f(M) = p =50 X 2 (each M person has 2 I M alleles) + 20 X 1 (each heterozygote has 1 I M allele) divided by 100 X 2 total alleles in this population =120/200 =0.6 then q = 1-p = 0.4 is the frequency of the I N allele. An alternate way of calculating p is f(MM) + 1/2 f(MN) = 0.5 + 1/2 X 0.2
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This note was uploaded on 04/29/2008 for the course GENE 310 taught by Professor Magill during the Spring '08 term at Texas A&M.

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popgen - Population Genetics: From previous lecture you...

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