Gene frequency

As explained in Section 5.2.3, p,p -DDE is much more persistent in food chains than either p,p -DDT or p,p -DDD, and dnring the 1960s when DDT was still extensively used, it was often the most abundant of the three compounds in birds and mammals found or sampled in the field. Since the widespread banning of DDT, very little of the pesticides has been released into the environment, and p,p -DDE is by far the most abnndant DDT residue found in biota. While discussing the ecological effects of DDT and related compounds, effects on population numbers will be considered before those on popnlation genetics (gene frequencies). 

Daday, H. 1954a. Gene frequencies in wild populations of Trifolium repens.. Distribution by lattitude. Heredity 8 61-78. 

This is, in essence, the modern synthesis of Darwin and Mendel achieved in the 1930s by Ronald Fisher and J. B. S. Haldane. Based on a series of relatively straightforward equations, it also took the study of evolution out of meticulously observed natural history and located it within a more abstract mathematised theory. Indeed, evolution itself came to be defined not in terms of organisms and populations, but as the rate of change of gene frequencies within any given population. One consequence has been a tendency for theoretical evolutionists to retreat further and further into abstract hypotheticals based on computer simulations, and to withdraw from that patient observation of the natural world which so characterised Darwin s own method . 

There must be ample information on the molecular genetic nature of the disorder (mode of inheritance, gene frequency, polymorphic nature of the locus, genetic heterogeneity of the disorder). 

Gene frequencies and allelic mutations for ADH vary considerably and significant interpersonal and ethnic variations have been described. Furthermore, functional ADH is a dimer, usually of identical subunits but heterodimers are also possible and to date approximately 20 ADH isoenzymes are known. There are notable kinetic differences between the various functional ADH isoenzymes for example, Km values differ almost 1000-fold and Vmax by approximately 40-fold (see Table 6.4). These differences account, to some extent, for variation in alcohol tolerance between individuals. 

An essential step in understanding genetic variation is to measure it in populations. This is done by estimating genotype and gene frequencies. 

The gene frequency measures the proportion of chromosomes that contain a specific allele. To continue the RFLP example given above, we wish to estimate the frequencies of alleles 1 and 2 in our population. Each individual with the 1-1 genotype has two copies of allele 1, and each heterozygote (1-2 genotype) has one copy of allele 1. Because each diploid somatic ceU contains... 

Genotype frequencies measure the proportion of each genotype in a population. Gene frequencies measure the proportion of chromosomes that contain a specific allele (gene). 

The same approach can be used to estimate the frequency of allele 2, which is 0.3. A convenient shortcut is to remember that the gene frequencies for all of the alleles of a given locus must add up to 1. Therefore, we can obtain the frequency of allele 2 simply by subtractir the frequency of allele 1 (0.7) from 1. 

This allele counting approach can be used for a locus that has any number of alleles. If, for example, our locus has three possible alleles, and if each genotype can be observed directly, we can still count the alleles within each genotype to obtain a gene frequency. Suppose the following genotype counts are observed ... 

If a population is large, and if individuals mate at random with respect to their genotypes at a locus, the population should be in Hardy-Weinberg equiUbrium, which means that there is a constant and predictable relationship between genotype frequencies and gene frequencies. This allows us to estimate genotype frequencies if we know gene frequencies, and vice versa. 

Because males have only one X chromosome, gene frequency estimation for X-Hnked traits differs from that of autosomal traits. Consider hemophilia A (Chapter 1), which is an X-linked recessive disease. If a male s X chromosome has a factor VIII mutation, he will have hemophilia A. If his X chromosome does not have the mutation, he will not develop the disease. Thus, the gene frequency for hemophilia A is obtained simply by counting the proportion of affected males in the population (i.e., the proportion of X chromosomes containing the mutation). Approximately one in 10,000 males has hemophilia A. Thus, the gene frequency for this disease, q, is 1/10,000. 

Females have two X chromosomes, so they must inherit two copies of the mutated chromosome to develop hemophilia A. Gene frequencies are similar in males and females, so we can use the gene frequency estimated in males to predict the genotype frequencies in females. Assuming Hardy-Weinberg equilibrium, the frequency of affected females is given by... 

On the basis of these genotype counts, what are the gene frequencies of alleles 1 and 2 ... 

Genotype fiequencies can be estiniated from gene frequencies, but the reverse is not... 

The incidence of Duchenne muscular dystrophy in North America is about 1/3,000 males. On the basis for this figure, what is the gene frequency of this X-linked recessive mutation ... 

Genotype frequencies can be estimated from gene frequencies (choice B), but gene frequencies can also be estimated from genotype frequencies (as in choice A). 

Answer A. This answer is obtained by taking the square root of the incidence (i.e., the frequency of affected homozygotes) to get a gene frequency for the disease-causing mutation (q) of 1/50 (0.02). The carrier frequency is given by 2pq, or approximately 2q, or 1/25. 

Answer A. Because males have only a single X chromosome, each affected male has one copy of the disease-causing recessive mutation. Thus, the incidence of an X-linked recessive disease in the male portion of a population is a direct estimate of the gene frequency in the population. 

The probability density P(r, t) of the position of the center of gravity of the mutant population can be roughly estimated by normalizing the mutant gene frequency ... 

Piazza, A. Menozzi, P. Cavall-Sforza, L. "The Making and Testing of Geographic Gene Frequency Maps" Dept, of Genetics, School of Medicine, Stanford University Stanford, California, 1979. 

---