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In any normal human population it is almost

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In any normal human population, it is almost impossibleto meet these demanding criteria. Although large popula-tions exist, collecting sample data from a significantly largeenough segment of a population that correctly represents themembers of the population is not always feasible. Also, mat-ing is not always random, and there is mixing of populationson a global scale now that leads to “gene flow” on a constantbasis. Recently, sequencing of the human genome has re-vealed that gene mutations occur much more commonlythan originally thought. Some of these mutations affect thephenotype of an individual, such as loss of enzyme function,and some do not. Despite these drawbacks, Hardy-Weinbergis still one of the best tools for studying inheritance patternsin human populations and is a cornerstone of populationgenetics.Most of the various genes controlling the inheritance ofblood group antigens can be studied using the Hardy-Weinbergequations. A relevant example that shows how to use theHardy-Weinberg formula is the frequency of the Rh antigen,D, in a given population. In this simple example, there aretwo alleles, D and d. To determine the frequency of eachallele, we count the number of individuals who have the cor-responding phenotype (remembering that both Dd and DDwill appear as Rh-positive) and divide this number by thetotal number of alleles. This value is represented by p in theHardy-Weinberg equation. Again, counting the alleles lets usdetermine the value of q. When p and q are added, they mustequal 1. The ratio of homozygotes and heterozygotes is de-termined using the other form of the Hardy-Weinberg equa-tion, p2+ 2pq + q2= 1. If in our example we tested 1,000random blood donors for the D antigen and found thatChapter 2Basic Genetics29ParentalRRYYRYGametesFirst-FilialSecond-FilialRYrryyryRYryWhere R = roundr = wrinkledY = yellowy = greenRyrYryRyrYRYRRYyRrYYRRYYRrYyRyRRyyRrYyRRYyRryyrYRrYyrrYYRrYYrrYyryRryyrrYyRrYyrryyFigure 2–3.A schematic illustration of Mendel’s law of independent assortmentusing seed types.Figure 2–4.Common inheritance patterns.pqp2pqpqpqq2p2+ 2pq + q2= 1.0The population studied must be large.Mating among all individuals must be random.Mutations must not occur in parents or offspring.There must be no migration, differential fertility, or mortality ofgenotypes studied.BOX 2–1Criteria for Use of the Hardy-Weinberg Formula
DD and Dd (Rh-positive) occurred in 84 percent of the pop-ulation, and dd (Rh-negative) occurred in 16 percent, thegene frequency calculations would be performed as follows:pgene frequency of Dqgene frequency of dp2DD, 2pqDd, which combined are 0.84q2dd, which is 0.16qsquare root of 0.16, which is 0.4pq1p1qp10.4p0.6This example is for a two-allele system only. A three-allelesystem would require use of the expanded binomial equationpq + r1 or p22pq2prq22qrr21. Morecomplex examples using this formula can be found in moreadvanced genetics textbooks.

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