0.3 Genetics in clasical understanding  (Page 6/6)

This figure, which can be thought of as either

• the chance that both maternal and paternal alleles at one locus are identical by descent or
• the proportion of all the individual's genes that are homozygous because of identity by common descent,

is known as the coefficient of inbreeding and is usually given the symbol F.

To see a compact and also clear description of using statiistics for pedigree genetic analysis, click MITOPENCOURSEWARE ( PDF ).

Lecture 33. population genetics

If genetics is a science studying structure, function and movement rules of genes, population genetics is the third part of it: a science studying movement rules of gene carriers – chromosomes and their effects and consequences. At least from the genetic point of view, population is a unit of evolution. In terms of breeding practice, populations are plant varieties and animal breeds.

D. S. Falconer (The quote is from Introduction to Quantitative Genetics by D. S. Falconer, 1960, Ronald Press.) wrote:

"A population in the genetic sense, is not just a group of individuals, but a breeding group; and the genetics of a population is concerned not only with the genetic constitution of the individuals but also with the transmission of the genes from one generation to the next. In the transmission the genotypes of the parents are broken down and a new set of genotypes is constituted in the progeny, from the genes transmitted in the gametes. The genes carried by the population thus have continuity from generation to generation, but the genotypes in which they appear do not. The genetic constitution of a population, referring to the genes it carries, is described by the array of gene frequencies, that is by specification of the alleles present at every locus and the numbers or proportions of the different alleles at each locus." (page 6).

In fact population genetics is studying the allele frequency distribution and change under the influence of the four evolutionary forces: natural selection , genetic drift , mutation and gene flow . It also takes account of population subdivision and population structure in space. As such, it attempts to explain such phenomena as adaptation and speciation . Population genetics was a vital ingredient in the modern evolutionary synthesis whose primary founders were Sewall Wright , J. B. S. Haldane and R. A. Fisher , they also laid the foundations for the related discipline of quantitative genetics .

For humans the applications of Mendelian genetics, chromosomal abnormalities, and multifactorial inheritance to medical practice are quite evident. Physicians work mostly with patients and families. However, as important as the work of physicians may be, genes also affect populations, and in the long run their effects in populations have a far more important impact on medicine than the relatively few families each physician may serve. It is important that certain polymorphisms are maintained so that the species may survive, even at the expense of individuals. Genetic polymorphisms often are detrimental to the homozygote, but they allow others of the species to survive. Before medical intervention was possible, populations that lacked the sickle cell anemia allele could not survive in the malaria regions of West Africa. Those that had the sickle cell anemia allele survived, and the gene remains in the population at high frequency today, even though the homozygous recessive phenotype was at a severe disadvantage in the past. The high rate of thalassemia in people of Mediterranean origin, the high rate of sickle cell anemia in people of West African descent, the high rate of cystic fibrosis in people from Western Europe, and the high rate of Tay-Sachs disease in ethnic groups from Eastern Europe may all owe their origin to environmental factors that cause changes in gene frequencies in large populations by giving some advantage to heterozygotes who carry a deleterious allele. Although one may never use the calculations of population genetics in medical practice, the underlying principles should be understood.

To have general understanding of population genetics, click ( PDF ) for studying Hardy-Weinberg Equilibrium; click ( PDF ) to see the role of Mutation and Selection in population structure, and click ( PDF ) for consideration of Inbreeding as a factor influencing the composition of populations.