Population genetics

population genetics, gene pool, gene frequency

1. Population Genetics Ikram Ullah M.Phil MLSc

2. Population Genetics Genetic change on an individual level takes the form of mutations and polymorphisms Genetic change occurs in populations in the form of allele ( gene variant ) frequencies, which provide a different type of information than that yielded from DNA sequences Changing allele frequencies also underlie evolution, which is the basis of all biology

3. Population : A population is any group of members of the same species in a given geographical area who are potentially capable of mating and producing fertile offspring Population Genetics: Population genetics is a branch of genetics that considers all the alleles in a population, which constitute the gene pool The “pool ” in gene pool refers to a collection of gametes, and an offspring represents two gametes from the pool. Alleles can move between populations when individuals migrate and mate . This movement, termed gene flow, underlies evolution

4. Population genetics enables us to trace our beginnings as well as understand our diversity today , and even predict the future Thinking about genes at the population level begins by considering frequencies—that is, how often a particular gene variant occurs in a particular population Such frequencies can be calculated for alleles, genotypes, or phenotypes For example , an allele frequency for the cystic fibrosis (CF) gene might be the number of Δ F 508 alleles among the residents of San Francisco

5. Δ F 508 is the most common allele that, when homozygous, causes the disorder The allele frequency derives from the two Δ F 508 alleles in each person with CF, plus those carried in heterozygotes, as a proportion of all alleles for that gene in the gene pool of San Francisco Genotype frequency  in a population is the number of individuals with a given  genotype  divided by the total number of individuals in the population The genotype frequencies are the proportions of heterozygotes and the two types of homozygotes in the population.

6. Finally , a phenotypic frequency is simply the percentage of people in the population who have CF (or who do not ). With multiple alleles for a single gene, the situation becomes more complex because there are many more phenotypes and genotypes to consider Phenotypic frequencies are determined empirically— that is, by observing how common a condition or trait is in a population. These figures have value in genetic counseling in estimating the risk that a particular inherited disorder will occur in an individual when there is no family history of the illness

7. Microevolution On a broader level, shifting allele frequencies in populations reflect small steps of genetic change, called microevolution These small, step-by-step changes underlie evolution

8. Genotype frequencies can change when any of the following conditions are met: 1. Individuals of one genotype are more likely to produce offspring with each other than with those of other genotypes ( nonrandom mating ). 2. Individuals migrate between populations . 3. Reproductively isolated small groups form within or separate from a larger population ( genetic drift ). 4. Mutation introduces new alleles into a population . 5. People with a particular genotype are more likely to produce viable, fertile offspring under a specific environmental condition than individuals with other genotypes ( natural selection ).

9. Genetic Drift Genetic drift  is a change in the frequency of an allele within a population over time. A population of rabbits can have brown fur and white fur with brown fur being the dominant allele. By random chance, the offspring may all be brown and this could reduce or eliminate the allele for white fur

10. In today’s world, all of these conditions, except mutation, are quite common Therefore, genetic equilibrium—when allele frequencies are not changing—is rare When enough microevolutionary changes accumulate to keep two fertile organisms of opposite sex in a population from successfully producing fertile offspring together, macroevolution, or the formation of a new species, has occurred C ertain allele frequencies stay constant, a condition called Hardy-Weinberg equilibrium