Continuous and discontinuous variation Genes in population

Discontinuous Variation The phenotypes we have consider so far have been due to two segregating alleles of single genes. This means that alternative phenotypes are clearly different. This variation or difference between phenotypes is known as discontinuous variation.

Continuous Variation Many phenotypes show continuous variation – there are many intermediate forms and the distribution of the phenotypes in a population is a bell curve. Traits that show continuous variation include: height and weight in humans, milk production in cows and the size of flowers. Continuous traits are usually polygenic, with alleles of many different genes contributing to produce the final phenotype. This results in considerable phenotypic variation, and further variation may also occur due to interactions with the environment.

Variation in Phenotype Variation in phenotype in populations is influenced by the following factors: One gene can have many alleles Meiosis involves independent assortment and genetic recombination of alleles Dominance may not be complete Several genes can affect the one trait One gene can determine more than one trait The expression of genes can be affected by other genes and by the environment

Genes in populations Different people in same community have different risks for disease. The reasons for this are a complex mix of genetic, environmental and social risk factors. Epidemiology is the description and analysis of the pattern of diseases in the population, the causes of these different patterns, and the use of this information to improve public health. We know that some diseases, both rare and common, seem to ‘run in families’. Genetic epidemiology attempts to determine the size of genetic influences on disease.

Heterozygote Advantage Some extremely serious and often fatal genetic diseases continue to be present in the population but why???? If individuals who are homozygous for such a disease normally die before reproducing, we would expect natural selection to lead to low frequencies for the disease allele in the population. This is not always the case – take the sickle cell anaemia gene and the cystic fibrosis gene as examples.

Heterozygote Advantage: Sickle Cell Anaemia High frequency of the mutant allele in people of African descent Homozygotes suffer from life-threatening anaemia The explanation for the unexpected frequency is that being heterozygous for this allele is advantageous in areas where malaria is present. The mutant allele results in defective haemoglobin molecules alter the red blood cells so that they are less susceptible to infection by the malarial parasite.

Heterozygote Advantage: Cystic Fibrosis High frequency of the mutant allele in people of Caucasian background (1 in 28 Caucasians are heterozygotes) Mutant copy of CFTR gene leads to defective channel proteins being produced in the cell membrane The bacteria Salmonella typhi requires the channel protein in order to get across the intestine wall This means that heterozygotes are less susceptible to typhoid fever The gene frequencies currently observed are a result of previous heterozygote advantage when typhoid fever was widespread in Europe

What is the gene pool The genetic information in an individual is a genotype. The genetic information in a population is a gene pool. A gene pool is described in terms of the allele frequencies (proportions) of each gene. When genotypes of all members of a population are known, allele frequencies may be calculated directly from genotype frequencies. When allele frequencies are not known, they may be estimated starting from the frequency of the homozygous recessive phenotype.