News tagged with genetic variations

Human genetic variation is the genetic diversity of humans and represents the total amount of genetic characteristics observed within the human species. Genetic differences are observed between humans at both the individual and the population level. There may be multiple variants of any given gene in the human population (alleles), leading to polymorphism. Many genes are not polymorphic, meaning that only a single allele is present in the population: that allele is then said to be fixed. No two humans are genetically identical, even monozygotic twins, who develop from a single zygote, have infrequent genetic differences due to mutations occurring during development and gene copy number variation has been observed. Differences between individuals, even closely related individuals, are the key to techniques such as genetic fingerprinting. Alleles occur at different frequencies in different human populations, with populations that are more geographically and ancestrally remote tending to differ more.

Causes of differences between individuals include the exchange of genes during meiosis and various mutational events. There are at least two reasons why genetic variation exists between populations. Natural selection may confer an adaptive advantage to individuals in a specific environment if an allele provides a competitive advantage. Alleles under selection are likely to occur only in those geographic regions where they confer an advantage. The second main cause of genetic variation is due to the high degree of neutrality of most mutations. Most mutations do not appear to have any selective effect one way or the other on the organism. The main cause is genetic drift, this is the effect of random changes in the gene pool. In humans, founder effect and past small population size (increasing the likelihood of genetic drift) may have had an important influence in neutral differences between populations. The theory that humans recently migrated out of Africa, is sometimes given as an example of this. It has been theorised that the migration out of Africa only represented a small fraction of the genetic variation in Africa and that this is a contibuting cause of the observed lower levels of diversity in all indigenous humans outside of Africa. Generally, more recent neutral polymorphisms caused by mutation are likely to be relatively geographically localised and rare, while older polymorphisms are more likely to be shared by a wider range of human groups. The large majority of observed genetic variation occurs within a population in any geographic region and not between populations in different regions, although it is still usually possible to accurately identify the geographic origins of any individual's ancestors by genetic means.

The study of human genetic variation has both evolutionary significance and medical applications. The study can help scientists understand ancient human population migrations as well as how different human groups are biologically related to one another. From a medical perspective the study of human genetic variation may be important because some disease causing alleles occur at a greater frequency in people from specific geographic regions.