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Like many other medical conditions, obesity is the result of an interplay between environmental and genetic factors. [2] [3] Studies have identified variants in several genes that may contribute to weight gain and body fat distribution; although, only in a few cases are genes the primary cause of obesity. [4] [5]
The extreme endpoint of this distribution are the so-called 'monogenic' obesities where most of the impact on body weight can be tied to a mutation in a single gene that runs in a single family. The classic example of such a genetic effect is the presence of mutations in the leptin gene. [29]
Over 200 genes affect weight by determining activity level, food preferences, body type, and metabolism. [36] Having two copies of the allele called FTO increases the likelihood of both obesity and diabetes. [37] As such, obesity is a major feature of a number of rare genetic conditions that often present in childhood:
The primacy of population thinking: the genetic diversity carried in natural populations is a key factor in evolution. The strength of natural selection in the wild was greater than expected; the effect of ecological factors such as niche occupation and the significance of barriers to gene flow are all important.
Population genetics is the branch of evolutionary biology responsible for investigating processes that cause changes in allele and genotype frequencies in populations based upon Mendelian inheritance. [3] Four different forces can influence the frequencies: natural selection, mutation, gene flow (migration), and genetic drift. A population can ...
Genetic load may increase when deleterious mutations, migration, inbreeding, or outcrossing lower mean fitness. Genetic load may also increase when beneficial mutations increase the maximum fitness against which other mutations are compared; this is known as the substitutional load or cost of selection .
Set point theory does not on its own explain why body mass index for humans, measured as a proxy for fat, tends to change with increasing age or why obesity levels in a population vary depending on socioeconomic or environmental factors (or why weight tends to change for an individual when socioeconomic status and environment change). [4]
Genetic effects are broadly divided into two categories: additive and non-additive. Additive genetic effects occur where expression of more than one gene contributes to phenotype (or where alleles of a heterozygous gene both contribute), and the phenotypic expression of these gene(s) can be said to be the sum of these contributions.