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The molecular basis of variation and inheritance: key words
- The human genome varies by about 0.5% between individuals. This variation arises from single nucleotide polymorphisms and from insertions, deletions, inversions and duplications of longer sequences.
- Patterns of sequence variation between populations provide information about human origins and the selective pressures that humans have experienced.
- Only a small proportion of sequence variation causes detectable phenotypic change.
- Although monogenic diseases such as cystic fibrosis tend to be rare, several mechanisms account for the fact that selection has not altogether eliminated the deleterious alleles from the population: recurrent mutation, delayed onset after reproductive age, and heterozygote advantage.
- Genetic susceptibility to common diseases such as type 2 diabetes probably arises from the combined effects of numerous alleles contributing to risk, each having small effects, coupled with environmental and developmental factors.
- Not all inheritance is genetic. Disease risk can be transmitted by cultural or behavioural factors, and there is increasing evidence for trans-generational persistence of epigenetic modifications of DNA and gene expression.
Bottlenecks and founder effects
A population bottleneck occurs when a previously larger population is reduced in number by some event – for example disease or famine. Even if the population size recovers, the survivors will have only a small proportion of the genetic diversity present in the original population. In a similar way, a founder effect occurs when a small part of a larger population becomes genetically isolated, perhaps by migration or by some geographical event such as rising sea level. The isolated subpopulation will contain only a fraction of the diversity present in the main population.