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Home > Population Genetics & Statistics > Population Theory > Hardy-Weinberg Principle > Inbreeding

Inbreeding is the mating of related individuals and can alter gene frequency in a population. When related individuals mate, the child can inherit identical copies of the gene through both parental lineages, resulting in an increase in homozygosity at any given locus.04

The degree of a relationship between two persons can be measured using the inbreeding coefficient, F. With inbreeding, the expected heterozygosity is reduced by a fraction, F, and that of homozygotes are increased (NRC).

Inbreeding Coefficient







First cousins



Population genetics studies show some substructure within racial groups. Mating tends to occur between persons who are likely to share some common ancestry. Allele frequencies have not yet been homogenized because people tend to mate within these subgroups (NRC). If there is subdivision within a population, it will lead to decreased heterozygosity within that subgroup. Population substructure may exist, and can be adjusted for with the use of a correction factor, theta (θ). (NRC). This will be covered in more detail in Subject 7, Module 2.

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Inbreeding can allow recessive alleles to become homozygous; therefore, unusual recessive diseases are much more common among the children. Inbred populations deviate from expected Hardy-Weinberg frequencies and there is a greater chance that certain alleles can become fixed within the population. When an allele becomes fixed in a population, all individuals are homozygous at this locus because no other alleles exist.

Click here to watch a video on inbreeding presented by Greggory LaBerge.

EVC Syndrome

The Amish population of Lancaster County, Pennsylvania, is an example of an inbreeding group of individuals. As a result, the Amish suffer from a variety of genetic disorders including Ellis - van Creveld (EVC) syndrome, a disease caused by inheritance of two mutated copies of the EVC gene. Symptoms of the disease include short-limbed dwarfism with polydactyly (additional fingers or toes), bone malformations in the wrist, heart defects, and prenatal eruption of the teeth.14

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© 2007 NFSTC Science Serving Justice®
NOTE TO USERS: The President’s DNA Initiative DNA Analyst Training program and assessment were completed and published in 2005, in cooperation with the National Institute of Justice. The science and techniques in the program are sound and proven, however, program content has not been updated to include tools and technologies developed and in use after 2005, including many kits and robots. Assessment questions address only content delivered in this program and may not contain the full range of tools in use in your laboratory.