Population Genetics & Single-gene Diseases (Mendelian Disorders)

A collection of key vocabulary terms and definitions related to population genetics and Mendelian disorders.

Locus

The unique chromosomal location that defines the position of a gene in DNA.

Genotype

The combination of alleles that a person possesses at a single locus.

Allele Frequency

The frequency with which alleles of a particular gene are present in a population.

Recessive Allele

An allele that only shows its effect if an individual has two copies of it.

Dominant Allele

An allele that shows its effect even if an individual only has one copy.

Homozygous

A person is homozygous if both alleles at a locus are the same.

Heterozygous

A person is heterozygous if the alleles at a locus are different.

Recurrence Risk

The chance that an inherited disease present in a family will recur in other persons from that family.

Horizontal transmission

An inheritance pattern where the disorder shows up in siblings but not in the parents.

Hardy-Weinberg Equilibrium

A principle that describes the relationship between allele frequencies and genotype frequencies in a population.

Genetic Drift

Random changes -fluctuations  in allele frequencies from generation to generation, particularly significant in small populations via chance. 

Genetic bottleneck - develops when a large population undergoes a drastic but transient reduction in numbers. The population may recover but its genetic diversity has been reduced. 

Founder effect - when a small number of individuals form a small population in an isolated area, the gene pool of all individuals in the next gerenations is derived from those of founders. 

Balancing Polymorphism (balancing selection)

The concept where the loss of a mutant allele is balanced by the loss of a wild-type allele due to selection.

Concept of balancing selection (balanced polymorphism) is defined as: the loss of a mutant allele (aa) due to selection, is balanced by the loss of the wild-type allele (AA) due to selection of a different kind. At the same time the carriers of the mutant allele have a higher fitness than normal individuals. Both the homozygous wildtype and homozygous mutant individuals are subjected to selected by malaria and anemia. Heterozygote advantage (Aa) survived more

Founder Effect

A phenomenon where small population founding leads to reduced genetic diversity.

When a small number of individuals form a small population in an isolated area, the gene pool of all individuals in the next gerenatiosn is derived from those of the founders

Natural Selection

The process where individuals with favorable traits are more likely to survive and reproduce.

Genetic Bottleneck

develops when a large population undergoes a drastic but transient reduction in numbers. The population may recover but its genetic diversity has been reduced.