One of the first Mendelian traits identified in humans was a dominant condition known as brachydactyly. This gene causes an abnormal shortening of the fingers or toes (or both). At the time, some researchers thought that the dominant trait would spread until 75 percent of the population would be affected (because the phenotypic ratio of dominant to recessive is 3 : 1). Show that the reasoning was incorrect.

Key Concepts

Mendelian Inheritance

Mendelian inheritance refers to the principles of heredity established by Gregor Mendel, which include the concepts of dominant and recessive traits. In a typical Mendelian trait, dominant alleles mask the expression of recessive alleles, leading to a phenotypic ratio of 3:1 in a monohybrid cross. However, this ratio applies to a specific scenario of random mating and does not account for factors like population dynamics or selective pressures.

Population Genetics

Population genetics studies the distribution and change in frequency of alleles within populations. It considers factors such as genetic drift, gene flow, and natural selection, which can significantly influence allele frequencies over time. The assumption that a dominant trait will reach 75% prevalence ignores these dynamics, as not all individuals will reproduce equally, and environmental factors can affect survival and reproduction.

Phenotypic Expression and Fitness

Phenotypic expression refers to the observable traits of an organism, which can be influenced by both genetic and environmental factors. The fitness of a trait determines its success in a given environment, affecting its prevalence in the population. If the dominant trait, such as brachydactyly, negatively impacts an individual's fitness or reproductive success, it may not spread as anticipated, contradicting the simplistic 3:1 ratio assumption.