Description of Quantitative Traits

Genetic and Environmental Effects

Quantitative Traits Statistics

Variance Components


Estimating Offspring Phenotype

Predicting Selection Response

Study Questions

Quantitative Genetics Overheads

Quantitative Genetics WWW Links

Genetic Topics

Description of Quantitative Traits

All of the traits that we have studied to date fall into a few distinct classes. These classes can be used to predict the genotypes of the individuals. For example, if we cross a tall and short pea plant and look at F2 plants, we know the genotype of s hort plants, and we can give a generalized genotype for the tall plant phenotype. Furthermore, if we know the genotype we could predict the phenotype of the plant. These type of phenotypes are called discontinuous traits.

Other traits do not fall into discrete classes. Rather, when a segregating population is analyzed, a continuous distribution of phenotypes is found. An example, is ear length in corn. Black Mexican Sweet corn has short ears, whereas Tom Thumb popcorn has long ears. When these two inbred lines are crossed, the length of the F1 ears are intermediate to the two parents. Furthermore, when the F1 plants are intermated, the distribution of ear length in the F2 ranges from the short ear Black Mexican Sweet size to the Tom Thumb popcorn size. The distribution resembles the bell-shaped curve for a normal distribution.

These types of traits are called continuous traits and cannot be analyzed in the same manner as discontinuous traits. Because continuous traits are often measured and given a quantitative value, they are often referred to as quantitative traits, and the area of genetics that studies their mode of inheritance is called quantitative genetics.

Many important agricultural traits such as crop yield, weight gain in animals, fat content of meat are quantitative traits, and much of the pioneering research into the modes of inheritance of these traits was performed by agricultural geneticists. Many human phenotypes such as IQ, learning ability and blood pressure also are quantitative traits. These traits are controlled by multiple genes, each segregating according to Mendel's laws. These traits can also be affected by the environment to varying degrees.

The following are examples of quantitative traits that we are concerned with in our daily life.

  • Crop Yield
  • Some Plant Disease Resistances
  • Weight Gain in Animals
  • Fat Content of Meat
  • IQ
  • Learning Ability
  • Blood Pressure

Here are some images of quantitative traits in plants:

This image demonstrates the variation for flower diameter, number of flower parts and the color of the flower Gaillaridia pilchella. Each trait is controlled by a number of genes and is a quantitative trait.

The two photographs above demonstrate variability for Indian Paintbrush flower color. The parents in the left photo are either yellow or reddish orange. The F2 individuals though show a distribution of colors from yellow to reddish orange. This range o f phenotypes is typcial of quantitative traits. This should be compared to flower color of Mendel's peas where the F2 individuals were either purple or white, the two parental phenotypes.

Copyright © 1997. Phillip McClean