Download Quantitative Traits

Quantitative Traits
Quantitative traits are determined by many genes spread across numerous chromosomes. The alleles of
quantitative genes are additive. So it is possible to have many combinations of the additive traits. What
results is a continuous range of variation. Traits which are controlled by genes that fall within the realm
of dominance-recessive, incomplete dominance, etc produce just a few categories of phenotypes (e.g.,
smooth vs. wrinkled). Quantitative traits can be depicted in a histogram. As the number of additive
genes which influence a trait increase so does the number of phenotypic classes. The histogram will
resemble a smooth bell-shaped curve if enough phenotypic classes exist.
Thus, quantitative traits lend themselves to descriptive statistics (measures of central tendency such as
the mean, median, and mode; measures of dispersion such as variance, standard deviation and
standard error of the mean).
Do not think of quantitative traits like dominate-recessive traits when constructing a Punnett Square.
Quantitative traits consist of alleles which add to the phenotype (A) and those that do not (a). AA adds
two doses, Aa one dose and aa no doses. A Punnett Square for a single gene trait is shown below.
Next we will cover a trait determined by two additive genes (A and B). For each capital letter in the
Punnett Square a “dose” of gene product influences the quantitative trait. The two corners of the
Punnett Square represent the extremes, 4 doses and 0 doses. For a 2 gene trait 1/16th of progeny will
have either of the two extreme phenotypes. The 2-gene AaBb cross below has five phenotype classes.
The number of distinct phenotypes is predictable and computed as classes = 2n +1 where n = the
number of additive genes.
The proportion expressing either extreme genotype can also be computed according to the formula:
Ratio or proportion = 1/4n where n is the number of genes. The role of number of genes on ratio of
extreme phenotypes and number of phenotypic classes is summarized in the table below.
Quantitative geneticists work to separate genotypic variance in a population from variation caused by
the environment and make estimates of heritability.