Some traits are discontinuous: they fall into distinct classes. Other traits do not –they are called continuous traits. These traits are often measured and given a quantitative value, so they are also called quantitative traits.
Causes of quantitative traits
There are two major causes of quantitative traits: multiple genes and/or environment. When one trait is controlled by two or more genes, these are called quantitative trait loci (QTL). Each allele of each gene may contribute differently to the trait, the end results is a normal distribution of the trait.
Statistic tools for quantitative genetics studies
Usually we take the mean value and the variance for the measurement of the quantitative traits. In order to correlate a trait with a QTL, covariance, correlation coefficient and regression tools are used.
Offspring phenotype estimation and response to selection
Heritability plays an important role in estimating the offspring phenotype and calculating the response to selection. In animal breeding, it is desirable to select the better population as breeding parents. If the heritability of the trait, the population mean and the selected mean is known, the mean of offspring population can be estimated.