A heritable feature in a population that varies continuously as a result of environmental influences and the additive effect of two or more genes (polygenic inheritance).
quaternary structure ...
(c) Typically s can vary over a significant range controlled by their underlying genetics
(d) For example, height and skin color (in humans) are s ...
Quantitative characters are those that vary along a continuum within a population.
For example, plant height in a wildflower population ranges from short to tall.
While Darwin was wrestling with pangenesis Galton was pioneering the analysis of the inheritance of s and he documented with extraordinary insight the properties of such inheritance.
 Completely homozygous genotypes are precisely repeatable and hence heritability of quantitative characters is increased leading to improved selection efficiency.
See also: Recessive, Sequence, Long, Allele, Environment