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 ...
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 quantitative characters 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: What is the meaning of Allele, Plant, Organ, Environment, Character?