Polygenic inheritance is the inheritance pattern of a trait controlled by two or more genes. Genes may be on the same or different chromosomes and each gene may have two or more alleles. The gene expression is influenced by a variety of factors including gender, nutrition, breed, rate of growth, and amount of exercise. These traits are quantitative traits - that is, there is a wide range within the population. Such traits include height, weight, character, working abilities, and some genetic defects.
One example is height in humans - there are a larger number of phenotypes, each differing slightly from the next and forming a graduated series. Some variation in height in humans is due to environmental factors such as diet, exercise and disease. However, if the environmental factors were constant, there would still be continuous variation in height due to such things as formation and hormone levels. The more genes that control a characteristic, the more possible gene combinations exist and the more phenotypes.
Other examples of polygenic inheritance include : skin colour in humans, colour in wheat kernels, egg weight in poultry, fleece weight in sheep. Kernal colour in wheat is determined by two gene pairs, so called polygenes that produce a range of colors from white to dark red depending on the combinations of alleles. Dark red plants are homozygous $A A B B$ and white plants are homozygous $a a b b$. When these homozygotes are crossed the $F_1$ offspring are all double heterozygotes $A a B b$. Thus crossing individuals with the phenotype extremes yield offspring that are a 'blend' of the two parents.