Polygenic Inheritance

Polygenic traits are controlled by three or more genes with cumulative additive effects showing continuous bell-shaped phenotypic variation (e.g. human skin colour, height), deviating from typical Mendelian monogenic ratios, hence following a non-Mendelian inheritance pattern.

Statement I incorrectly describes multiple alleles (many alleles of a single gene); polygenic inheritance actually refers to a character governed by more than one gene (multiple genes), so Statement I is false. In polygenic inheritance, the contribution of each gene/allele is additive, producing a continuous range of phenotypes, so Statement II is true.

Human skin colour is not controlled by a single gene; it is controlled by three (or more) genes, say A, B and C.
Such traits, which are controlled by several genes, are called polygenic traits.
The dominant alleles A, B and C add to the amount of melanin, and the effect of each allele is additive.
The genotype with all dominant alleles (AABBCC) has the darkest skin colour and the one with all recessive alleles (aabbcc) has the lightest; the genotype with three dominant and three recessive alleles has an intermediate colour.
The phenotype therefore shows continuous gradation instead of sharp classes. Human height is another example.
It is not due to a point mutation, codominance or a chromosomal aberration.
So the inheritance of skin colour in humans is an example of polygenic inheritance.

In polygenic inheritance a character is controlled by three or more genes, each allele having a small additive effect, so the phenotype shows a continuous range.
Human skin colour is controlled by at least three genes (A, B and C). The more dominant alleles a person has, the more melanin is produced and the darker the skin; this is the classic example of polygenic inheritance.
Flower colour in Mirabilis jalapa (four o'clock plant) is an example of incomplete dominance, with red, pink and white flowers in a 1 : 2 : 1 ratio.
The production of male honey bees (drones) from unfertilised eggs is haplo-diploid sex determination (parthenogenesis).
Pod shape in garden pea is controlled by a single gene and shows simple Mendelian inheritance.
So the example of polygenic inheritance is skin colour in humans.