Explain how the inheritance patterns of many traits (such as petal color in snapdragons, or skin color in humans) cannot be accounted for by Mendelian genetics.

Petal color in snapdragon is an example of incomplete dominance and deviates from Mendel's law of dominance.

Skin color in humans exhibits a continuous variation and is regulated by more than one genetic loci.

Explanation:

Two alleles of a gene do not always exhibit the dominant-recessive inheritance as shown by Mendel. The alleles controlling the petal color in Snapdragon exhibit incomplete dominance since the allele "R" is not able to produce enough pigment in heterozygous condition to completely mask the effect of the recessive allele "r" Therefore, the heterozygous genotype "Rr" gives pink color to petals.

Mendel showed that each genetic trait is regulated by one gene and exhibit discontinuous variations. This is not followed by skin color in humans which is a polygenic trait with multiple genes regulating it. Here, a range of phenotype is present which is regulated by the sum total of the dominant alleles of all the genes involved controlling the melanin production.