Answer:
a. polygenic inheritance
Explanation:
There are some phenotypes which have a range of variability rather than having discreet differences. Such phenotypes exist due to polygenic inheritance. Polygenic inheritance is also known as quantitative inheritance. Such inheritance is controlled by more than one gene and each contributing allele has little but equal effect. In short, there is additive/cumulative impact of many alleles.
The frequency distribution of such traits exhibits a bell shaped curve.
For example, human skin color is a polygenic trait which shows a long range of variability. The skin color has color range like negro, very dark, dark, agouti, fair, very fair and albino with negro (darkest) being produced by additive effect of 6 dominant alleles (AABBCC) which produce huge amount of melanin while albino (lightest) is produced if there are all recessive alleles (aabbcc) which produce extremely low amount of melanin. All other intermediate colors are produced by a combination of dominant and recessive alleles like AaBbcc, AabbCc etc.
