The a2 and b2 alleles are null (non-functional) alleles, contributing 0 units of color to wheat kernels. Hence the correct option is a.
Null alleles, such as a2 and b2 in the context of wheat kernel color, indicate non-functional or significantly impaired versions of the genes responsible for coloration. These alleles are often associated with mutations that result in the loss of the gene's normal function. In the additive trait system for wheat kernel color, where the a1 and b1 alleles add 1 unit of color, the null alleles a2 and b2 are functionally inactive, leading to the absence of color contribution (0 units).
Understanding null alleles is crucial in genetic analysis, as they help elucidate the impact of specific gene variants on phenotypic traits. In this case, the null alleles a2 and b2 reveal a lack of color expression, contrasting with the functional alleles a1 and b1. This information is valuable for researchers studying the genetic basis of wheat kernel color and can be applied to breeding programs aimed at manipulating color characteristics in wheat varieties. Hence the correct option is a.