In corn, blue kernels are produced by a dominant allele of a coloration gene, and white kernels are produced in individuals homozygous for a recessive allele of the same coloration gene. Another gene has two alleles for shape, with smooth kernels being dominant to wrinkled. A plant heterozygous for both genes is testcrossed (crossed to a homozygous recessive white, wrinkled strain). The testcross offspring consist of the following types: 1,447 blue smooth; 1,436 white wrinkled; 150 blue wrinkled; 145 white smooth. Explain the inheritance of the coloration and shape traits.

This is a dihybrid cross in which two genes are involved. Usually in a dihybrid test cross involving a heterozygous plant offspring are produced in 1:1:1:1 ratio which means that all the four type of offspring are produced in almost same numbers.

In this case the four type of offspring would be blue smooth, white wrinkled, blue wrinkled and white smooth. However they are not produced in 1:1:1:1 ratio. Blue smooth and white wrinkled are produced in far greater numbers than blue wrinkled and white smooth. This phenomena is the result of linkage.

The genes for kernel colour and shape are located near to each other on the same chromosome. So they defy independent assortment and have the tendency to get inherited together. Parental type offspring are produced more than recombinants. Here, blue smooth and white wrinkled phenotypes were present in original parents hence they are produced more in number.