You discover that the number of kernels per cob in a type of corn varies continu

Question

You discover that the number of kernels per cob in a type of corn varies continuously in a wild population and appears to be controlled by equal and additive traits. You take true-breeding corn with few kernels per cob (variety A) and true breeding varieties with many kernels per cob (variety B), breed them to create an F1 with an intermediate number of kernels, and then interbreed the F1 plants to generate an F2 population. You find that of the 1028 F2 plants you examine, none have as few kernels per cob as variety A, nor do any have as many as variety B. Based on this, how many genes do you estimate are involved in determining this trait?

The answer is 6 or more. But why?

Explanation / Answer

Number of kernels per cob is an example of polygenic inheritance. The F1 generation had cobs with intermediate number of kernels. Interbreeding of F1 generation. The number of genes involved in trait can be calculated by a simple formula – 2n*2. Here, n stands for type of cross. In a monohybrid cross n = 1, dihybrid n = 2. When 5 genes are involved; 2(5*2) = 1024,

And for 6 genes, 2(6*2) = 212 = 4096.

Since the plants examined in F2 had 1028 (< 1024) such plants which did not resemble either of the variety, atleast 6 genes or more are involved in determining this trait.

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