For a trait contolled by two genes, these ratios will differ from the 9:3:3:1 that would be expected. These altered ratios are called modified Mendelian ratios.
Example 1: 15:1 Ratio (Duplicate Gene Action)
Phenotypes: Kernel Color in Wheat
The following pathway depicts how duplicate genes can work.
|
Gene A |
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| Enzyme A | ||
| Precursor |
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Product |
|---|---|---|
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Enzyme B |
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Gene B |
The problem is to explain the following result.
| Parental Cross |
Colored kernels (AABB) x Colorless kernels(aabb) |
|---|---|
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| F1 |
Colored Kernels |
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| F2 |
15 Colored:1 Colorless |
The following table can be used to figure out the observed F2 ratio. All that is need is a single functioning gene to obtain the needed product. Individuals that are genotypically A_B_ , A_bb, or aaB_ can produce an enzyme. Also these individuals comprise 15/16 of the F2 offspring. Only 1/16 have the genotype aabb. This is the basis for the observed 15:1 ratio. Because either of the genes can provide the wild type phenotype, this interaction is called duplicate gene action.
| Genotype | Kernel phenotype | Enzymatic activities |
|---|---|---|
| 9 A_B_ | colored kernels | functional enzymes from both genes |
| 3 A_bb | colored kernels | functional enzyme from the A gene |
| 3 aaB_ | colored kernels | functional enzyme from the B gene |
| 1 aabb | colorless kernels | non-functional enzymes produced at both genes |
