Genetics Picture

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Polygenes

Sometimes two or more genes can contribute to one specific characteristic. This has been studied for example on wheat. Wheat seed has as a rule a red hue, but even pure white seed exist. When crossing between a stable red seed variety and a white one, one only gets red seed. In the next generation the white seeds surface again. This depends on that wheat have two different genes for the color of the seeds, which are placed in one chromosome each (inherited hence independently of each other). The first crossing can be written as R1R1 R2R2 (red) x r1r1 r2r2 (white), which the gives R1r1 R2r2 (red). The dominance by R1 and R2 is incomplete, and due to this the effect of the genes can be added. This results in that the red colored wheat seeds can have different hues of red. Crossing between two of the genotype R1r1 R2r2 will give the following crossing table and colors:

________________________________________________________________________________
|      |                 |                 |                 |                 |
|      | R1R2            | R1r2            | r1R2            | r1r2            |
|______|_________________|_________________|_________________|_________________|
|      |                 |                 |                 |                 |
| R1R2 | R1R1 R2R2       | R1R1 R2r2       | R1r1 R2R2       | R1r1 R2r2       |
|      | dark red        | medium-dark red | medium-dark red | medium red      |
|______|_________________|_________________|_________________|_________________|
|      |                 |                 |                 |                 |
| R1r2 | R1R1 R2r2       | R1R1 r2r2       | R1r1 R2r2       | R1r1 r2r2       |
|      | medium-dark red | medium red      | medium red      | light red       |
|______|_________________|_________________|_________________|_________________|
|      |                 |                 |                 |                 |
| r1R2 | R1r1 R2R2       | R1r1 R2r2       | r1r1 R2R2       | r1r1 R2r2       |
|      | medium-dark red | medium red      | medium red      | light red       |
|______|_________________|_________________|_________________|_________________|
|      |                 |                 |                 |                 |
| r1r2 | R1r1 R2r2       | R1r1 r2r2       | r1r1 R2r2       | r1r1 r2r2       |
|      | medium red      | light red       | light red       | white           |
|______|_________________|_________________|_________________|_________________|

We then get a scale of five different nuances while only having two pair of genes involved. When it comes to for example the size of a cat, length of fur or the amount of undercoat, there certainly are many more genes involved. The result will be a continuous scale between two extremes.

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