Genetics Picture

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Sex-linked heredity

Color blindness in humans is an example of a predisposition that is a sex linked inheritance. The predisposition is recessive and placed on the x-chromosomes. To mark this we can call it Xf, and the dominant gene for normal color viewing we can call XF. It is commonly known that there are a significant amount of men who are color blind than there are color blind women. The reason for this is that a color blind woman needs to have the defect on both her x-chromosomes, otherwise the dominant gene for normal color vision would override so that she didn't show the defect. A man on the other hand only has one x-chromosome and if the gene for color blindness is situated on it then he is color blind. There is no gene for color vision on the y-chromosome that could correct the defect.

The consequence of this will then that for a woman to be color blind she must have inherited the defective gene from both her father and from her mother. The father is then color blind, since the gene cannot be hidden with men. The mother on the other hand doesn't have to be color blind; she can carry it as a hidden gene.

A color blind man must have inherited the disposition from his mother, since he has have inherited the y-chromosome from his father and hence the x-chromosome must come from his mother, and with the x-chromosome comes the defective gene.

For example: we make a mating scheme between a color blind man and a woman who carries a hidden predisposition:

____________________________________
|    |              |              |
|    | Xf           | Y            |
|____|______________|______________|
|    |              |              |
| XF | XFXf         | XFY          |
|    | girl, normal | boy, normal  |
|    | color vision | color vision |
|____|______________|______________|
|    |              |              |
| Xf | XfXf         | XfY          |
|    | girl,        | boy,         |
|    | color blind  | color blind  |
|____|______________|______________|

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