Red-green colour blindness
Red-green colour blindness is inherited in exactly the same way as haemophilia. This time run the cross with the father affected instead of the mother carrying, because the result is the one people find surprising.
Red-green colour blindness
A condition in which red and green are hard to tell apart. It is caused by a recessive allele on the X chromosome, written Xc, and is much more common in men than in women.
An affected father and an unaffected mother
Take a man who is colour blind, XcY, and a woman who is homozygous for normal vision, XCXC. Both her ova are the same, so both rows of the grid repeat.
| X CX C × X cY | Xc | Y |
| XC | XCXc | XCY |
| XC | XCXc | XCY |
Every ovum carries XC; the father contributes either his Xc or his Y.
Not one affected child
- Every daughter is XCXc — normal vision, but a carrier. She received her father's only X, so she has his allele whether he liked it or not.
- Every son is XCY — normal vision and not a carrier, because he received his father's Y and his mother's XC.
The allele travels sideways before it reappears
None of this man's children is colour blind, and none of his sons even carries the allele. But every daughter carries it, and each of her sons then has a one-in-two chance of being colour blind. The condition surfaces in the grandsons, through the daughters — which is exactly the skipping pattern of activity 20.1.
Figure to be added
A difference in vision, not an illness
People with red-green colour blindness see perfectly well; they distinguish certain colours differently. It matters for a few jobs that depend on colour signals, and for very little else. As with the rare characteristics in the first skill, inherited and unusual does not mean unwell.