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Colour Blind Awareness Day 2017
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Real Family Tree

The CH/CA/AW family is a real family which demonstrates how chance can determine the fate of future generations.

At the start of this family tree there are no colour blind genes in the AW family but the wives of both the CA and CH families are colour blind carriers (XX).

In the CA family, Mrs CA is a colour blind carrier and Mr CA is not colour blind. They have 3 children, two girls and one boy. None are colour blind.

The eldest daughter marries the youngest AW son and they have a son and a daughter. Their son is colour blind but their daughter is not. This tells us young Mr AW is not colour blind but his wife is a carrier.

Their daughter may be a carrier or she may not, time will tell! If their daughter marries a non-colour blind man and she IS a carrier, she will have a 50% chance of having a colour blind son or a carrier daughter. If she is NOT a carrier the line will die out with her. The line could continue through her brother, BUT ONLY if he has daughters (because he can’t pass on his colour blind gene to his sons).

If their son marries a non colour blind woman, all of his daughters will be carriers but none of his sons will be either colour blind or colour blind carriers.

Meanwhile the younger CA sister marries the eldest son of the CH family. Mr and Mrs CH have had 3 children – one colour blind son, one non-colour blind son and a colour blind carrier daughter. The younger CA sister and her husband have 4 sons, but amazingly none of them are colour blind. The younger CA sister may or may not be a colour blind carrier. Each of the boys would have taken their Y chromosome from their father (he gives them the Y to make them male but cannot give them the colour blind gene) and so each had a 50% chance of getting the X chromosome from their mother (if she is a carrier). So, it would appear that the younger colour blind sister is probably not a colour blind carrier, but she could be and her sons have just been lucky to have escaped the X chromosome. Nevertheless the colour blind line dies out here on the male side of the CH family.

However, the CH daughter has married a non colour blind man and they have had two boys, both of whom are colour blind, so the line will continue through the CH grandsons, if they have daughters of their own.