MadSci Network: Genetics |

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Hi Carrie,
Thanks for the question. It's actually rather complicated to answer; if
you search the MadSci archives you'll find several answers about the
genetics of red hair. For this answer I'm going to assume that red hair
is a recessive trait. It's not as simple as this, but let's assume it for
now.

You probably know that we have 2 copies of each gene. Let's call the gene
for red hair color "r" and let's say it comes in 2 forms, recessive ("r")
and dominant ("R"). To have red hair, you need 2 copies of the recessive
form (rr). Any other combination (RR, Rr or rR) won't give you red hair.
Now, you say that your grandfather had red hair, so he was 'rr'. This
means that your mother must have inherited one 'r' copy from him (but your
mother isn't red-headed, so must have inherited 'R' from her mother). So
your mother is 'Rr'. Your father is also not red-headed, so he must be
either 'RR' or 'Rr'.

Let's look at the first case, where your father is 'RR':

father | ||

mother | R | R |

R | RR | RR |

r | rR | rR |

In this case, you have a 2/4 (or 1/2) chance of carrying the recessive copy. Your child has a 2/8 (or 1/4) chance of inheriting it from you and a 1/1 (100%) chance of inheriting from your partner, who must be 'rr'. So in this case the chance of your child having red hair is (chance he inherits from you times chance he inherits from your partner) = 1/4 x 1 = 1/4.

Now the second case, where your father is 'Rr':

father | ||

mother | R | r |

R | RR | Rr |

r | rR | rr |

Now you have a 3/4 chance of carrying the recessive copy, and a 1/4 chance of being redheaded yourself. But we can discount the 'rr' case for you, as you are not redheaded. This would make you 'RR', 'Rr' or 'rR', giving your child a 2/6 (or 1/3) chance of receiving the 'r' copy from you. With the 100% chance of getting 'r' from your partner, this raises the odds for your child slightly, to 1/3.

So we might say your child has a 1/3 to 1/4 chance of being redheaded and
you might want to look into your father's side of the family for
redheads. Note also that your first cousin doesn't really enter the
equation; he or she must have inherited a recessive copy from your
mother's brother/sister, who in turn got it from your grandparent, but the
other copy came from the partner of your mother's brother/sister.

Now for the caveats; red hair inheritance is actually rather complex and
not necessarily a simple recessive/dominant trait. Also the simple
Mendelian genetics discussed above assumes that the copies can separate
and recombine entirely randomly, which isn't necessarily so. But
basically, your mother must have at least some "red-headed trait" thanks
to your grandfather, giving you a good chance of having some too, backed
up by the certainty that your partner does, so I'd say the odds are good,
especially if you have 3 or more children!

Neil

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