MadSci Network: Genetics

Re: how long do you carry recessive traits?

Date: Sat Oct 7 21:36:14 2000
Posted By: Layne Johnson, Undergraduate
Area of science: Genetics
ID: 967395471.Ge


Recessive traits can be passed on for a very long time. In fact, there's no 
limit to the length of time they can be passed down. But the chance of 
receiving one decreases each generation. Let me explain.

Each of your parents contribute one eye color gene to you. If both genes 
are blue, you have blue eyes. If both genes are brown, you have brown eyes. 
But if one gene is blue and one gene is brown, you have brown eyes (since 
brown genes are dominant) and a recessive blue gene. When you have a child, 
you pass on one of your two genes to that child. If you have blue eyes, you 
can only pass on a blue gene since both your genes are blue. Likewise, you 
can only pass on a brown gene if both your genes are brown. But if you are 
brown eyed with a blue recessive gene, your child has a 50/50 chance of 
inheriting either gene from you.

I'm actually a very good test case. My mother is Native American, and blue 
recessive genes among Native Americans are extremely rare. My father's 
father has brown eyes, and his mother has blue eyes. My father has brown 
eyes, but he must have received a blue recessive gene from his mother. So 
the father-mother combination I have could have been either blue-brown or 
brown-brown. I married my blue eyed sweetheart, so our children's father 
mother combination could either be brown-blue or blue-blue, depending on 
which color gene my father gave to me. Our three children all have their 
mother's blue eyes. (Unfortunately, they all have their dad's gigantic 
nose, but let's just stick to the question at hand.)

I must have received a recessive blue gene from my father, and passed it to 
my three children. If my wife had been brown eyed with no blue recessive 
gene, my children would have been brown eyed too. But they would still 
carry the blue recessive gene they inherited from me. And they could pass 
that blue gene to their children, and it could continue generation after 
generation. But there's a 50% chance each generation that the blue 
recessive gene won't get passed on.

A blue recessive gene introduced into a pure brown gene population could 
get passed down, but the chances diminish each generation, like this -

Generation 1: 50%
Generation 2: 25%
Generation 3: 12.5%
Generation 4: 6.25%
Generation 5: 3.125%
Generation 6: 1.5625%

By the time of the seventh generation, the chance is less than 1%. But the 
chance still exists.

I hope this answers your question,

Layne Johnson

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