I'm trying to understand more about inheritance of an autosomal recessive trait that runs in my girlfriend's family. How could I start looking at this?

My girlfriend's sister, C, is affected with a recessive trait. Her parents, A and B, are confirmed unaffected carriers. Am I correct to assume my girlfriend, D, is a carrier with probability 50%? Or must I take into consideration that we know the state of C given she is affected.

Furthermore, if I (E) have a child with my girlfriend, what is the probability that our children are either carriers or affected? My carrier status is unknown.

  • 2
    $\begingroup$ Welcome to Biology SE! Although questions on human genetics are most welcome here, please note that we do not answer personal medical questions or give health advice at this site. For advice on genetic testing etc, you should consult your physician. $\endgroup$
    – Roland
    Commented Mar 1, 2017 at 18:56
  • $\begingroup$ @Roland Sure thing. More interested in the probability aspect of inheritance, and how it works in general $\endgroup$
    – ensnare
    Commented Mar 1, 2017 at 19:18

2 Answers 2


Real life is not always as simple as the paradigms you learn in high school.

However, the high school level answer is this: Since you know your girlfriend is not affected, there is a 67% chance she is a carrier, and a 33% chance she is not.

The state of the sister is irrelevant, if you are sure both parents are carriers.

Without knowing the prevalence of the recessive allele in your population, it is impossible to know the odds of you being a carrier. But if you are not, then your kid has a 33% chance of being a carrier.


Assuming the trait is caused by a single gene, she would indeed have a roughly 50% chance of being a carrier. If both parents are carriers but unaffected they will both be Aa (with a being the recessive mutation). For your girlfriend to be a carrier she will have to be Aa, of the 4 possible combinations her parents could have passed on (AA,Aa,aA,aa) two lead to this outcome, thus 1/2. If you are sure that she is unaffected the aa option is eliminated and she thus has a 2/3 chance of being a carrier.

If you are not a carrier but your girlfriend is it will be impossible for your child to express the trait (but would still have a 50% chance to be a carrier!). If you are both carriers you loop back to the original situation of her parents, your child would have a 25% chance of being affected and have a 50% chance of becoming a carrier. I must emphasize that real life genetic inheritance is more complex than these textbook calculations, chromosomal crossing over and whatnot can seriously alter these percentages.

I am not a health professional, consult your physician and/or get your girlfriend tested (if possible).

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    $\begingroup$ That calculation is only correct if the girlfriend does not know whether or not she is affected. $\endgroup$
    – swbarnes2
    Commented Mar 1, 2017 at 19:19
  • $\begingroup$ @swbarnes2 Yeah I apologize I misread carrier for affected. $\endgroup$
    – Koen vd H
    Commented Mar 1, 2017 at 19:21

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