What's your blood type?
The Rh-antigens classify people into Rh-positive or Rh-negative groups, and each of us acquires a pair of Rh-controlling genes from our parents (one from father and one from mother).
The Rh-negative individuals have 2 Rh-negative genes presented in their DNA, resulting in no Rh-antigens on the red blood cells. On the other hand, the Rh-positive gene is dominant over the negative one: if one or two Rh-positive genes are presented, that person will be Rh-positive. The persons with 1 Rh-positive gene & 1 negative gene are called carriers, for they can pass on the Rh-negative gene despite being Rh-positive. A carrier parent has a 50:50 chance of giving his or her child a Rh-positive or negative gene.
If both parents are such carriers, what is the chance that their child will be a carrier, too?
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1 solution
In genetics, this is a very common question about autosomal recessive genes, where the affected babies (Rh-negative in this case) acquire the affecting genes from each of their parents. From the diagram above, we can see that a carrier parent can either give his or her child a Rh-positive or negative gene, each having a chance of 50%.
With 2 choices of Rh+ and Rh- from both parents, we come up with 4 different permutations: Rh+Rh+, Rh+Rh-, Rh-Rh+, and Rh-Rh-.
Since a carrier has 1 Rh-positive and 1 negative gene, 2 out of 4 outcomes fit the criteria.
Thus, the chance of bearing a carrier child = 2/4 = 50%.
Bonus: the chance of bearing a Rh-negative child = 1/4 = 25% (in case you're wondering).