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I found somewhere on the Internet that, if parents are both Rh-positive, their children have 93.75% chance of being RH-positive and 6.25% chance of being RH-negative.

My question is: How is this calculated?

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There are four possible parental genotype combinations (R = Rh+ allele, r = Rh- allele):

m  f
RR RR
Rr RR
RR Rr
Rr Rr

Only the last can produce Rh- offspring. The probabilty that the Rr Rr combination leads to a rr genotype in the F1 generation is $1 \over 4$. Since there are four combinations, the probability that the Rh+ parents are Rh Rh is also $1\over4$. Thus the probability of the offspring to be Rh- is $${1\over 4} \cdot {1 \over 4} = {1 \over 16} = 0.0625$$ Thus, the probability of Rh+ in the F1 generation is $$1 - {1 \over 16} = {15 \over 16} = 0.9375$$

Note: This is the rationale behind the values you state in your question. I am pretty sure that these four combinations are not equally distributed in a population. You would have to need to know the allele frequencies.

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