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Using Rules of Probability to Solve Genetics Problems

I'm totally lost in this subject especially when i have to solve problems using probability for trihybrid. I have of not knowing when to use the Rule of Addition and when to us the rule om Multiplication. I will try to show you what i know and maybe you can help me impron on what i already know.

R r
R(RR)(Rr)
r(rR)(rr)

In this examble I know that when Rr x Rr mates, half the gamets will recieve the "R" allele and the other 1/2 the "r" allele. So for the probabilty of "RR" to occur it would be 1/4 by rules of Multiplication. 1/2 x 1/2.

that's about how much i understand so far
i need help in
problems using addition rule
problems for trihybrids
and beyond
if you can help me with the following and maybe direct me to a good website with questions and answeres.

Solution Preview

The "AND" rule for probability: If you want to find the probability of multiple things that would occur together or at the same time. You've given a correct example of this using the Rr x Rr cross.

Here's another: What's the probability that in the Rr x Rr cross you get an individual that is homozygous recessive and male? So here, we need to find...

the probability of receiving:
an r allele (1/2) AND anohter r allele (1/2) AND being a male (1/2) = (1/2)(1/2)(1/2) = 1/8

The "OR" rule for probability: If you want to find the probability of multiple possible outcomes. For example, what's the probabililty of having a male or female offspring? The probability ...

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