2.6 Predicting Ratios in Genetic Crosses – Rules of Probability

So far, in our discussion of Mendel’s Laws, we have mentioned various (predicted) ratios in offspring produced from monohybrid and dihybrid crosses. A predicted ratio simply indicates the probability of a particular outcome (genotype or phenotype) we should expect in a genetic cross.  As such, Mendel’s results have been shown to reflect the basic rules of Probability. In genetics, we use Probability (the likelihood of the occurrence of a particular event) to predict the outcome of a genetic cross. The following two rules of Probability are very useful in conducting genetic crosses:

1. Multiplication or Product rule: The product rule of probability can be applied to the phenomenon of the independent transmission of traits. It states that the probability of two independent events occurring together can be calculated by multiplying the individual probabilities of each event occurring alone.

For example, the Probability of event A occurring AND event B occurring is = P (A and B) = P(A) x P(B)

The word “and” indicates you should apply the product rule. 

2. Addition or sum rule: The sum rule is applied when considering two mutually-exclusive outcomes that can result from more than one pathway. It states that the probability of the occurrence of one event or the other, of two mutually-exclusive events, is the sum of their individual probabilities.

For example, the Probability of event A occurring OR event B occurring is = P(A or B) = P(A) + P(B)

The word “or” indicates that you should apply the sum rule. 

Try the following question to test your understanding!

Take a look at the video, Probability in Genetics: Multiplication and Addition Rules, by Bozeman Science (2011) on YouTube, which explains these two rules of probability further.


Bozeman Science. (2011, December 13). Genetics: Multiplication and addition rules [Video file]. YouTube. https://www.youtube.com/watch?v=y4Ne9DXk_Jc


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Introduction to Genetics Copyright © 2023 by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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