Genetics & Probabilities

Applying Laws of Probability While a Punnett square is great it really isn’t practical when looking at multiple traits Math is better alternative and all you need to do is apply probability The probability scale ranges from 0 to 1 0 indicates certainty that an event is not going to occur 1 indicates certainty that an event will occur

The Multiplication Rule Used to determine the probability of random events occurring together in a specific combination

The Addition Rule Used to determine the probability that any one of at least two mutually exclusive events will occur Using the previous example… The probability of the offspring being heterozygous uses the addition rule ¼ + ¼ = ½

A Simple Example Consider a cross between two individuals with YyRr for seed color & shape Look at each character independently to determine the probability of getting any particular genotype YY=1/4, Yy=1/2, yy=1/4 RR=1/4, Rr=1/2, rr=1/4 Now we can determine the probability of getting any genotype in F2 generation by using multiplication YYRR=1/4 * 1/4=1/16 YyRR= 1/2 * 1/4=1/8

A Harder Example Now let’s look at three characters! (flower color, seed color, & seed shape) If we cross PpYyRr with Ppyyrr what are the chances the offspring will have the recessive phenotype for at least two of the three characters? First, which genotypes would work? ppyyRr, ppYyrr, Ppyyrr, PPyyrr, ppyyrr Now determine the probability of each genotype using multiplication Then add together the different probabilities of the mutually exclusive events Answer: 6/16 or 3/8!