Probability and Punnett Squares Chapter 11.2 Biology Mr. Hines

Probability and Punnett Squares After Mendall crossed the various pea plant alleles, he had to count the offspring and their traits. He noticed that the numbers could be related to the principles of Probability. Probability – The likelyhood that a particular event will occur.

If you flip a coin, the probability that you will get heads is ½. If you want to calculate the probability of getting heads 3 times in a row, you would perform this math ½ x ½ x ½ = 1/8 This means that you have a 1 in 8 chance of flipping a coin 3 times and getting heads all 3 times. The principles of probability can be used to predict the outcomes of genetic crosses.

A Punnett Square can be used to show how the various crossed traits will combine to form offspring. A Punnett Square can also show the probability of each outcome. Important terms Homozygous – organisms have 2 identical alleles for a particular trait (TT for 2 dominant alleles or tt for 2 recessive traits.)

Heterozygous - organisms that have 2 different alleles for the same trait (Tt – one dominant and one recessive) Phenotype – physical characteristics Genotype – genetic characteristics

Example 1 Dogs with big feet and small feet. F = dominant Big feet (capital letter) f = recessive small feet (lower case) Genotype Phenotype FF Ff ff

Probability and segregation Probability and segregation is best learned by practicing with Punnet squares. In this chapter, we will do monohybrid crosses. Monohybrid cross – a cross that deals only with one trait (gene).

Example 2 Dogs with Large feet F (dominant) Dogs with small feet f (recessive) Parents genotypes Ff x Ff Draw the phenotypes of both parents Determine the probability of each trait after crossing both parents.