11.2 Probability and Punnett Squares

Probability

• The likelihood that a particular event will occur is probability

Probability• Example: flipping a coin– 50% chance you will get heads, 50% chance you will

get tails– Also called a 1 in 2 chance (1/2)

Probability• IMPORTANT: Each event does not effect the

event before or the event after– You have EXACTLY the same probability each time

Probability of Many Events

• Multiply the probabilities for each flip.

• Example: if you want to see what your probability is to get heads 3 times in a row, you would calculate (½) x (½) x (½) = 1/8

Probability and Genetics

• The principles of probability can be used to predict the outcomes of genetic crosses.

• Example: predict the offspring of a cross between two individuals.

Probability Activity• State the question and hypothesis.• Answer the pre-activity questions together as a class.• Flip a coin 100 times.• Record how many times the coin landed on heads and how

many times the coin landed on tails.• Calculate the frequency (totals) of heads and tails.• Calculate the percentage of heads and tails.• Answer the post-lab questions.

Probability Worksheet

• Things to remember:–One event does NOT effect a later event–Probability of multiple events, you must

multiply each probability

Punnett Squares

1. Punnett Squares• Diagram used to

determine the results of a genetic cross

Punnett Squares• Genes have two different

possibilities– Two different alleles– These alleles separate

during gamete formation (segregation)

2. Terminology• Dominant Allele: the

trait that will always be seen if it is present– It is “stronger” than

the recessive trait– Represented by a

capitol letter

Terminology

• Recessive Allele: the trait that will only be seen if a dominant allele is not present– Overpowered by

dominant alleles– Represented by a lower

case letter

Phenotype and Genotype

• Phenotype– Physical

characteristics• Example: hair

color, height, eye color

• Genotype– Genetic makeup

Phenotypes and Genotypes

• Important: Some individuals may have the same phenotype with different genotypes– Example: Aa will look like AA• Same phenotype but different genotypes!

Terminology• Homozygous: two identical alleles for a trait– Also called “pure”– “homo-” means “same”

Terminology• Heterozygous: two different alleles for a trait– Considered “hybrid” for that trait– “hetero-” means “different”

Dominant and Recessive• Homozygous dominant: two dominant alleles– Will show the dominant trait– Example: AA

• Homozygous recessive: two recessive alleles– Will show the recessive trait– Example: aa

• Heterozygous– One dominant and one recessive allele– Will show the dominant trait– Example: Aa

Practice with Genetics Terminology

• Get a piece of paper.• Cut the paper into 4 sheets.• On each sheet write:– Two sheets with a capitol A– Two sheets with a lower case a

• Terms: Heterozygous, Homozygous, Homozygous recessive, Homozygous dominant

Punnett Squares

1. Baby Steps Through the Punnett Square

• Determine the genotypes of the parents• Write down your cross• Draw a punnett square• Split the letters of the genotype for each parent and put them

outside the punnett square• Fill in the punnett square to determine the genotypes of the

offspring• Summarize – percentages of genotypes and phenotypes

(these may be different percentages!)• Keep on practicing!

Making Babies!

• This activity involves using the terms we just learned in order to create an individual with each trait.

• Once you have gotten each trait, you will draw your baby.

• Yes, you may name it!