ProbabilityProbability

Probability…Probability…• is the likelihood (chance) that an event will happen.

• is expressed as a percentagepercentage or fractionfraction.

Everyday examples of probability:• weather!weather! Lots of times we hear predictions like Lots of times we hear predictions like ““80% chance of rain,80% chance of rain,”” or or ““20% chance of snow.20% chance of snow.””

• the the lotterylottery ““ThereThere’’s a 1 in 10 million chance s a 1 in 10 million chance your ticket will be a winner.your ticket will be a winner.””• geneticsgenetics

Calculating ProbabilityProbabilityProbabilityProbability is calculated by using this formula:

# of outcomes that meet the conditions x 100 = %

# Possible outcomes

# of outcomes that meet the conditions # of possible outcomes

This gives you a fraction.To turn the fraction into a percent,

multiply by 100.

Probability ExamplesProbability ExamplesWhat is the probability when flipping a penny of it landing heads up?

# outcomes that meet the conditions is 1 (heads)# possible outcomes is 2 (heads or tails)Probability is 1 ÷ 2; ½, or 50%

What is the probability when rolling a die of it getting a 6?

# outcomes that meet the conditions is 1 (6)# possible outcomes is 6 (1,2,3,4,5,6)Probability is 1 ÷ 6; 1/6, or 16.7%

Probability ExamplesProbability ExamplesOK, let’s try some more difficult examples.

What is the probability, if I pick a marble out of a bag containing 4 red, 3 blue, 2 green, and 5 yellow marbles, that I will pick a green marble?

# outcomes that meet the conditions is 2 (2 green marbles)# possible outcomes is 14 (total # of marbles in bag)

Probability is 2/14, or 1/7, or 14%

Probability ExamplesProbability ExamplesI have a can holding sticks with the names of my students on them. There are 22 sticks in the can; 12 girls and 10 boys. What is the probability, if I pick a stick, that I will get your name?

1/22, or 4.5%

What is the probability that I will pick a girl’s name?

12/22, or 55%

What is the probability that I will pick a boy’s name?

10/22, or 45%

multiple probability = likelihood of 2 or more events happening at the same time

For example: the probability of flipping two coins and getting 2 heads is:

½ x ½ = ¼ or 25%

Multiple Probability = Prob of Event 1 x Prob. of Event 2

Formula:

Multiple ProbabilitiesMultiple Probabilities

Probability PracticeProbability Practice1. What is the probability that, if I roll a pair of dice, I will get 2 sixes?

2. What is the probability that, if I flip a penny 3 times, I will get heads all 3 times?

1/6 x 1/6 = 1/36, or 2.8%

½ x ½ x ½ = 1/8 or 12.5%

2 Important Things To Know About Probability

1. Probability is the likelihood of an outcome, but it is not a certainty. The outcome often does not match the prediction. (Think of the weather report!!!)

2. The more data you have, the closer the outcome will be to the predicted probability.

ProbabilityProbabilityWhen flipping 2 pennies, there are 4 When flipping 2 pennies, there are 4

possible outcomes:possible outcomes:

H-H T-TH-T T-HSame thing

Probability 1/41/4 or 25%25%

Probability 1/41/4 or 25%25%

Probability 1/41/4 or 25%25%

Probability 1/41/4 or 25%25%

25% + 25% = 50%25% + 25% = 50%

How does this relate to genetics?

In hybrid (heterozygous) crosses, the probabilities are the same as when flipping

pennies!