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Probability & Punnett Squares
Probability
• A concept that can be used to predict the results of a particular event– Examples
• Chance of a specific team winning a sporting event
• Chance of a coin landing on heads in a coin toss
– Predict what is likely to occur, not necessarily what will actually occur
Probability and Heredity
• Think back to Mendel’s plant experiments– He noticed that traits were
inherited in patterns
• For example, when he crossed two plants that were heterozygous for stem height (Tt) he noticed their offspring would inherit this trait in a predictable pattern, with 3 out of 4 having tall stem height.
Probability and Heredity
• Each time Mendel repeated the cross, he would obtain similar results
• He could say that the probability, or chance, of the cross producing a tall plant was 3 in 4 and the probability of producing a short plant was 1 in 4.
Mendel noticed the same pattern of inheritance in other traits as well
Probability and Punnett Squares
• Punnett Square – A chart that shows how parents’ alleles might combine in an offspring– a tool that can help you understand the patterns of
heredity– Geneticists use Punnett Squares to show all the
possible outcomes of a genetic cross and to determine the probability of a particular outcome
– Punnett Squares Explained at http://www.siskiyous.edu/class/bio1/genetics/monohybrid_v2.html
Probability and Punnett Squares
• What is the probabilty of…..– The offspring having a
yellow seed color?
2 in 4
– The offspring having a
green seed color?
2 in 4
Probability and Punnett Squares
• How else can probability be written?
– The offspring having a yellow seed color?
2 in 4 or 50%
– The offspring having a
green seed color?
2 in 4 or 50%
Using a Punnett Square
• In rabbits, black fur color is dominant to white. What is the probability of producing a white rabbit if two heterozygous rabbits mate?
Using a Punnett Square
• Step 1: figure out the genotype of the parents.
• Black is dominant to white. – B - black– b - white
• Parents are heterozygous.– Bb - dad– Bb - mom
Using a Punnett Square
• Step 2: set up a Punnett Square.
B b
B
b
BbBb
Using a Punnett Square
• Step 3: Fill in the Punnett Square.
B b
B
b
Using a Punnett Square
• Step 3: Fill in the Punnett Square.
B b
B
b
BB
Using a Punnett Square
• Step 3: Fill in the Punnett Square.
B b
B
b
BB Bb
Using a Punnett Square
• Step 3: Fill in the Punnett Square.
B b
B
b
BB Bb
Bb
Using a Punnett Square
• Step 3: Fill in the Punnett Square.
B b
B
b
BB Bb
Bb bb
Using a Punnett Square
• Step 4: Count the results.
• Genotypes – 1 homozygous
dominant
B b
B
b
BB Bb
Bb bb
Using a Punnett Square
• Step 4: Count the results.
• Genotypes – 1 homozygous
dominant– 2 heterozygotesB b
B
b
BB Bb
Bb bb
Using Punnett Square
• Step 4: Count the results.
• Genotypes – 1 homozygous
dominant– 2 heterozygotes– 1 homozygous
recessive
B b
B
b
BB Bb
Bb bb
Using a Punnett Square
• Step 4: Count the results.
• Genotypes – 1 homozygous
dominant– 2 heterozygotes– 1 homozygous
recessive
• Phenotypes
B b
B
b
BB Bb
Bb bb
Using a Punnett Square
• Step 4: Count the results.
• Genotypes – 1 homozygous
dominant– 2 heterozygotes– 1 homozygous
recessive
• Phenotypes– 3 black fur
B b
B
b
BB Bb
Bb bb
Using a Punnett Square
• Step 4: Count the results.
• Genotypes – 1 homozygous
dominant– 2 heterozygotes– 1 homozygous
recessive
• Phenotypes– 3 black fur– 1 white fur
B b
B
b
BB Bb
Bb bb
Probability and Punnett Squares
• In rabbits, black fur color is dominant to white. What is the probability of producing a white rabbit if two heterozygous rabbits mate? – 1 in 4 or 25%
Representing Ratios
• Probability can also be represented in ratios– A ratio compares or shows the relationship
between a part to the whole• Example the probability that a coin will land on
heads in a single coin toss is 1 in 2 or 50%
– As a ratio, the probability would be written as 1:2 and would be read as, “One to two.”
– In genetics we use rations to represent probability
Using a Punnett Square
• Step 4: Count the results.
• Genotypes =– 1 homozygous
dominant– 2 heterozygotes– 1 homozygous
recessive– Genotypic Ratio =
1:2:1
• Phenotypes– 3 black fur– 1 white fur
B b
B
b
BB Bb
Bb bb
Using a Punnett Square
• Step 4: Count the results. • Genotypes =– 1 homozygous dominant– 2 heterozygotes– 1 homozygous recessive– Genotypic Ratio = 1:2:1
• Phenotypes– 3 black fur– 1 white fur– Phenotypic Ratio = 3:1
B b
B
b
BB Bb
Bb bb
Let’s Practice1. In pine trees, long
needles are dominant over short needles. Cross a homozygous dominant plant with a plant that is heterozygous. What is the probability that the parents will produce offspring that have short needles? Give the genotypic and phenotypic ratios.
Let’s Practice 2. In cats, a striped coat
is dominant over a solid coat. Cross a cat with a solid coat with a cat that is heterozygous for a striped coat. What is the probability that the parents will produce offspring with a solid coat? Give the genotypic and phenotypic ratios.
Let’s Practice 3. In humans, dark hair is
dominant over light hair. Cross a man that is homozygous recessive with a woman that is homozygous dominant. What is the probability that the parents will produce offspring with light hair? Give the genotypic and phenotypic ratios.
