Chapter 3Section 2Chapter 3Section 2

Probability and Punnett Squares

Probability and Punnett Squares

Genetics and ProbabilityGenetics and Probability

The likelihood that a particular event will occur is called probability

Ex: Flipping a coin

There are 2 possible outcomes…heads or tails

The chances, or probabilities, of either outcome are equal

The likelihood that a particular event will occur is called probability

Ex: Flipping a coin

There are 2 possible outcomes…heads or tails

The chances, or probabilities, of either outcome are equal

Therefore, the probability that a single coin flip will come up heads is 1 chance in 2

This is 1/2 or 50%

Therefore, the probability that a single coin flip will come up heads is 1 chance in 2

This is 1/2 or 50%

Let’s say you flip a coin 3 times in a row, what is the probability it will land heads up every time?

Think of each coin flip as an independent event

The probability of the coin landing heads up is 1/2…we said this already

The probability of flipping 3 heads in a row is:

1/2 x 1/2 x 1/2 = 1/8

• You have a 1 in 8 chance of flipping 3 heads in a row

Let’s say you flip a coin 3 times in a row, what is the probability it will land heads up every time?

Think of each coin flip as an independent event

The probability of the coin landing heads up is 1/2…we said this already

The probability of flipping 3 heads in a row is:

1/2 x 1/2 x 1/2 = 1/8

• You have a 1 in 8 chance of flipping 3 heads in a row

These same principles of probability can be used to predict the outcomes of genetic crosses!

These same principles of probability can be used to predict the outcomes of genetic crosses!

Punnett SquaresPunnett Squares

The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a punnett square

Punnett squares can be used to predict and compare the genetic variations that will result from a cross.

The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a punnett square

Punnett squares can be used to predict and compare the genetic variations that will result from a cross.

The parents are shown on the top and left side of the box

The possible gene combinations for the offspring appear in the 4 boxes that make up the square.

The letter in the the punnett square represent alleles.

T T

T t

T t

t t

T t

T

t

HomozygousHomozygous

Organisms that have 2 identical alleles for a particular trait are said to be HOMOZYGOUS

Example: TT or tt

Organisms that have 2 identical alleles for a particular trait are said to be HOMOZYGOUS

Example: TT or tt

HeterozygousHeterozygous

Organisms that have 2 different alleles for the same trait are referred to as HETEROZYGOUS.

They are hybrid for a particular trait

Example: Tt

Organisms that have 2 different alleles for the same trait are referred to as HETEROZYGOUS.

They are hybrid for a particular trait

Example: Tt

Phenotype vs. GenotypePhenotype vs. Genotype

What the heck is the difference???

Phenotype refers to the physical characteristics of an organism• Tall, short, wavy, straight, etc.

Genotype refers to the genetic makeup of an organism• The actual alleles that give an organism

its specific phenotype

• Genoytype tells us if it is TT, Tt, or tt

What the heck is the difference???

Phenotype refers to the physical characteristics of an organism• Tall, short, wavy, straight, etc.

Genotype refers to the genetic makeup of an organism• The actual alleles that give an organism

its specific phenotype

• Genoytype tells us if it is TT, Tt, or tt

All tall plants have the same PHENOTYPE, but they do not have the same GENOTYPE necessarily.

How can we explain this???

Plants can be TT or Tt and still be a tall plant!

As long as there is a dominant allele present, the dominant form of that trait is visible!

All tall plants have the same PHENOTYPE, but they do not have the same GENOTYPE necessarily.

How can we explain this???

Plants can be TT or Tt and still be a tall plant!

As long as there is a dominant allele present, the dominant form of that trait is visible!

Probability & Segregation

1/4 of the offspring have 2 alleles for tallness (TT)

2/4 or 1/2 have one allele for tallness and one allele for shortness (Tt)

1/4 of the offspring have 2 alleles for shortness (tt)

Overall 3/4 of the plants should be tall

There should be 3 tall plants and 1 short plant

3:1 ratio!

T t

T

t

T T T t

T t t t

For each of the 7 crosses Mendel completed and studied, 3/4 of the plants showed the trait controlled by the dominant allele

About 1/4 showed the trait controlled by the recessive allele.

Segregation did indeed occur!

Mendel was RIGHT!

For each of the 7 crosses Mendel completed and studied, 3/4 of the plants showed the trait controlled by the dominant allele

About 1/4 showed the trait controlled by the recessive allele.

Segregation did indeed occur!

Mendel was RIGHT!