Chapter 11-2 Probability and Punnett Squares Notes

Every time Mendel performed a cross with his pea plants, he carefully counted the offspring (over 20,000 plants)

This why he noticed there was a pattern! LARGE NUMBERS

Whenever he crossed 2 plants that were hybrid for stem height (Tt), about ¾ (75%) of the resulting plants were Tall and ¼ (25%) were short (3:1 ratio)

He realized that the principles of probability could be used to explain his results!

Probability

Definition: the likelihood that a particular event will occur

Examples:

Flip a coin: ½ or 50% chance it will come up heads

If you flip a coin 3 times in a row, what is the probability it will come up heads 3 times in a row?

Each coin flip is an independent event therefore it is:

½ x ½ x ½ = 1/8

Past outcomes do not influence future outcomes

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

Punnett Squares

The gene combinations that MIGHT result from a genetic cross can be determined by drawing a PunnettSquare

1. The parents go on the outside of the square along the side (male alleles) and top (female alleles

2. The possible outcomes of the cross are on the inside of the square

3. The letters in the Punnett Square represent alleles

Alleles in a Punnett Square

-“T” represents the dominant allele: TALL

-“t” represents the recessive allele: short

Organisms with 2 identical alleles (TT or tt) for a trait are said to be HOMOZYGOUS

Mendel called these: true-breeding

Organisms with 2 different alleles for a particular trait are said to be HETEROZYGOUS

Mendel called these hybrids

Phenotype: the physical characteristics of the organism (what the organism looks like)

Ex: Tall; short

Genotype: the actual genetic makeup of the organism (the 2 alleles the organism inherited)

Ex: TT; Tt: tt

Probabilities Predict Averages

Probabilities predict the outcomes of a LARGEnumber of events

Probabilities cannot predict precise outcomes of an event

-Ex: flip a coin twice you may get 100% heads

-You need to flip the coin many, many times to get close to 50%

Punnett Squares

Tt Tt

Tt Tt

The genes from the female parent go here.

The genes from the male parent go here.

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

Punnett Squares

T T

t Tt Tt

t Tt Tt

F1 generation

T T

t Tt Tt

t Tt Tt

Interpreting the ResultsThe genotype for all the offspring is Tt.

The genotype ratio is:

Tt – 4:0 = 100%

The phenotype for all the offspring is tall.The phenotype ratio is:

tall – 4:0 = 100%

Punnett Squares

T t

T ?? ??

t ?? ??

Your Turn!!

Punnett Squares

T t

T TT Tt

t Tt tt

F2 generation

Punnett Squares

T t

T TT Tt

t Tt tt

Next, give the genotype and phenotype ratios of the offspring (F2 generation).

Punnett Squares

T t

T TT Tt

t Tt tt

Genotype ratio: TT - 1

Punnett Squares

T t

T TT Tt

t Tt tt

Genotype ratio: TT - 1, Tt - 2

Punnett Squares

T t

T TT Tt

t Tt tt

Genotype ratio: TT - 1, Tt - 2, tt - 1

Genotype ratio: 1: 2: 1

Punnett Squares

T t

T TT Tt

t Tt tt

Genotype ratio: TT - 1, Tt - 2, tt - 1

Phenotype ratio: Tall - 3

Punnett Squares

T t

T TT Tt

t Tt tt

Genotype ratio: TT - 1, Tt - 2, tt - 1

Phenotype ratio: Tall - 3, short - 1

Punnett Squares

T t

T TT Tt

t Tt tt

This is a monohybrid cross. We worked with only onetrait. The height of the plant.

One

Punnett Squares

T t

T TT Tt

t Tt tt

This is a monohybrid cross. We worked with only onetrait. The height of the plant.

Later we will work with a dihybrid cross, using theheight of the plant and either seed color or seedshape.

two

3. What is the expected ratio for this crosses? Assume the first written is from the female….

Tt x tt Female TtM

ale

t

t

T t

t

t

Tt t t

Tt t t

Tt x tt

Genotypic Ratio:

Phenotypic Ratio:

0 TT, 2 Tt, 2 tt = 2 Tt : 2 tt

2 Tall : 2 short or 50% Tall : 50% short

50% Tt : 50% tt

XX and XY

Female XXM

ale

X

Y

XX XX

XY XY

X X

X

Y

XX x XY

Phenotypic Ratio: 50% Females and 50% Males

The Testcross

Homozygous dominant and heterozygous individuals are indistinguishable (phenotypically)

A testcross allows us to determine the genotype of an individual that has the dominant trait.

We cross the parent showing the

dominant trait with a recessive

parent.

-If all (100%) of the offspring are dominant type then we assume the initial parent was homozygous dominant.

-If even one offspring shows the recessive type then we know the initial parent was heterozygous.