PROBABILITY & PUNNETT SQUARES

BELL RINGER

What do these pictures show?What is unusual about this hand and foot? Look closely!!

BELL RINGER CONT.

•The hand and foot have six fingers and toes instead of five.

•This is the result of a genetic disorder referred to as Polydactyly – having more than five digits.

BELL RINGER CONT.

What religious group do these people

belong to?

They are Amish - lead a simple life centered on

family and religion. Large population resides in Lancaster County, PA.

BELL RINGER CONT.The Amish live separately from the rest of society. They typically marry only other Amish people.

What if I told you Polydactyly is more common among the Amish than in the

general population. Why do you think that

is true?

Why might it someday be important for you to know if you are a carrier for a genetic disease?

OBJECTIVESAs a result of this lesson you (my students) will be able to:

•Explain how geneticists use the principles of probability •Describe how geneticists use Punnet Squares •Distinguish between the concepts heterozygous and homozygous •Solve genetic problems using a Punnet Square

WHAT IS PROBABILITY?

Probability is a number that describes the likelihood that a

particular event will occur

Example: A coin flip

What are the possible outcomes when you flip a

coin?

Answer: There are two – it could land heads up or tails up

WHAT IS PROBABILITY? CONT.Probability is the likelihood that a particular event will

occur

Example: A coin flip

What are the chances a coin will land heads up?

Answer: 50% or ½

What are the chances a coin will land tails up?

Answer: 50% or ½

WHAT IS PROBABILITY? CONT.If you flip a coin 3 times in a row, what is the probability it will land

heads up every time?

Each coin flip is an independent event – each time the

coin is flipped, the probability of it landing heads up is ½ .

The probability of flipping 3 heads in a row is:

½ x ½ x ½ = 1/8 Remember: Past outcomes do not

effect future outcomes!

HOW DOES PROBABILITY RELATE TO GENETICS?

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

Coin Toss Lab

• Suppose that you were to toss a coin 20 times. Predict how many times the coin would land on heads and how many times would it land on tails.

• Let’s do it.

COIN TOSS LAB DISCUSSION Did your results equal your calculated

probabilities?

What did you notice as your number of tosses increased?

Answer: The more times you tossed your coin, the closer your actual results should have been to your predicted probability. Probable outcomes are not always equal to

actual outcomes; however, the two should get closer as the number of trials

increases.

GENOTYPE AND PHENOTYPE REVIEW What is genotype?

An organism’s genetic makeup. An organisms particular combination of alleles.

What is phenotype?An organism’s physical appearance. The phenotype is determined by the genotype. When an organism is heterozygous the dominant allele will be expressed.

FFFfff

FF or Ff ff

HUMAN TRAITS REVIEW

When an organism has two identical alleles for a particular trait (ex. TT or tt), it is said to be

_______.Answer: Homozygous

When an organism has two different alleles for the same trait (ex. Tt), it is said to be

_________.Answer: Heterozygous

HUMAN TRAITS REVIEW

This girl is displaying one of the traits we spoke of. Which one?

Answer: Widow’s Peak

What is her phenotype?Answer: She has a Widow’s Peak

What is a possible genotype given that Widow’s Peak is a dominant allele?

Answer: WW or Ww

PUNNETT SQUARESLet’s combine our knowledge of probability

with our understanding of genotype and phenotype.

WHAT IS A PUNNET SQUARE? The Punnett square is a diagram that is used

to predict an outcome of a particular cross or breeding experiment.

It is named after Reginald C. Punnett, who devised the approach, and is used by biologists to determine the probability of an offspring having a particular genotype.

HOW DO YOU CONSTRUCT A PUNNETT SQUARE? Start by drawing a box and dividing it into

four squares.

HOW TO CONSTRUCT A PUNNETT SQUARE (CONT.) Write the male parent’s alleles (RR) along the

top of the square and the female parent’s alleles along the left side (Rr).

R R Dad

Mom

R

r

HOW TO CONSTRUCT A PUNNETT SQUARE (CONT.) Copy the female parent’s alleles into the

boxes to the right.

R r Dad

Mom

R

r

R

r r

R

HOW TO CONSTRUCT A PUNNETT SQUARE (CONT.) Copy the male parent’s alleles into the boxes

beneath them.

R r Dad

Mom

R

r

R

r r

RR r

rR

PUNNET SQUARES The completed Punnett square shows all the

possible combinations in the offspring.

R r Dad

Mom

R

r

R

r r

RR r

rR

WHY USE A PUNNET SQUARE? You can use a Punnet square to calculate the

probability that offspring with a certain combination of alleles will result.

For example in: R r

R

r

R

r r

RR r

rR

What is the probability that the offspring will be RR?

1 in 4 or 25%

What is the probability that the offspring will be rr?

1 in 4 or 25%

What is the probability that the offspring will be Rr?

2 in 4 or 50%

NOW LET’S SEE WHAT HAPPENS… We are going to mate a black guinea pig (homozygous:

BB) with a white guinea pig (homozygous: bb).

Create a Punnet square to determine the probability of having a white guinea pig (remember that uppercase letters mean the trait is dominant, this means uppercase will mask the recessive trait b)

HOWEVER If we mate a black guinea pig (heterozygous: Bb)

with a white guinea pig (homozygous: bb).

What is the probability of having a white guinea pig? (once again, remember that uppercase letters mean the trait is dominant, this means uppercase will mask the recessive trait b)

CLOSURELet’s return to our Bell Ringer…

What is the genetic disorder that is displayed in this picture?Answer: Polydactyly

Polydactyly is a dominant geneticdisorder. John is heterozygous for

Polydactyly – What is his genotype?Answer: Pp

What is John’s phenotype?Answer: He displays the symptoms

of the disorder – he has six toes.

CLOSURE CONT.John (Pp) marries Diane who is

homozygous for Polydactyly. What are the chances they will

have a child who is homozygous for Polydactyly?

Use a Punnett Square to answer the question!

CLOSURE CONT.Answer: 2/4 or 50%P p

PP PpPP Pp

PP

ANSWER THE FOLLOWING QUESTIONS IN YOUR NOTEBOOK What is probability? If you know the parents’ alleles for a trait

how can you use a Punnet square to predict the probable genotypes of the offspring?

Define genotype and phenotype. Explain how two organisms can have the

same phenotype but different genotypes. Give an example.