Transcript Probability & Punnett Squares - AdVENTUREScience-7th

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?
OBJECTIVES
As 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?
FF
Ff
ff
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.
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 SQUARES
Let’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
r
Mom
R
Dad
HOW TO CONSTRUCT A PUNNETT SQUARE
(CONT.)

Copy the female parent’s alleles into the boxes to
the right.
R
r
R
R
R
r
r
r
Mom
Dad
HOW TO CONSTRUCT A PUNNETT SQUARE
(CONT.)

Copy the male parent’s alleles into the boxes
beneath them.
R
r
R
RR
Rr
r
rR
rr
Mom
Dad
PUNNET SQUARES

The completed Punnett square shows all the
possible combinations in the offspring.
R
r
R
RR
Rr
r
rR
rr
Mom
Dad
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
RR
Rr
r
rR
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)
CLOSURE
Let’s return to our Bell Ringer…
What is the genetic disorder that is
displayed in this picture?
Answer: Polydactyly
Polydactyly is a dominant genetic
disorder. 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.
P
P
Answer: 2/4 or 50%
P
p
PP
Pp
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.
