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Probability and Punnett
Squares
Tossing Coins
• If you toss a coin, what is the probability of
getting heads? Tails? If you toss a coin 10
times, how many heads and how many
tails would you expect to get? Working
with a partner, have one person toss a
coin ten times while the other person
tallies the results on a sheet of paper.
Then, switch tasks to produce a separate
tally of the second set of 10 tosses.
Tossing Coins
(continued)
• 1. Assuming that you expect 5 heads and 5 tails in 10 tosses, how
do the results of your tosses compare? How about the results of
your partner’s tosses? How close was each set of results to what
was expected?
• 2. Add your results to those of your partner to produce a total of 20
tosses. Assuming that you expect 10 heads and 10 tails in 20
tosses, how close are these results to what was expected?
• 3. If you compiled the results for the whole class, what results would
you expect?
• 4. How do the expected results differ from the observed results?
Probability
 Likelihood that a particular event will occur is
called probability
 Ex. Coin tossing – probability that a coin will land
heads up is 1 chance in 2 tries = ½ or 50%
chance
 Toss the coin 3 times, probability of each coin
landing heads up is ½ for each of the three
tosses:
½ x ½ x ½ = 1/8
(flip 1) (flip 2) (flip 3) = 1/8
1/8 chance of flipping heads 3 times in row
Probability
(continued)
• Past outcomes do not affect future ones.
Alleles separate completely random, like a
coin flip. The Principles of Probability can
be used to predict the outcomes of genetic
crosses.
Punnett Squares
• Gene combinations that might result from
a genetic cross can be determined by
diagrams called Punnett Squares.
Parents’ alleles are placed along the top
and sides of the squares while the
offsprings’ alleles are inside the squares.
Capital letters = dominant allele while
lowercase letters = recessive alleles
Punnett Squares
“T” = tall plant
“t” = short plant
Punnett Squares
“T” = tall plant
“t” = short plant
Punnett Squares
(continued)
• Two identical alleles for a trait (TT or tt) =
homozygous (true-breeding)
• Two different alleles for a trait (Tt) =
heterozygous (hybrids)
• Phenotype = physical, observable
characteristics
• Genotype = genetic make-up, NOT
observable
Probability and Segregation
“T” = tall plant
“t” = short plant
Genotype Ratio: 1 TT : 2 Tt : 1 tt
•1/4th of F2 plants have TT
•½ of F2 plants have Tt
•1/4th of F2 plants have tt
Phenotype Ratio: 3 Tall : 1 Short
•3 Tall plants (1 TT, 2 Tt) for every 1
short plant (1 tt)
•Ratio of 3 : 1 for tall to short plants
Probabilities Predict Averages
– Probability cannot predict precise outcomes,
but is usually close
– The larger the sample size of offspring, the
closer the observed data will be to the
expected data
Setting Up Punnett Squares
Cross a Homozygous Yellow Seed Plant (YY) with
a Heterozygous Yellow Seed Plant (Yy)
YY x Yy
Genotype Ratio:
Y
Y
2 YY : 2 Yy
y = green seed
Reduces to
1 YY : 1 Yy
Y
YY
YY
yellow
yellow
Yy
Yy
yellow
yellow
Phenotype Ratio:
All yellow seeds
100 % yellow seeds
Y = yellow seed
y