1. II. Probability and Punnett Squares
-Mendel used the principles of probability to
explain the results of his genetic crosses.
A. Genetics and Probability
Probability – the likelihood that a particular
event will occur, ex : coins – have a 50/50
shot of heads or tails.
Problem : Determine the probability of
flipping heads on a coin 3 times in a row.
½ x ½ x ½ = 1/8
*Past outcomes DON’T affect future
outcomes.
-Segregation of alleles is completely
random like coin tossing.
-The principles of probability can be used
to predict the outcomes of genetic crosses.

2. B. Punnett Squares *Gene combinations that might result from a genetic
cross can be determined
by using a Punnett square.
Punnett square – a
diagram showing the gene
combinations that might
result from a genetic cross.

3. In a Punnett square the :
1. Types of gametes produced by P or F1 generations are shown along the top & left sides of the square.
2. Possible gene combinations for the F1 or F2 offspring appear in the 4 boxes that make up the square.

4. Homozygous vs. Heterozygous
-Organisms with 2 identical alleles for a trait
(TT or tt) are called homozygous, homo = same.
-Organisms with 2 different alleles for the same
trait (Tt) are called heterozygous, hetero =
different.
-Homozygous organisms are true-breeding or
pure for a trait & heterozygous organisms are
hybrid for a trait.

5. Genotype vs. Phenotype *Genetic make-up of an organism & its
physical characteristics can be referred to
as its genotype & phenotype.
Genotype – the actual genetic make-up of
an organism or the genes it has.
Phenotype – physical characteristics that
can be seen & described when looking at
an organism.

6. Genotype vs. Phenotype What are the genotypic &
phenotypic ratios of the F1
generation of plants in this
picture (assuming A = yellow
seeds/a = green seeds)?
Genotypic = 1 AA : 2 Aa : 1 aa
Phenotypic = 3 yellow seeds : 1 green seed

7. C. Probabilities Predict Averages
*Probabilities predict the average outcome of a
large # of events.
-Probability CAN’T predict the precise outcome of
an individual event, ex : flipping a coin 2x &
getting heads twice as opposed to heads once &
tails once. The same is true of genetics & the
probability of expected ratios in a population.
-In order to get close to expected values, one
must flip a coin many times/in genetics produce
many offspring.