Biology
November 7, 2012
11.2 Probability and Punnett Squares
I. Genetics and Probability
Probability
The likelihood that a particular event
will occur
***The principles of probability can be
used to predict the outcome of
genetic crosses
II. Punnett Squares
The gene combinations that might
result from a genetic cross can be
determined by drawing a diagram
***Punnett squares can be used to
predict and compare the genetic
variations that will result from a cross
Homozygous
Organism that have two identical
alleles for a particular trait – TT, tt
1) True-breeding for a particular trait
Heterozygous
Organisms that have two different
alleles for the same trait
1) Hybrid for a particular trait
Phenotype
Genotype
All tall plants have the same physical
characteristics but have different
genotypes
Genetic makeup; only 1/3 of the tall
plants have this makeup
III. Probabilities and Segregation
Mendel’s Experiment
For each of his seven crosses, about ¾
of the plants showed the trait
controlled by the dominant allele. ¼
showed the trait controlled by the
recessive allele.
Ration 3: 1
IV. Probabilities Predict Averages
The larger number of offspring the
closer to the expected values