7.2.9 Students know plant and animal cells
contain many thousands of different genes and
typically have two copies of every gene. The two
copies (or alleles) of the gene may or may not be
identical, and one may be dominant in
determining the phenotype while the other is
recessive.
I. Principles of Probability
 Probability- a number that describes the likelihood of
a certain event happening
 Laws of probability predict what is likely to happen but
not always what does happen
 The more times you do the activity the closer the
actual results will get to the predicted results
 Independence of Events- the results of one trial does
not effect the next trial
II. Probability and Genetics
 How is probability related to genetics?
 Mendel’s pea plants had ¾ tall and ¼ short- each time
he repeated the cross he got similar results
 Mendel was the first scientist to recognize that
probability can be used to predict results of genetics
crosses
 Punnett squares- a chart that shows all the possible
combinations of alleles that can come from a genetic
cross
Punnett Squares
The diagrams show how to make a Punnett square. In this cross, both
parents are heterozygous for the trait of seed shape. R represents the
dominant round allele, and r represents the recessive wrinkled allele.
 Punnett square is used to calculate probability
 Key concept- In a genetic cross, the allele that each
parent will pass on to its offspring is based on
probability.
 Remember that a dominate allele masks the recessive
allele
 Look at Figure 8
III. Phenotypes and Genotypes
 Phenotypes- physical
appearance or what it looks
like
 Genotypes- genetic makeup
or allele combinations
 Look at figure 9
 Homozygous- two identical
alleles for the same trait (TT)
 Heterozygous- two different
alleles for the same trait (Tt)
IV. Codominance
 Codominance- alleles are neither dominant nor
recessive so both allelles are present
 Neither allele is masked
 Codominant alleles are written as capital letters with
subscript
 What is probability and how does it help
explain the results of genetic crosses?
 What is meant by genotype and phenotype?
 What is codominance?