Chapter 4
Probability and
Heredity
Section 2
I can… explain the role of
probability in a genetic cross
 Warm
Up: If a mother rabbit is hybrid for
black fur, and black fur is dominant, how
would you write her alleles?
 Wrap
Up: Draw a punnett square for the
cross: Hh x Hh
Principles of Probability
 Probability:
the number that describes
how likely it is that an event will occur
 Coin toss: two possible ways that the coin
will land (heads up or tails up)

1 in 2 chance, ½, 50%
 The
Laws of Probability predict what is
LIKELY to occur
Independence of Events
 When
you toss a coin more than once,
the results of one toss do not affect the
results of the next toss
Principles of Probability with
Genetics
 Mendel
realized that the mathematical
principles of probability applied to his
work
 He could say that the probability of
producing…


A tall plant was 3 in 4
A short plant was 1 in 4
HOW?!
Something called….
PUNNETT SQUARES
 Punnett
square: a chart that shows all the
possible combinations of alleles that result
from a genetic cross

They show all the possible outcomes of a
genetic cross and determine the probability
of that outcome
Example
 Hybrid


(one dominant, one recessive)
R: round seed (dominant)
r: wrinkled seed (recessive)
R
r
R
r
RR
Rr
Rr
rr
Using a Punnett Square
 Use
a Punnett square to calculate the
probability
 In a genetic cross, the allele that each
parent will pass on to its offspring is based
on probability

What is the probability that the
offspring will be RR?


What is the probability that the
offspring will be rr?


50% or 2 in 4
What is the probability that the
offspring will be a round seed?


25% or 1 in 4
What is the probability that the
offspring will be Rr?


25% or 1 in 4
75% or 3 in 4
What is the probability that the
offspring will be a wrinkled seed?

25% or 1 in 4
Predicting Probabilities
 Dominant:
black fur
 Recessive: white fur
 What
are the possible
allele combinations?

Bb
 What
is the probability
the guinea pig has black
fur?

100%
Phenotypes and Genotypes



Phenotype: physical appearance, or visible
traits
Genotype: genetic makeup, or allele
combination
What is the dominant trait?


What is the recessive trait?


Smooth pods
Pinched
How many different
phenotypes? How many
different genotypes?
Homozygous and
Heterozygous
 Purebred

SS or ss
 Hybrid

= two identical alleles = homozygous
Ss
= two different alleles = heterozygous
Codominance
 Codominance:
the
alleles are neither
dominant nor
recessive

Both results are
expressed in the
offspring, black and
white feathers
 Will
the offspring be
heterozygous? Or
homozygous?