Probability and Heredity Notes
Probability: A number that describes how likely it is that an event will occur
Example: Flip a Coin, what is the probability that you will get heads?
1 out of 2 or 50%
*The laws of probability predict what is likely to occur, not necessarily what will
occur
Punnet Square: a chart that shows all the possible combinations of alleles that can
result from a genetic cross (determines the probability that a certain genetic
combination of alleles will occur for specific physical traits)
1. Start by drawing a box and dividing it into four squares
2. Write the Male Parent’s alleles along the top of the square and the Female
Parent’s alleles along the left side
3. Copy the female parent’s alleles into the boxes to their right
4. Copy the male parent’s alleles into the boxes beneath them
5. The completed Punnett Square shows all the possible allele combinations in
the offspring.
Or Use the “Box Method” from Algebra
Example: Cross two Hybrid Pea Plants with round seeds (Rr). The allele for
round seeds (R) is dominant over the allele for wrinkled seeds (r). Each parent can
pass either of its alleles R or r to its offspring
R
r
R
r
RR
Rr
Rr
rr
What is the probability that the allele combination will be RR? Rr? rr?
RR =1out of 4 or 25%,
Rr= 2 out of 4 or 50%
rr =1 out of 4 or 25%
What is that probability that the plant will have round seeds? 3 out of 4 or 75%
What is the probability that the plant will have wrinkled seeds? 1 out of 4 or 25%
Want More Examples?
Example:
Brown Eyes are Dominant to Blue Eyes
Male Parent = Bb (Heterozygous Dominant)
Female Parent = bb (Homozygous Recessive)
What is the probability that the offspring will have brown eyes?
What is the probability that the offspring will have blue eyes?
What is the probability that the offspring will have the BB allele combination?
What is the probability that the offspring will have the Bb allele combination?
What is the probability that the offspring will have the bb allele combination?
B
b
b
b
Bb
bb
Bb
bb
Harder Example (you have to figure out what each parent’s alleles are first)
Brown hair (B) is dominant to red hair (r). The male parent is homozygous for
brown hair and the female parent is homozygous for red hair. Will any of the
offspring produced have red hair? Why or Why Not?
If the male parent was heterozygous dominant instead, could any of the offspring
have red hair? Why or Why Not?
r
r
B
B
Br
Br
Br
Br
B
r
r
Br
rr
r
Br
rr