2. Genetics & Probability
Mendel’s laws:
segregation
independent assortment
reflect same laws of
probability that apply to
tossing coins or rolling dice
AP Biology
3. Probability & genetics
Calculating probability of
making a specific gamete
is just like calculating the
probability in flipping a
coin
probability of tossing
heads?
probability making a B
gamete?
B
100%
BB
B
AP Biology
B
50%
Bb
b
4. Probability & genetics
Outcome of 1 toss has no
impact on the outcome of the
next toss
probability of tossing heads
each time? 50%
probability making a B gamete
each time? 50%
B
Bb
b
AP Biology
5. Calculating probability
Pp x Pp
P
p
egg
offspring
P
male / sperm
sperm
P
PP
P
p
p
P
1/2 x 1/2
female / eggs
1/2 x 1/2
P
PP
Pp
p
Pp
pp
AP Biology
1/2 x 1/2
p
p
1/2 x 1/2
=
=
=
=
1/4
Pp
1/4
+
1/4
1/2
pp
1/4
6. Rule of multiplication
Chance that 2 or more independent
events will occur together
probability that 2 coins tossed at the
same time will land heads up
1/2 x 1/2 = 1/4
probability of Pp x Pp → pp
1/2 x 1/2 = 1/4
AP Biology
P
Pp
p
7. Calculating probability in crosses
Use rule of multiplication to predict crosses
YyRr x YyRr
x
Yy
Yy
Rr
yyrr
x
?%
1/16
yy
AP Biology
rr
1/4
x
1/4
Rr
8. Apply the Rule of Multiplication
AABbccDdEEFf
x
AaBbccDdeeFf
AabbccDdEeFF
Got it?
Try this!
AP Biology
AA x Aa → Aa
Bb x Bb → bb
cc x cc → cc
Dd x Dd → Dd
EE x ee → Ee
Ff x Ff → FF
1/2
1/4
1
1/2
1
1/4
1/64
9. Rule of addition
Chance that an event can occur
2 or more different ways
sum of the separate probabilities
probability of Bb x Bb → Bb
sperm
egg
offspring
B
b
Bb
1/2 x 1/2 =
b
B
1/2 x 1/2 =
AP Biology
1/4
Bb
1/4
1/4
+ 1/4
1/2
10. Chi-square test
Test to see if your data supports
your hypothesis
Compare “observed” vs. “expected” data
is variance from expected due to
“random chance”?
or is there another factor influencing data?
null hypothesis
degrees of freedom
statistical significance
AP Biology
