1. Punnett Squares Honors Genetics Ms. Gaynor
Chapter 11 (Part 2)
Punnett Squares
Honors Genetics
Ms. Gaynor

2. What is “Probability”? Why did Mendel get his results?
What is the probability that Mendel would get his results EACH time?
PROBABILITY = likelihood (chance) that an event will happen

3. Probability Rules Probability Rule #1 The multiplication rule (“and”)
States that the probability that 2 or more independent events will occur together is the product (x ing) of their individual probabilities

4. Sperm R r 1⁄2 1⁄4 Eggs Rr Segregation of alleles into eggs Rr
Segregation of
alleles into eggs
alleles into sperm R r
1⁄2
1⁄4
Sperm
Eggs

5. Probability Rule #2 The rule of addition (“or”)
States that the probability that any 1 of 2 or more exclusive events will occur is calculated by adding (+ ing) together their individual probabilities

6. IN SUMMARY: 2 Probability Rules
Multiplication Rule
What are the chances of _?_ AND _?_ happening at the same time?
MULTIPLY probability of EACH event happening TOGETHER
Addition Rule
What are the chances of _?_ OR _?_ happening at same time?
ADD the probability of EACH event happening TOGETHER

7. Probability Example Problems
Ms. Gaynor has 4 sisters. What is the probability that this will happen (her mother having 5 girls in a row)?
½ x ½ x ½ x ½ x ½ =??
or % chance

8. Mendel’s Results
Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses?
We can answer this question using a Punnett square, which shows a genetic cross

9. What is a Punnett Square?
a diagram (box) used to predict the probabilities of the possible outcomes for offspring that will result from a cross between 2 parents
SHOWS EXPECTED RESULTS (not necessarily actual)

10. Monohybrid Cross Dihybrid Cross
a cross between 2 individuals that looks at 1 trait
There are 4 different types
Ex: freckles OR dimples
a cross between 2 individuals that looks at the possibilities of inheriting 2 DIFFERENT traits at one time
Ex: Freckles AND dimples
Dihybrid Cross

11. Monohybrid Punnett Square
Mom’s genotype (Hh) x Dad’s genotype (hh)
Tall Short
Mom’s allele # Mom’s allele #2
H h
Dad’s
Allele # h
Allele # h Hh hh

12. Genotypic Ratio vs. Phenotypic Ratio
Genotypic Ratio = the GENOTYPE probabilities (expected results) of offspring
Ex: 50 % AA and 50 % Aa
Phenotypic Ratio = the PHENOTYPE probabilities (expected results) of offspring
Ex: 100% Tall

13. Ratios Can be expressed as a: DECIMAL Ex: 0.75 and 0.25 FRACTION
PRECENTAGE
Ex: 75% and 25%
RATIO
Ex: 3: 1

14. Punnett Square Hh hh Genotypic ratio = 50 % Hh, 50% hh
Phenotypic ratio = 50 % Tall, 50% short Hh hh

15. 4 different types of monohybrid crosses
COMPLETE DOMINANCE:
4 different types of monohybrid crosses

16. #1: Homozygous Dominant x Homozygous Recessive
Mom’s genotype (HH) x Dad’s genotype (hh)
Tall Short
H H h
Genotypic ratio = 100% Hh (4:0)
Phenotypic ratio = 100% Tall (4:0) Hh

17. #2: Homozygous Recessive x Homozygous Recessive
Mom’s genotype (hh) x Dad’s genotype (hh)
short Short
h h h
Genotypic ratio = 100% hh (4:0)
Phenotypic ratio = 100% short (4:0) hh

18. #3: Homozygous Dominant x Heterozygous
Mom’s genotype (HH) x Dad’s genotype (Hh)
Tall Tall
H H H h
Genotypic ratio = 50% HH, 50% Hh (2:2)
Phenotypic ratio = 100% Tall (4:0) HH Hh

19. #4: Heterozygous x Heterozygous
Mom’s genotype (Hh) x Dad’s genotype (Hh)
Tall Tall
H h H h
Genotypic ratio = 25% HH, 50% Hh, 25% hh (1:2:1)
Phenotypic ratio = 75% Tall, 25% short (3:1) HH Hh hh

20. 3 Genotype Phenotype Purple 1 White 2 Ratio 3:1 Ratio 1:2:1 PP
Figure 14.6 3 1 2
Phenotype
Purple
White
Genotype PP
(homozygous) Pp
(heterozygous) pp
Ratio 3:1
Ratio 1:2:1

21. The Testcross
Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
Genotype is not immediately obvious
Cross an individual with the dominant phenotype with an individual that is homozygous recessive for a trait

22. Conduct a test cross, where the unknown dominant individual is crossed with the known recessive .
H _ ?_ x hh

23. Mom’s genotype (H?) x Dad’s genotype (hh)
Test Cross
Mom’s genotype (H?) x Dad’s genotype (hh)
Tall Short
H ? h
Genetypic ratio =
Phenotypic ratio = HH ?h Hh

24. Mendel’s 2nd Law: Law of Independent Assortment
says that the genes are inherited independently of other genes
Occurs during Metaphase I
Mendel assumed traits occur on different chromosomes!

25. Dihybrid Crosses
How are two characters transmitted from parents to offspring?
As a package?
Independently?

26. Dihybrid Crosses
crosses involveing crossing 2 DIFFERENT traits at one time
Example: Mate 2 parents and look at the probability of seeing 2 traits, such as:
eye color AND hair color
freckles AND dimples
Produces four possible phenotypes in the F2 generation

27. Using the information from a dihybrid cross, Mendel developed the law of independent assortment
Each pair of alleles segregates independently during gamete formation

28. For the following genotype, how many DIFFERENT gametes can be formed?
AaBBCcDdeeFf
10 different
A, a, B, C, c, D, d, e, F, f

29. How do You Do Dihybrid Crosses?
2 ways
1. one BIG Punnett Square (16 boxes instead of 4)
2. Make 2 separate Punnett Squares
1 square for EACH trait
Then use PROBABILITY RULES and MULTIPLY

30. One BIG Punnett Square
1. Find the possible gametes using F.O.I.L method
AaBb x AABb
AB AB
Ab Ab
aB AB
ab Ab

31. 2. Place the gametes in the Punnett Square & fill in boxes
AB Ab aB ab AB Ab
AABB
AABb
AaBB
AaBb
AAbB
AAbb
AabB
Aabb

32. Genotypic Ratio AAbB = 8/16 or 1/2 AaBB = 2/16 or 1/8

33. Phenotypic Ratio A= freckles, a= no freckles
B = Dimples, b= no dimples
AAbB = 8/16 (50%) freckles, dimples
AaBB = 2/16 (12.5%) freckles, dimples
Aabb = 2/16 (12.5%) freckles, no dimples
AABB= 2/16 (12.5%) freckles, dimples
AAbb = 2/16 (12.5%) freckles, no dimples
So…12/16 (75%) freckles AND dimples
4/16 (25%) freckles AND no dimples

34. Now…the easier way! Write out a key for EACH trait
AA, Aa= Freckles, aa= No freckles
BB, Bb= Dimples, bb= No dimples
Figure out the genotypes of EACH parent and put them together
Draw 2 Punnett squares & figure out the FRACTION ratio of possible genotype and phenotypes
Draw punnett square #1 for trait #1
Draw punnett square #2 for trait #2
Figure out genotype/phenotype possibilities of offspring by multiplying fractions

35. The Laws of Probability Control Mendelian Inheritance
Mendel’s laws of segregation and independent assortment
Reflect the rules of probability

36. Probability Rules and Dihybrid Crosses
A dihybrid or other multicharacter cross
Is equivalent to two or more independent monohybrid crosses occurring simultaneously
In calculating the chances for various genotypes from such crosses
Each character first is considered separately and then the individual probabilities are multiplied together