1. Punnett Squares Honors Biology Ms. Day
Lecture #25
Punnett Squares
Honors Biology
Ms. Day

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. 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

4. Probability Example Problem
Ms. Day 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

5. 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

6. 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!!!!!)

7. Law of Segregation
Germ Line Cell a A
Gamete
(sperm or egg) a A

8. 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

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

10. Probabilities can be expressed as a:
DECIMAL
Ex: 0.75 and 0.25
FRACTION
Ex: ¾ and ¼
PRECENTAGE
Ex: 75% and 25%
RATIO
Ex: 3: 1

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

12. Ex#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

13. Ex #2: 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

14. 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

15. The Testcross
Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
Genotype is not immediately obvious
Cross:
dominant phenotype individual x homozygous recessive for same trait (H _ ?_ x hh)

16. 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 =?
IF THERE ARE ANY SHORT OFFSPRING, THEN WE KNOW THE UNKNOWN GENOTYPE HAS TO BE Hh HH ?h Hh

17. 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!

18. DIHYBRIDS
Review…
Mendel’s 1st Law: law of segregation says that the each gamete gets 1 allele PER GENE
Mendel’s 2nd Law: law of independent assortment says that the genes are inherited independently of other genes

19. Dihybrid Crosses
crosses involving 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

20. 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

21. One BIG Punnett Square
1. Find the possible gametes using F.O.I.L method (ONLY 1 “letter” per gene)
AaBb x AABb
AB AB
Ab Ab
aB AB
ab Ab

22. 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

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

24. 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

25. 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

26. AaBb x aabb AA, Aa= Tall aa= short BB, Bb= Black bb= white
½ Bb ½ bb ½ Aa ½ aa
½ Black ½ white ½ Tall ½ short
So… babies could look:
25% Black & Tall (½ * ½) % white & tall (½ * ½)
25% Black & short (½ * ½) % white & short (½ * ½) Bb bb Aa aa

27. Probability Rules and Dihybrid Crosses
A dihybrid or other multicharacter cross
Is equivalent to two or more independent monohybrid crosses occurring simultaneously (AT THE SAME TIME)