1. Punnett Square Catalyst
Is it possible for two brown eyed people to have a blue eyed child? Explain your answer.

2. Objective 5
SWBAT solve simple genetics problems using monohybrid Punnett squares -I can identify homozygous and heterozygous genotypes -I can determine an organism’s genotype for a trait based on a given scenario -I can use a cross to predict the genotype/phenotype ratio of potential offspring -I can ‘work backwards’ to determine the parents of a given offspring.

3. Key Words Allele Homozygous Heterozygous Dominant Recessive Genotype
Phenotype
Cross
Purebred
Hybrid
Probability
Ratio
Offspring
Key Words

4. An EOCT Problem might look like this…
Which of the following describes a phenotype? A Gg B homozygous C having freckles D heterozygous

5. Comparing Genotypes: BB Homozygous: -Same alleles -Purebred bb
Heterozygous:
-Different alleles
-Hybrid Bb
For Complete Dominance:
A heterozygous genotype will code for the DOMINANT phenotype

6. Homozygous vs Heterozygous Practice
For each genotype below, indicate whether it is a heterozygous (He) OR homozygous (Ho).
TT _____ Bb _____ DD _____ Ff _____ tt _____ dd _____
Dd _____ ff _____ Tt _____ bb _____ BB _____ FF _____

7. Using Genotype to Determine Phenotype
To be successful at this, you have to know which trait is DOMINANT!
If it is a Complete Dominance example:
Heterozygous genotype = Dominant phenotype
If it is a Incomplete Dominance example:
Heterozygous genotype = Blend of the two traits
If it is a Codominant example:
Heterozygous genotype = Spotted/patches of both traits

8. Think: What is the Heterozygous phenotype?
Complete Dominance
RR=red flower
rr= white flower
Incomplete Dominance
RR= red flower
Codominance
RR= Red flower
R’R’= white flower
Rr=
Rr=
RR’=

9. Punnett squares let you predict the genotypes of the children

10. If you are looking at a single trait, you must use a single letter from the alphabet!
Ex: eye color = letter b

11. Baby Steps to Solving Punnett Squares
BABY STEPS: 1. determine the genotypes of the parent organisms
*For long scenarios, make a T-chart to jot down notes about each parent

12. Green peas (G) are dominant to yellow peas (g) in pea plants
Green peas (G) are dominant to yellow peas (g) in pea plants. A heterozygous pea plant is crossed with a yellow pea plant.

13. BABY STEPS: 2. write down your "cross" (mating):
__________ x____________

14. Baby Steps #3
Draw a punnett-square

15. Baby Steps #4
“Split" the letters of the genotype for each parent & put them "outside" the p-square (This represents the process of Meiosis and the potential GAMETES!*) Tt x tt

16. Baby Steps #5
Determine the possible genotypes of the offspring by filling in the p-square

17. Baby Steps #6
Summarize results in a KEY *letters will vary depending on problem Genotypes Ratio Phenotype Ratio

18. Shortcut #1
Homozygous Dominant + Homozygous recessive = 100% heterozygous
TT x tt
100 % Tt
(100% dominant phenotype)

19. Shortcut #2: Heterozygous Cross (1:2:1 Rule)
If you cross TWO HETEROZYGOUS, you will always get…
1 homozygous dominant (BB)
2 heterozygous (Bb)
1 homozygous recessive (bb)
Bb x Bb

20. Shortcut #3: Half and Half
Homozygous crossed with heterozygous =
HALF AND HALF
hh x Hh
-50 % hh
-50% Hh

21. Punnett Square Practice Problem
In elephants, the gene for long trunks is dominant over the gene for short trunks. A mom who is homozygous recessive is crossed with a heterozygous dad. What are the genotypes and phenotypes of their offspring?