1. Section 6-5 Part 1 Punnet Squares

2. The same gene can have many versions
A gene is a piece of DNA that tells a cell to make a certain protein
Locus - a specific position
on a pair of
homologous chromosomes 2

3. Allele is an alternative form of a gene
Each parent donates one allele for every gene
Pea plants have 2 alleles for shape - they receive one from each parent
For shape, there is a wrinkled allele and a round allele 3

4. Dominant vs. Recessive
Dominant traits: the ones you can see –cover up other traits
Recessive traits: are hidden by dominant “disappear”
Rule of Dominance: When a dominant trait is present the recessive trait is hidden

5. Phenotype: The way an organism looks
No matter what genes are present, phenotype of a tall pea plant is tall and a short pea plant is short

6. Genotype: The gene combination of an organism It consists of 2 alleles
For example:
Pure dominant, 2 dominant genes
Pure recessive, 2 recessive genes
Hybrid, 1 dominant and recessive gene

7. When the 2 alleles are the same
Homozygous:
When the 2 alleles are the same
2 dominant genes = Homozygous dominant
2 recessive genes = Homozygous recessive

8. 1 dominant gene and 1 recessive gene
Heterozygous:
When the 2 alleles for
a trait are different
It’s a hybrid
1 dominant gene and 1 recessive gene

9. Punnet Square time!!!

10. Punnett Squares
Punnett squares allow you to predict the ratios, or percentages of offspring possible in a cross.

11. Lettering Dominant Genes Recessive Genes Always get capital letters.
The letters will be the 1st letter of the trait.
Recessive Genes
Always get lower case letters.
The letter used is the same as the one used for the dominant gene.

12. Punnett Squares All problems will consist of 5 steps:
Labeling dominant and recessive letters
The Cross- Who are the parents
The Square- draw and fill in the square
Genotype ratios or percentages
Phenotype ratios or percentages

13. Example Problem: Red flowers are dominant over purple flowers in a certain type of plant. What will the possible genotypes and phenotypes of the offspring of a hybrid red flower and a purple flower? There are five steps to this problem

14. Step 1: R = Red flower r = Purple flower Label the dominant
Red flowers are dominant over purple flowers in a certain type of plant.
Label the dominant
and recessive traits
R = Red flower
r = Purple flower

15. Write the genotypes of each
Step 2:
Write the genotypes of each
parent to be crossed
Heterozygous Red
(means Hybrid)
Homozygous recessive Purple
(since it is recessive it has to be homozygous) Rr rr
Rr x rr
P1 generation: _______ _______

16. Rr rr R r R r r r r r

17. Set up the punnett square.
Step 3:
Set up the punnett square.
Each parent will give up one gene or the other so there will be one gene per box for each parent
R r r
Rr rr

18. Step 4: Analyze the genotypes
Rr rr
R r r 2 50
How many are heterozygous? ______ (_____%)
How many are homozygous dominant? ______ (_____%)
How many are homozygous recessive?______ (_____%) 2 50

19. Step 5: Analyze the phenotypes
Rr rr
R r r
50% of the offspring
will be red
50% of the offspring
will be purple

20. Practice Problem # 1
Tall plants are dominant over short plants in a tomato plant. What will the possible genotypes and phenotypes of the offspring of a Homozygous Tall plant and a Homozygous short plant?

21. Step 1: T = Tall plant t = short plant Label the dominant
and recessive traits
T = Tall plant
t = short plant
Remember, it’s NOT T and S

22. Write the genotypes of each
Step 2:
Write the genotypes of each
parent to be crossed
Homozygous Tall
(homo means same)
Homozygous Short
(homo means same) TT tt
TT x tt

23. Set up the punnett square. Write the parents first
Step 3:
Set up the punnett square.
Write the parents first
T T t
Tt Tt

24. Step 4: Analyze the genotypes
Tt Tt
T T t
4 of the 4 (100%) offspring will be heterozygous (or hybrid) for plant height

25. Step 5: Analyze the phenotypes
Tt Tt
T T t
100% of the offspring
will be Tall
because having at least one T makes it tall because T is dominant over short (t)

26. Practice Problem # 2
Let’s say Brown eyes are dominant over Green eyes. What will the possible genotypes and phenotypes of the offspring of a
Heterozygous Brown eyed person and a
Heterozygous Brown eyed person?

27. Step 1: B = Brown eyes b = green eyes Label the dominant
and recessive traits
B = Brown eyes
b = green eyes
Remember, it’s NOT B and G

28. Write the genotypes of each
Step 2:
Write the genotypes of each
parent to be crossed
Heterozygous Brown
Heterozygous Brown
(hetero means different) Bb Bb
Bb x Bb

29. Set up the punnett square. Write the parents first
Step 3:
Set up the punnett square.
Write the parents first
B b B b
BB Bb
Bb bb

30. Step 4: Analyze the genotypes
1 of the 4 (25%) offspring will be homozygous Dominant
2 of the 4 (50%) offspring will be heterozygous
1 of the 4 (25%) offspring will be homozygous Recessive
BB Bb
Bb bb
B b B b

31. Step 5: Analyze the phenotypes
3 of the 4 (75%) offspring will be Brown
1 of the 4 (25%) offspring will be Green
BB Bb
Bb bb
B b B b

32. Testcross A testcross is a cross between an
organism with an unknown genotype and an organism with the recessive phenotype.
The organism with the recessive phenotype must be homozygous recessive.
The offspring will show whether the organism with the unknown genotype is heterozygous, as above, or homozygous dominant.