1. Notes #24 I can: Predict the offspring of two parents
Punnett Squares
Notes #24
I can:
Predict the offspring of two parents

2. How can we predict what the offspring of two parents look like?

3. How can we predict what the offspring of two parents look like?

4. How can we predict what the offspring of two parents look like?

5. The principles of probability can be used to predict the outcomes of genetic crosses.
What is the probability of landing on heads both times if you flip a penny twice?

6. What is the probability of landing on heads both times if you flip a penny twice?
First we must figure out all the possible combi-nations of coin flips.
H/T
Which combos meet our criteria?
¼ = 25% T H H T H T T H

7. How to complete a Punnett Square:
The allele combinations that might result from a cross can be determined by drawing a Punnett Square.
father = Bb, mother = bb
How to complete a Punnett Square: B b
1. Draw the Punnett Square.
2. Place the father's alleles on the top of the Punnett square, one letter per box. B b b b b
3. Place the mother's alleles on the left side of the square, one letter per box. B b b b b
4. Drop the father's alleles down into the squares and bring the mother's across.
Now you have all possible allele combinations for the offspring.

8. Solving Genetics Word Problems
The steps:
The problem: Black fur in guinea pigs is dominant over white fur. If two heterozygous guinea pigs are crossed, what percentage of their children will be black?
1. Read the problem. Underline the known alleles.
2. Choose a letter to represent the alleles in the problem.
3. Circle and/or write down the genotypes of the parents.
4. Set up and solve Punnett Square.
B = black (dominant)
b = white (recessive)
5. Highlight and answer what the question is asking.
parents: Bb x Bb
75% black

9. Step 5: Highlight and answer what the question is asking.
The problem: Black fur in guinea pigs is dominant over white fur. If two heterozygous guinea pigs are crossed, what is the genotypic ratio of their offspring?
genotypic ratio = the proportion of offspring with each genotype
BB =
1/4 =
25%
There is a 1:2:1 ratio of genotypes
Bb =
2/4 =
50%
bb =
1/4 =
25%

10. Step 5: Highlight and answer what the question is asking.
The problem: Black fur in guinea pigs is dominant over white fur. If two heterozygous guinea pigs are crossed, what is the phenotypic ratio of their offspring?
phenotypic ratio = the proportion of offspring with each phenotype
black =
3/4 =
75%
There is a 1:3 ratio of white to black offspring.
white =
1/4 =
25%
Phenotypic ratios only care about what the offspring look like!

11. Dihybrid Cross
What happens if we have two genes that are close together on the chromosome? We perform a DIHYBRID CROSS! That’s SIXTEEN SQUARES instead of four! They are said to be LINKED.

12. Dihybrid Cross Example
A pea plant is heterozygous for both seed shape and seed color. S is the allele for the dominant, spherical shape characteristic; s is the allele for the recessive, dented shape characteristic. Y is the allele for the dominant, yellow color characteristic; y is the allele for the recessive, green color characteristic. What is the genotypic ratio of the offspring?

13. Dihybrid Cross Example

14. Punnett Square Practice
Alleles?
Parental genotypes?
Answer?