1. Selective Plant Breeding

2. Objectives Students will be able to:
Explain the heritability of traits using common genetic terminology
Predict the genotypes and phenotypes from monohybrid and dihybrid crosses by using the Punnett square.

3. List the traits of corn #1, #2 & #3

4. What could cause the differences between corn #1, #2 & #3?

5. Genetics! Genes determine the traits plants have.
When plants reproduce the offspring receives a copy of each gene from the male and female parent. This means the individual has 2 copies of each gene.
The different versions of the gene that the plant can receive are called alleles.
The two alleles make up the genotype of the plant

6. The genotype is the combination of alleles within a plant
The phenotype is the physical appearance of the plant

7. Simple Modes of Inheritance
Alleles can be dominant or recessive
Dominant alleles only need one copy of the allele to be present to be expressed. They are represented by a capital letter. (Ex. A)
Recessive alleles: must have two copies of the allele to be expressed. They are represented by a lower case letter. (Ex. a)

8. Homozygotes have two copies of the same allele. (Ex. AA)
Heterozygote have two different alleles (Ex. Aa)

9. What does this mean to a Plant Breeder?
Understanding genetics can help explain or predict plant traits!
Plant breeders use a tool called the Punnett Square to estimate the probability that a trait will occur.

10. Scenario
Wheat is one of the major field crops grown in the United States. It is used to make many breads and cereals. More foods are made out of wheat than any other grain.
Wheat seeds can be either red or white. Red seed color is dominant to white seed color.
What will happen when you cross red wheat with white wheat?

11. Using a Punnett Square A A a a
The genotype of the male plant is placed along the top of the square
The genotype of the female plant is placed along the left side of the square
Example:
Male genotype: AA (Red Wheat)
Female genotype: aa (White Wheat) A A a a

12. Using a Punnett Square A A a Aa Aa a Aa Aa
The alleles for the male genotype are written in the boxes below them
The alleles for the female genotype are written in the boxes across from them A A a Aa Aa a Aa Aa

13. Understanding the Punnett Square
The Punnett square calculates the expected ratio for a specific trait.
In Punnett crosses with a single trait, each box represents 25% A A a Aa Aa a Aa Aa

14. Determining the Phenotypic Ratio
What percentage of the offspring will be red wheat? 100%
What percentage of the offspring will be white wheat? 50% A A a Aa Aa a Aa Aa

15. Determining the Genotypic Ratio
What percentage of the offspring will be AA? 0%
What percentage of the offspring will be aa? 0%
What percentage of the offspring will be Aa? 100% A A a Aa Aa a Aa Aa

16. Using the Punnett Square to Calculate F2 Generation
The first cross between parents is called the F1. In the F1 cross all of the offspring were Aa, or heterozygotes for the red wheat trait.
The F2 generation uses the offspring produced in the F1 generation as parents.

17. Calculating the F2 Generation
The genotype of the male plant is placed along the top of the square
The genotype of the female plant is placed along the left side of the square
Example:
Male genotype: Aa (Heterozygous Red Wheat)
Female genotype: Aa (Heterozygous Red Wheat) A A A a

18. Determining the Phenotypic Ratio
What percentage of the offspring will be red wheat? 75%
What percentage of the offspring will be white wheat? 25% A a A AA Aa a Aa aa

19. Determining the Genotypic Ratio
What percentage of the offspring will be AA? 25%
What percentage of the offspring will be aa? 25%
What percentage of the offspring will be Aa? 50% A A a Aa Aa a Aa Aa

20. How a Plant Breeder Uses Punnett Squares
What are the two phenotypes for the wheat field?
What can observations of the plot explain?
What was the genotype of the F2 plants?
Is herbicide resistance dominant or recessive?
What percentage of the F2 offspring will be resistant to herbicide?
What is the genotype of the F3 parents?
How many generations did it take to create an herbicide resistant wheat crop?

21. Dihybrid Punnett Squares
Punnett Squares can be used to calculate the possibilities with numerous traits.
In order to predict the occurrence of two traits happening simultaneously, Punnett Square with 16 squares can be used.

22. Scenario
A plant breeder wants to create a wheat crop that is resistant to herbicides and is white in color. Both traits are recessive. The plant breeder is only able to find plants that have one of each of the desired traits.
By crossing the two parent plants, what percentage of the offspring will have both of the desired traits?

23. Dihybrid Punnett Squares
Male parent plant is white in color, but is not herbicide resistant: aaRR
Female parent plant is red in color, and is herbicide resistant: AArr
Find all possible allelic combinations for each of the parents.
Male: aR
Female: Ar

24. Dihybrid Punnett Squares
Across the top, write the male combinations
Male genotype (aaRR)
Combinations: aR
Down the left side, write the female combinations
Female genotype (AArr)
Combinations: Ar aR aR aR aR Ar
AaRr
AaRr
AaRr
AaRr Ar
AaRr
AaRr
AaRr
AaRr Ar
AaRr
AaRr
AaRr
AaRr Ar
AaRr
AaRr
AaRr
AaRr

25. Percentages of Possible Combinations
AARR: 0/16
AaRR: 0/16
aaRR: 0/16
AARr: 0/16
AaRr: 16/16
aaRr: 0/16
AArr: 0/16
Aarr: 0/16
aarr: 0/16 aR aR aR aR Ar
AaRr
AaRr
AaRr
AaRr Ar
AaRr
AaRr
AaRr
AaRr Ar
AaRr
AaRr
AaRr
AaRr Ar
AaRr
AaRr
AaRr
AaRr
What percentage of the offspring had both the desired traits?

26. Making the F2 Cross
Two individuals from the F1 generation are crossed to make the F2 generation
Male: AaRr
Female: AaRr

27. Dihybrid Punnett Squares
Across the top, write the male combinations
Male genotype (AaRr)
Combinations: AR, Ar, aR, ar
Down the left side, write the female combinations
Female genotype (AaRr) AR Ar aR ar AR
AARR
AARr
AaRR
AaRr Ar
AARr
AArr
AaRr
Aarr aR
AaRR
AaRr
aaRR
aaRr ar
AaRr
Aarr
aaRr
aarr

28. Percentages of Possible Combinations
AARR: 1/16
AaRR: 2/16
aaRR: 1/16
AARr: 2/16
AaRr: 4/16
aaRr: 2/16
AArr: 1/16
Aarr: 2/16
aarr: 1/16 AR Ar aR ar AR
AARR
AARr
AaRR
AaRr Ar
AARr
AArr
AaRr
Aarr aR
AaRR
AaRr
aaRR
aaRr ar
AaRr
Aarr
aaRr
aarr

29. What is the expected percentages for phenotypes?
AARR: 1/16
AaRR: 2/16
aaRR: 1/16
AARr: 2/16
AaRr: 4/16
aaRr: 2/16
AArr: 1/16
Aarr: 2/16
aarr: 1/16
Red & Not Herbicide Resistant: 9/16
Red & Herbicide Resistant: 3/16
White & Not Herbicide Resistant: 3
White & Herbicide Resistant: 1