Selective Plant Breeding

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.

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

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

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

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

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)

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

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.

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?

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

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

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

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

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

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.

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

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

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

How a Plant Breeder Uses Punnett Squares https://vimeo.com/57950005 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?

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.

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?

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

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

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?

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

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

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

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

http://agronomy.unl.edu/documents/EC%201278.pdf