studying genetics to make the perfect plant

Noble Research Institute, LLC • 2510 Sam Noble Parkway • Ardmore, OK 73401 • www.noble.org • 580-223-5810

An education and outreach program of:

TEACHER GUIDE

LESSON OVERVIEW:Biologists have known for a long time that heredity is associated with the nucleus of cells and in particular with the passing on of chromosomes. Plant breeders have taken advantage of this feature of heredity to help improve plants and create new varieties. The fact that even plants have parents means that one or more desired characteristics or traits from one plant can be crossed with desir-able traits from another.

ESSENTIAL QUESTION:How are traits passed from generation to generation?

TOPICAL ESSENTIAL QUESTION:How does the genotype of an organism affect its phenotype?

SAFETY PRECAUTIONS:No specific safety precautions required.

MATERIALS:• 14 Paper sandwich/lunch bags of

two different colors (example: sev-en white bags, seven brown bags)

• Multiple copies of uppercase and lowercase alleles (see Appendix 1)

TOTAL DURATION:10 min. pre-lab prep time; 40 min. class time

LESSON OBJECTIVES:Students will be able to:

1. Understand the relationship between genes and alleles.

2. Understand the difference be-tween phenotype and genotype.

3. Understand what it means for an organism to be homozygous or heterozygous for a particular trait.

4. Understand traits can be inher-ited in dominant and recessive ways.

5. Understand the use of a Punnett square to predict the outcome of a specific cross.

PLANTS HAVE PARENTS TOO!Studying Genetics to Make the Perfect Plant


TEACHER GUIDE


Middle School Grades 6-8:MS-LS1-5: Students who demonstrate un-derstanding will be able to:Construct a scientific expla-nation based on evidence for how environmental and genetic factors influence the growth of organisms.

MS-LS3-2:Students who demonstrate un-derstanding will be able to:Develop and use a model to de-scribe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation.

MS-LS4-4:Students who demonstrate un-derstanding will be able to:

Construct an explanation based on evidence that describes how genetic variations of traits in a population increase an individu-al’s probability of surviving and reproducing in a specific environ-ment.

MS-LS4-5:Students who demonstrate un-derstanding will be able to:Gather and synthesize informa-tion about the technologies that have changed the way humans influence the inheritance of de-sired traits in organisms.

Biology:HS-LS3-1:Students who demonstrate un-derstanding will be able to:Ask questions to clarify relation-ships about the role of DNA and

chromosomes in coding the in-structions for characteristic traits passed from parents to offspring.

HS-LS3-3:Students who demonstrate un-derstanding will be able to:Apply concepts of statistics and probability to explain the vari-ation and distribution of ex-pressed traits in a population.

HS-LS4-3:Students who demonstrate un-derstanding will be able to:Apply the concepts of statis-tics and probability to support explanations that organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait.

Science and Engineering Practices:1. Asking questions

2. Developing and using models

3. Planning and carrying out investigations

4. Analyzing and interpreting data

5. Using mathematics and computational thinking

6. Constructing explanations and designing solutions

7. Engaging in argument from evidence

8. Obtaining, evaluating and communicating evidence

Crosscutting Concepts:1. Patterns

2. Cause and Effect: Mechanisms and explanations

3. Scale, Proportion and Quantity

4. Systems and System Models

5. Energy and Matter: Flows, cycles and conservation

6. Structure and Function

7. Stability and Change

KEY VOCABULARY: Gene Allele Homozygous Heterozygous Chromosome Dominant Recessive Selective Breeding Gamete Traits Genotype Phenotype Locus

STANDARDS


TEACHER GUIDE


LAB BACKGROUND INFORMATION:NOTE: This is background information for the teacher to assist in facilitating learning. A modified version is included in the Student Guide.

Ever since people started to look at the world around them, they have puzzled and wondered about heredity. Why do the offspring of all living things – whether it’s bullfrogs, dogs, humans or oak trees – always resemble their parents? Why do you perhaps have your mother’s eyes or your father’s chin or even grandpa’s nose? Biologists have known for a long time that heredity is associated with the nucleus of cells and in particular with the passing on of chromosomes. Plant breeders have taken advantage of this feature of heredity to help improve plants and create new varieties. The fact that even plants have parents means that one or more de-sired characteristics or traits from one plant can be crossed with desirable traits from another. This process, called selective breeding, leads to offspring with many possible beneficial characteristics, ultimately creating a new variety. Almost every agricultural crop on the market today is the result of this selective breeding pro-cess. What traits would you choose to mix if you were a plant breeder? With which crops would you work?

Corn, also known as maize (Zea mays), is the most widely grown crop in the USA. It is a large grain crop first domesticated 10,000 years ago in Mexico. Explorers and traders carried many varieties of corn back to Europe and introduced it to other countries. Corn spread to the rest of the world because of its ability to grow in diverse climates. Corn plants are diploid, which means they carry two sets of homologous (match-ing) chromosomes or 10 pairs for a total of 20 chromosomes. One set comes from the female parent and is carried in the egg cell. The other set comes from the male parent and is carried in the sperm nucleus. Corn plants reproduce sexually, which means male structures of the plant known as “tassels” produce pollen containing sperm and female structures of the plant know as styles or “silks” lead to an egg positioned on the ear of the plant that will become the seed. These gametes have half the genetic information of the par-ents (haploid) and when fused will produce a seed with a complete set of 20 chromosomes (diploid). Each chromosome has hundreds of genes; since there are two copies of each chromosome, one from each parent, there are two copies of every gene found in the plant. Alternate forms of a gene are called alleles, and there are different alleles for different genes. The combination of alleles an organism inherits from its parents is called its genotype; these determine the plant’s physical characteristics, also known as its phenotype.

The branch of genetics which deals with traits and their inheritance is referred to as Mendelian genetics, in recognition of the work of Gregor Mendel, a monk born in 1822. Mendel crossed pea plants showing one form of a trait with pea plants showing the other form, for example, tall plants crossed with short, purple-flowered plants crossed with white-flowered plants. Mendel observed the trait’s appearance in the following genera-tions. He observed in every case he studied that one of the traits could not be seen in the next generation. For example, the offspring of the tall plant crossed with a short plant were all tall. Mendel called these traits that appeared in the next generation (F1 generation) dominant traits. The trait that was hidden was called a recessive trait. When the plants of the F1 generation were allowed to self-pollinate, the recessive trait reap-peared in the next generation (F2 generation) in the ratio of three dominant to one recessive (see Fig. 1). Today biologists represent a dominant allele with a capital letter and a recessive allele with a lowercase letter. If an organism has two copies of the same allele, for example, AA or aa, it is homozygous for that trait. If the organism has one copy of two different alleles, for example, Aa, it is heterozygous. Keep in mind that a whole organism can't simply be “homozygous,” or “heterozygous.” For each of the organism's thousands of genes, it is either homozygous or heterozygous at a genetic position on the chromosome, known as a locus. This means genetics can get complicated, but, for a plant breeder, it also means the possibilities of plant varieties are endless!


TEACHER GUIDE


So how do plant breeders make predictions and keep track of all those possible traits and inheritance patterns? They use Punnett squares to show the possible gene combinations resulting from crossing specific genotypes. Using this information, mathematical probabilities can be calculated to determine the possible genotypes and phenotypes of the offspring. Figure 1 shows the resulting gene combinations from a cross between two Tall hetero-zygous (Tt) pea plants. The Punnett square shows 25 per-cent or one out of four plants would be expected to have a TT genotype, 50 percent (two out of four) would have the Tt genotype, and 25 percent (one out of four) would have the tt genotype. The Punnett square also shows that 75 percent of three out of four would have the Tall phe-notype and 25 percent (one out of four) would have the short genotype since Tall is the dominant trait.

Just like Mendel’s pea plants, corn plants lend themselves to crossing and selective breeding; because there are so many naturally occurring varieties, the combination of traits is almost endless. Farmers value traits that may increase yield and make plants easier and cheaper to grow. Figure 2 shows seven potential corn traits a plant breeder may be interested in studying to generate corn varieties with combinations of these traits.

Figure 2. Corn traits, alleles, and the effect on the plant and importance of those traits to the farmer.

Trait Allele Effect/Importance

Root length Long roots Better use of water and resistant to drought

Short roots May require irrigation and is susceptible to drought

Plant height Tall plants Better pollination, greater yield, but requires more water

Dwarf plants Reduced pollination, easier harvesting, require less water

Kernel number High kernel # per cob Improved yield provides more income per acre

Low kernel # per cob Lower yield causes reduced income per acre

Season length Short day flowering Flower early in season, increase cob production

Long day flowering Flower late in season, fewer cobs, more leaves

Leaf width Wide leaves Increased photosynthesis but requires more water

Narrow leaves Less photosynthesis but requires less water

Chemical synthesis Makes DIMBOA Naturally produced chemical that protects against bacteria, fungi and insects

No DIMBOA Plant has to be sprayed to protect from pests

Phosphate use Efficient use Requires less fertilizer, reduces cost per acre

Inefficient use Requires more fertilizer, increases cost per acre

Fig. 1. Punnett Square of heterozygous pea plant cross

Tt

Tt

T t

T

t

TT 25% Tt 25%

Tt 25% tt 25%


TEACHER GUIDE


ENGAGE:Show the students several varieties of the same type of flower, such as roses, carnations or petunias. Ask them to construct a list of how they are similar and how they are different. Invite the students to share their observations and discuss why the same type of flower can have different characteristics.

Show the students a list of some classic dominant-recessive human inheritance traits such as attached and unattached earlobes, hitchhiker’s thumb, rolling the tongue, or blue eyes versus brown eyes, and ask for a show of hands to see who does or doesn’t have these traits. Record numbers on a chart or whiteboard.

Discuss the idea of whether dominant means “common” and recessive means “rare.” Many students may think recessive means rare. Ask the students to show their hands and count their fingers. Five fingers is a recessive trait, however, it is very “common.” Explain that only one in every 500 people has six fingers (polydactylism). Explain that dwarfism is a dominant trait, but it is very “rare” with only one in 50,000 people having that condition.

IntroductionIn this activity, you will selectively breed a new corn plant variety. You will simulate fertilization, the process by which the parent’s genes are passed on to the offspring by selecting one allele for each of the seven dif-ferent traits from each parent and combining them.

EXPLORE:Activity 1: Parent gamete probability

1. Assuming the parent plants in this activity are heterozygous (Aa) for all traits, write in the circles the possible alleles carried in the gametes.

2. Using the gamete information, complete the Punnett square.

3. Count and record the number of AA, Aa and aa offspring that are possible.

Female (Aa)Produces 2 possible gametes

Mal

e (A

a)P

rod

uces

2 p

oss

ible

g

amet

es

F1 Genotype #:

AA __________

Aa __________


TEACHER GUIDE


MOTHERChromosome

1Root Length

MOTHERChromosome

2Plant Height

MOTHERChromosome

3Kernal

Number

MOTHERChromosome

4SeasonLength

MOTHERChromosome

5Leaf Width

MOTHERChromosome

6ChemicalSynthesis

MOTHERChromosome

7Phosphate

Use

FATHERChromosome

1Root Length

FATHERChromosome

2Plant Height

FATHERChromosome

3Kernal

Number

FATHERChromosome

4SeasonLength

FATHERChromosome

5Leaf Width

FATHERChromosome

6ChemicalSynthesis

FATHERChromosome

7Phosphate

Use

Activity 2: Selective breeding the F1 generation

Teacher Preparation Instructions:1. Label seven brown paper bags with chromosome numbers 1-7 and the appropriate trait carried by that

chromosome (see Table 1). These seven bags will represent the seven “Mother” chromosomes and seven traits.

2. Label seven white paper bags with chromosome numbers 1-7 and the appropriate trait carried by that chromosome (see Table 1). These seven bags will represent the seven “Father” chromosomes and seven traits.

3. Make two copies of the Appendix 1 allele cards in two different colors.

4. Cut along the dotted lines, placing one set of the R and r alleles into the Mother bag labelled Chromo-some 1 “Root length.”

5. Cut along the dotted lines of the second set of R and r alleles, placing them into the Father bag labelled Chromosome 1 “Root length.”

6. Repeat steps 4 and 5 with the remaining six allele sets, placing them in the appropriately labelled Mother- and Father-labelled trait bags.

7. Place the “Mother” chromosome bags on one side of the room and the “Father” chromosome bags on the other side.


TEACHER GUIDE


1. Select one allele from each of the Mother corn plant’s seven chromosome bags at the front of the class. Bring the seven alleles back to your station and record the symbol in Table 1.

2. Select one allele from each of the Father corn plant’s seven chromosome bags at the front of the class. Bring the seven alleles back to your station and record the symbol in Table 1.

3. Match and combine the 14 alleles collected from the female and male parent, and write the genotype of your offspring corn plant in Table 1, writing the capital letter first.

Table 1. Alleles from selected gametes

Chromosome Trait Symbol Allele (Mother) Allele (Father) Genotype (offspring)

1 Root length R or r

2 Plant height H or h

3 Kernal # N or n

4 Season length F or f

5 Leaf width M or m

6 Chemical synthesis Q or q

7 Phosphate use P or p

4. Use the genotype decoder chart (Table 2) to decode your corn plant’s genotype into a phenotype, and record the results in Table 3.


TEACHER GUIDE


Table 2. Genotype decoder

Chromosome Trait Genotype Phenotype

1 Root length RRRrrr

Long rootsLong rootsShort roots

2 Plant height HHHhhh

Tall plantsTall plantsDwarf plants

3 Kernel # NNNnnn

High kernel numberHigh kernel numberLow kernel number

4 Season length FFFfff

Short day flowering plantShort day flowering plantLong day flowering plant

5 Leaf width MMMmmm

Wide leavesWide leavesNarrow leaves

6 Chemical synthesis QQQqqq

Disease resistantDisease resistantNot disease resistant

7 Phosphate use PPPppp

Efficiently uses soil phosphorusEfficiently uses soil phosphorusInefficiently uses phosphorus

Table 3. Corn plant phenotype results


1 Rooth length

2 Plant height

3 Kernel #

4 Season length

5 Leaf width

6 Chemical synthesis

7 Phosphate use


TEACHER GUIDE


Activity 3: Draw the new corn variety1. Using the data collected and your understanding of the seven different corn plant traits selected, draw

a picture of the corn plant your selective breeding technique has generated in Table 4. Make sure your diagram clearly displays and/or labels the phenotypes expressed.

Table 4. Diagram of "New Variety" Corn Plant

Traing Phenotype Drawing of selectively bred corn plant

Root length

Plant height

Kernel #

Season length

Leaf width

Chemical synthesis

Phosphate use

Activity 4: Punnett square analysis1. Choose any one of the traits you bred into your plant and ask another group to share the genotype data

of their plant for the same trait.

Trait: _____________ Group 1 genotype: _______ Group 2 genotype: _______

2. Construct a Punnett square below to represent the outcome of the cross if you and the other group selectively bred your two plants to create the next generation (F2).


TEACHER GUIDE


3. List the possible phenotypes and probabilities of the plants generated from your cross of that trait below.

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

Teacher Note: Student answers will vary depending on the alleles selected. Use Table 2 to check for correct decoding of genotype to phenotype

EXPLAIN: (SEE LAB BACKGROUND)The Lab Background Information from the Teacher Guide is repeated in the Explain section of the Student Guide.

ELABORATE:Optional Extension ActivitiesPurchase the following kits from Carolina Biological (www.carolina.com) that demonstrate corn genetics and allow students an opportunity to look at simple Mendelian genetics in action. Choose lab kits or basic refills that allow students to observe monohybrid and dihybrid crosses. With these kits, students can be introduced to the concept of observed versus expected genotype ratios. Mendelian Genetics of Corn Kit cat#176360 Corn Segregating Ear, R Color Alleles 3:1 cat#176500 Genetic Corn Seed, Green:Albino cat#177130 Wisconsin Fast Plants 72-hour Dihybrid Genetics Kit cat#158939

EVALUATE:1. You have investigated seven chromosomes with seven traits in this activity. Each parent corn plant actual-

ly has a total of 20 chromosomes with hundreds of traits. How many chromosomes would be present in a corn gamete? 10 chromosomes

2. How many chromosomes would be in each F1 generation plant? 20 chromosomes

3. List the homozygous genotypes in your new corn variety. Answers will vary

4. List the heterozygous genotypes in your new corn variety. Answers will vary

5. Why does the F1 plant have two letters (alleles) for each trait? The plant has inherited two alleles for each trait, one allele from the father and one allele from the mother


TEACHER GUIDE


6. What are gametes? Gametes are sex cells, sperm and egg, and carry half the genetic information of the parent

7. How much genetic information is found in a gamete? Half of the genetic information is found in a

gamete

8. What genetic traits did your new corn variety gain which are beneficial to the plant’s growth and develop-ment? Answers will vary

9. Did your corn plant inherit any traits which are not beneficial to the plant’s growth and development? Answers will vary

10. Rainfall amounts have dropped over the last four years, leaving your area in a drought condition. What phenotypes would have the greatest advantage in this environment? What genotypes would the parents need to have in order for this offspring to be possible? Please explain and use diagrams if necessary.

Answers may include: Long roots, short plants, narrow leavesParent genotypes would include: Long roots - RR or Rr

Short Plants - hh Narrow leaves - mm


TEACHER GUIDE


Appendix 1. Allele template (make two copies on different colored paper.)

R R R R R R R R R R

r r r r r r r r r r

H H H H H H H H H H

h h h h h h h h h h

N N N N N N N N N N

n n n n n n n n n n

F F F F F F F F F F

f f f f f f f f f f

M M M M M M M M M M

m m m m m m m m m m

Q Q Q Q Q Q Q Q Q Q

q q q q q q q q q q

P P P P P P P P P P

p p p p p p p p p p

Noble Research Institute would like to thank the following people for their contributions to this lesson:• Quentin Biddy • Susie Edens • Kay Gamble • Janie Herriott • Fiona McAlister



STUDENT GUIDE

MATERIALS:• Seven alleles collected from each

parent corn plant genome

LESSON OBJECTIVES:You will be able to:

1. Understand the relationship be-tween genes and alleles.

2. Understand the difference be-tween phenotype and genotype.

3. Understand what it means for an organism to be homozygous or heterozygous for a particular trait.

4. Understand traits can be inher-ited in dominant and recessive ways.

5. Understand the use of a Punnett square to predict the outcome of a specific cross.

LESSON OVERVIEW:Biologists have known for a long time that heredity is associated with the nucleus of cells and in particular with the passing on of chromosomes. Plant breeders have taken advantage of this feature of heredity to help improve plants and create new varieties. The fact that even plants have parents means that one or more desired characteristics or traits from one plant can be crossed with desir-able traits from another.

ESSENTIAL QUESTION:How are traits passed from generation to generation?

TOPICAL ESSENTIAL QUESTION:How does the genotype of an organism affect its phenotype?

SAFETY PRECAUTIONS:No specific safety precautions required.

PLANTS HAVE PARENTS TOO!Studying Genetics to Make the Perfect Plant


STUDENT GUIDE


KEY VOCABULARY:Gene Allele Homozygous HeterozygousChromosome Dominant Recessive Selective Breeding Gamete Traits Genotype PhenotypeLocus

ENGAGE:Observe the plants or images provided by your teacher of several varieties of the same type of flowers, such as roses, carnations or petunias. Construct a list of how they are similar and how they are different.

Similar Different

Observe and check whether you have the dominant or recessive human inheritance traits list below:

Dominant Recessive___ attached earlobes ___ unattached earlobes, ___ hitchhiker’s thumb ___ straight thumb___ able to roll tongue ___ cannot roll tongue___ brown eyes ___ blue eyes

What do you think the terms dominant and recessive mean?

Dominant: _______________________________________________________

Recessive: ________________________________________________________

INTRODUCTION:In this activity, you will selectively breed a new corn plant variety. You will simulate fertilization, the process by which the parent’s genes are passed on to the offspring by selecting one allele for each of the seven dif-ferent traits from each parent and combining them.


STUDENT GUIDE


EXPLORE:Activity 1: Parent gamete probability

1. Assuming that the parent plants in this activity are heterozygous (Aa) for all traits, write in the circles the possible alleles carried in the gametes.

2. Using the gamete information, complete the Punnett square.

3. Count and record the number of AA, Aa and aa offspring that are possible.

Activity 2: Selective breeding the F1 generation1. Select one allele from each of the Mother corn plant’s seven chromosome bags at the front of the

class. Bring the seven alleles back to your station and record the symbol in Table 1.

2. Select one allele from each of the Father corn plant’s seven chromosome bags at the front of the class. Bring the seven alleles back to your station and record the symbol in Table 1.

3. Match and combine the 14 alleles collected from the female and male parents, and write the geno-type of your offspring corn plant in Table 1, writing the capital letter first.

Female (Aa)Produces 2 possible gametes

Mal

e (A

a)P

rod

uces

2 p

oss

ible

g

amet

es

F1 Genotype #:

AA __________

Aa __________


STUDENT GUIDE


Table 1. Alleles from selected gametes

Chromosome Trait Symbol Allele (Mother) Allele (Father) Genotype (offspring)

1 Root length R or r

2 Plant height H or h

3 Kernal # N or n

4 Season length F or f

5 Leaf width M or m

6 Chemical synthesis Q or q

7 Phosphate use P or p

4. Use the genotype decoder chart (Table 2) to decode your corn plant’s genotype into a phenotype, and record the results in Table 3.

Table 2. Genotype decoder


1 Root length RRRrrr

Long rootsLong rootsShort roots

2 Plant height HHHhhh

Tall plantsTall plantsDwarf plants

3 Kernel # NNNnnn

High kernel numberHigh kernel numberLow kernel number

4 Season length FFFfff

Short day flowering plantShort day flowering plantLong day flowering plant

5 Leaf width MMMmmm

Wide leavesWide leavesNarrow leaves

6 Chemical synthesis QQQqqq

Disease resistantDisease resistantNot disease resistant

7 Phosphate use PPPppp

Efficiently uses soil phosphorusEfficiently uses soil phosphorusInefficiently uses phosphorus


STUDENT GUIDE


Table 3. Corn plant phenotype results


1 Rooth length

2 Plant height

3 Kernel #

4 Season length

5 Leaf width

6 Chemical synthesis

7 Phosphate use

Activity 3: Draw the new corn variety1. Using the data collected and your understanding of the seven different corn plant traits selected, draw

a picture of the corn plant your selective breeding technique has generated in Table 4. Make sure your diagram clearly displays and/or labels the phenotypes expressed.

Table 4. Diagram of "New Variety" Corn Plant

Traing Phenotype Drawing of selectively bred corn plant

Root length

Plant height

Kernel #

Season length

Leaf width

Chemical synthesis

Phosphate use


STUDENT GUIDE


Activity 4: Punnett square analysis1. Choose any one of the traits you bred into your plant and ask another group to share the genotype data

of their plant for the same trait.

Trait: _____________ Group 1 genotype: _______ Group 2 genotype: _______

2. Construct a Punnett square below to represent the outcome of the cross if you and the other group selectively bred your two plants to create the next generation (F2).

3. List the possible phenotypes and probabilities of the plants generated from your cross of that trait below.

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

EXPLAIN:Ever since people started to look at the world around them, they have puzzled and wondered about he-redity. Why do the offspring of all living things – whether it’s bullfrogs, dogs, humans or oak trees – always resemble their parents? Why do you perhaps have your mother’s eyes or your father’s chin or even grand-pa’s nose? Biologists have known for a long time that heredity is associated with the nucleus of cells and in particular with the passing on of chromosomes. Plant breeders have taken advantage of this feature of he-redity to help improve plants and create new varieties. The fact that plants have parents means that one or more desired characteristics or traits from one plant can be crossed with desirable traits from another. This process, called selective breeding, leads to offspring with many possible beneficial characteristics, ultimate-ly creating a new variety. Almost every agricultural crop on the market today is the result of this selective breeding process.

Corn, also known as maize (Zea mays), is the most widely grown crop in the USA. It is a large grain crop that was first domesticated 10,000 years ago in Mexico. Explorers and traders carried many varieties of corn back to Europe and introduced it to other countries. Corn spread to the rest of the world because of its abil-ity to grow in diverse climates. Corn plants are diploid, meaning there are two copies of each chromosome,


STUDENT GUIDE


one from each parent. When the male pollen (gamete) and the female egg (gamete) join, each contribute 10 chromosomes for a total of 20. Each chromosome is comprised of hundreds of genes that control specific traits. Within the chromosomes, there are two copies of every gene. There are also alternate forms of the genes, called alleles. The combination of alleles an organism inherits from its parents is called its genotype; these determine the plant’s physical characteristics, also known as its phenotype.

Gregor Mendel crossed pea plants and showed that some alleles are inherited and expressed as dominant traits while others are recessive. For example, the offspring of the tall plant crossed with a short plant were all tall. Mendel called these traits that appeared in the next generation (F1 generation) dominant traits. The trait that was hidden was called a recessive trait. When the plants of the F1 generation were allowed to self-pol-linate, the recessive trait reappeared in the next generation (F2 generation) in the ratio of three dominant to one recessive (See Fig. 1). Today biologists represent a dominant allele with a capital letter and a recessive allele with a lowercase letter. If an organism has two copies of the same allele, for example, AA or aa, it is homozygous for that trait. If the organism has one copy of two different alleles, for example, Aa, it is hetero-zygous. Keep in mind a whole organism can't simply be “homozygous” or “heterozygous.” For each of the organism's thousands of genes, it is either homozygous or heterozygous at a genetic position on the chromo-some, known as a locus. This means genetics can get complicated, but, for a plant breeder, it also means the possibilities of plant varieties are endless!

Plant breeders make predictions and keep track of all those possible traits and inheritance patterns by using Punnett squares to show the possible gene combinations that could result when crossing specific genotypes. Using this information, mathematical probabilities can be calculated to determine the possible genotypes and phenotypes of the offspring.

Just like Mendel’s pea plants, corn plants lend themselves to crossing and selective breeding; because there are so many naturally occurring varieties, the combination of traits is almost endless. Farmers value traits that may increase yield and make plants easier and cheaper to grow. Figure 1 shows seven potential corn traits that a plant breeder may be interested in studying to generate corn varieties with combinations of these traits. What traits would you choose to mix if you were a plant breeder? What other crops would you work with?


STUDENT GUIDE


Figure 1. Corn traits, alleles, and the effect on the plant and importance of those traits to the farmer.

Trait Allele Effect/Importance

Root Length Long roots Better use of water and resistant to drought

Short roots May require irrigation and is susceptible to drought

Plant height Tall plants Better pollination, more yield, but requires more water

Dwarf plants Reduced pollination, easier harvesting, requires less water

Kernel number High kernal # Improved yield provides more income per acre

Low kernal # Lower yields cause reduced income per acre

Season length Short day flowering Flowers early in season, increased cob production

Long day flowering Flowers late in season, fewer cobs, more leaves

Leaf width Wide leaves Increased photosynthesis but requires less water

Narrow leaves Less photosynthesis but requires less water

Chemical synthesis Makes DIMBOA Naturally protects against bacteria, fungi and insects

No DIMBOA Plant has to be sprayed to protect from pests

Phosphate use Efficient use Requires less fertilizer, reduces cost per acre

Inefficient use Requires more fertilizer, higher cost per acre

EVALUATE:

Name: ____________________________________________________________

1. You have investigated seven chromosomes with seven traits in this activity. Each parent corn plant actu-ally has a total of 20 chromosomes with hundreds of traits. How many chromosomes would be present in a corn gamete?

_______________________________________________________________________________________

2. How many chromosomes would be in each F1 generation plant?

_______________________________________________________________________________________


STUDENT GUIDE


3. List the homozygous genotypes in your new corn variety.

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

4. List the heterozygous genotypes in your new corn variety.

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

5. Why does the F1 plant have two letters (alleles) for each trait?

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

6. What are gametes?

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

7. How much genetic information is found in a gamete?

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

8. What genetic traits did your new corn variety gain that are beneficial to the plant’s growth and develop-ment?

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________


STUDENT GUIDE


9. Did your corn plant inherit any traits that are not beneficial to the plant’s growth and development? If so, which one(s) and explain why.

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

10. Rainfall amounts have dropped over the last four years, leaving your area in a drought condition.

a) What phenotypes would have the greatest advantage in this environment?

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

b) What genotypes would the parents need to have in order for this offspring to be possible? Please explain and use diagrams if necessary.

_______________________________________________________________________________________

_______________________________________________________________________________________

_______________________________________________________________________________________

studying genetics to make the perfect plant

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