Plant Breeding and Predicting Offspring Traits

By Patty Hain - croptechnology.unl.edu


Introduction
Kernel color in an easily observable trait. There are different alleles of the kernel color gene that code for different colored kernels. For example, one allele represented with Y (upper-case) encodes yellow colored kernels. Another allele for the color trait encodes white colored kernels represented with the y (lower-case).

In this lesson students will plant bicolor sweet corn (sweet corn with yellow and white kernels) which can be purchased at any garden store. While all the seeds they plant will be yellow, the kernels of corn on the ears they harvest will be observed with a ratio of 1 white: 3 yellow.

Students will use their observations to determine the genotype and phenotype of the original parents and how the traits are passed on to the offspring. Students will also learn how trait expression is affected by whether an allele is dominant or recessive and whether an organism is homozygous or heterozygous for that allele.

This experiment may be continued for a second generation. The kernels harvested from the first generation can be planted and grown. When the plants reach the proper stage, students will practice corn breeding techniques to cross (mate) plants. They will make a series of crosses between plants grown from white and yellow kernels and also make self crosses. Using Punnett squares students will predict the genotypic and phenotypic ratios for the offspring of each cross and compare their predictions with observed results at harvest.

 

 

Purpose:

The purpose of this activity is to demonstrate trait expression and how traits are passed from one generation to the next. Students will practice corn breeding techniques and use Punnett squares to predict the phenotypes and genotypes of offspring and their parents.

Objectives (Year 1)
Following completion of this lesson, the students will:
-identify the parts of a corn plant and how they reproduce.
-explain the process of corn breeding.
-demonstrate how traits are passed to offspring
-identify what gametes are and how their importance in sexual reproduction.
-define DNA, genes, alleles, and chromosomes and explain their function in controlling and passing on traits.
-explain how genes control traits, i.e. seed color.
-identify the genotype and phenotype of an organism.
-differentiate between dominant and recessive alleles
-differentiate between homozygous and heterozygous genotypes

Objectives (Year 2)
-predict the phenoptypic and genotypic ratios of offspring
-determine the genotype of parents based on observations of offspring.

Materials
1.
Seed for bicolor (yellow and white) kerneled sweet corn. Available at any garden store.
2. Soil
3. Flower pots (12 or larger)
4. Fertilizer
5.
Water
6.
Warm sunny location (a greenhouse is optimal)

7. Small paper bags
8. Stapler
9.
Marker
10.
Worksheet

Year 1
Methods

1.
Fill pots with soil up to 1 from the top of the pot. Plants may also be grown outside in a field setting. Check seed packet for specific planting instructions.
2. Poke 1 hole in soil in center of each pot approximately 1-1 deep (read seed packet for exact depth).
3. Drop one seed into each hole
4. Gently cover seeds with soil.
5. Record the date the corn was planted for completing the worksheet.
6. Water seeds until soil is moist
7. Allow plants to grow for several weeks fertilizing as recommended on fertilizer container. Water regularly (when soil is dry).
8. The rate of plant growth directly depends on the temperature of its environment. Corn needs a certain number of days to reach maturity, and this is based on how much heat the plants get. Warmer conditions may speed up plant growth and shorten the number of days necessary. Similarly, cooler conditions can lengthen the season. Check the seed packet when purchasing, and try to choose a variety with the shortest growing season to decrease the time it takes to complete the experiment.
9. Allow plants to grow to maturity.

When to pick corn:
After the plants begin to tassel, watch for when approximately half of the plants are showing silks and record this date. The field will be ready to pick about 18-22 days after the half silk date, depending upon temperature conditions (source: Earl May Flower & Vegetable Guide 2002). Kernels should be juicy yet firm.

***NOTE: If you plan to continue the experiment for a second year, or would like to be able to plant some of the kernels harvested, allow plants to reach full maturity before harvesting. The stalks should be dry and brown and kernels will be hard.

Analyzing observations:
Give each student or student group an ear and have them score and record the data using the worksheet provided.
Discuss observations and results.

Year 2 (optional)
Follow planting instructions outlined for year 1. Label each pot (or row if planted in a field) as to whether the seed planted was yellow or white. Make sure the labels stay with the plants throughout the entire season.

Corn Breeding:
After several weeks (time depends upon the corn variety) you will notice an ear shoot developing about halfway up the side of the plant. Cover the tip of the ear shoot before any silks emerge to prevent self-pollination. You will also notice a tassel beginning to emerge from the top of the plant.

 

 

 

 

Once the tassel begins to shed pollen place a paper bag over the tassel and grip/fold it closed over the base of the stem of the tassel to collect any pollen that falls within the bag. Shake the bag vigorously to loosen any remaining pollen from the tassel. Carefully bend the top of the stalk over and remove the bag of pollen. *The silks must be ready to pollinate (emerging from the ear). If they are not ready when the tassel begins to emerge, put a bag on the tassel to collect any pollen that falls. Staple the bag closed around the base of the tassel to keep the pollen within the bag and wait until silks are ready. When the silks have begun to emerge, shake the tassel bag to remove remaining pollen from the tassel and dump the bag of pollen onto the silk as outlined in the next step.

Three different crosses can be made to demonstrate the possible results.
Cross 1: yellow X yellow (self-cross)
Cross 2: yellow X white (out cross)
Cross 3: white X white (self-cross)

Determine the plant to be pollinated, remove the bag over the ear (silks should be at least partially emerged from the ear) and pour the pollen from the bag on the silks. Place the pollen bag on the ear and staple the base shut to protect the ear from being contaminated with other pollen. Write on the bag what cross was made (yellow X white).

For plants that are self-crossed or self-pollinated, pour the bag of pollen on the ear of the plant it came from. Label the plant as such. (yellow X yellow)

Continue to grow the plants to maturity.

When to pick corn:
Follow harvesting instructions for year 1
***IMPORTANT: label each ear with the cross that was made (the parents that were mated to make this offspring). A piece of masking tape wrapped around the ear with the cross written with a permanent marker works well.

Analyzing observations:
Give each student or student group an ear from each cross and have them score and record the data using the worksheet provided. Students should complete the the Year 2 section of the breeding lesson worksheet. Students and teachers should compare the predicted results with the observed results from the breeding exercise.

Additional Articles for Review
National Corn Growers Association Education resources http://www.ncga.com/education/guide/guide.html
Nebraska Cooperative Extension http://ianrpubs.unl.edu/insects/nf425.htm NF00-425 Revised November 2000.

Assessment
Students are assessed based on participation in the experiment and adequate completion of the worksheets.

 

 

 

 

 

 

WORKSHEET #1 Trait Inheritance - Observations

NAME_____________________________

 

 

Date planted: _______________________

 

What was the color of the seed planted (phenotype)?

 

A: yellow

 

Use the letter y to denote this gene. The dominant allele is Y. The recessive allele is y.

 

What are the possible genotypes of the seed for this trait?

 

A: YY, Yy, yy We do not know yet if the yellow allele is dominant or recessive.

 

From what is observed, can the genotype and phenotype of the parents that made this seed be determined yet? Why or why not?

 

A: No. Because we do not know if the yellow allele is dominant or recessive. Therefore we dont know the genotype of the offspring (seed planted) and cant determine the genotype or phenotype of the parents.

 

Half silk date: ____________________

 

Harvest date: _____________________

 

 

Collect and record data from a single ear:

 

How many yellow kernels?

 

A: Numbers will vary

 

How many white kernels?

 

A: Numbers will vary

 

What is the ratio of yellow to white kernels?

 

A: 3 yellow:1 white (may need to round or approximate to get exact ratio)

 

Which allele does this indicate is dominant, yellow or white?

A: yellow

 

Use the letter y to denote this gene. The dominant allele is Y. The recessive allele is y.

 

What would be the genotype(s) of the white kernels? Why?

 

A: yy. The white kernel color allele is recessive meaning two copies of the allele are necessary for expression of the trait.

 

Would the genotype(s) be homozygous or heterozygous for this trait?

 

A: homozygous

 

What would be the genotype(s) of the yellow kernels? Why?

 

A: YY and Yy. Since they are yellow in color, they obviously have a Y allele. However, the fact that some of the offspring were white, yy, indicates that the parents also had the white allele, y. Therefore, it is possible that some of the yellow kernels have a YY genotype and others have Yy.

 

Would the genotype(s) be homozygous or heterozygous for this trait?

 

A: some homozygous, some heterozygous

 

y

 
What would the genotypes of the parents be? Use a Punnett square to determine this based on the genotypes of the offspring.

 

y

 

 

y yy

 
A: We know that the white kernels are homozygous recessive. Therefore each of the parents had to donate a y allele. We can then

deduce that two of the other offspring (out of

four) also received one copy of the y allele.

 

 

 

 


y

 

 

 

 

 

 

 

 

 


y

 

Y

 

 


Y

 
We also know that he yellow kernels must each have a yellow

YY Yy

 

 

 

Yy yy

 
allele giving them the yellow color. Therefore each of the parents

Had to donate a Y allele.

 

y

 
 

 

 

 

 

 


Therefore, the genotypes of both the parents was Yy,

 

What was the phenotype(s) of the parents?

 

A: yellow

 

Can you determine if a yellow kernel is homozygous or heterozygous by looking at it?

 

A: No

 

How might you be able to determine this?

 

A: Plant seed from several yellow kernels, self-pollinate them, and look at the kernels of the progeny. If the kernels on a plant are all yellow, the original plant and all the offspring (kernels) are homozygous for the yellow allele. If the kernels have a 3 yellow: 1 white kernel ratio, then the original plant was heterozygous and the offspring (kernels) are a mixture of the genotypes.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

WORKSHEET #2 CORN BREEDING Parts of a corn plant

 

 

This worksheet will have an unlabeled copy of the image of a corn plant mentioned in the lesson plan above (yet to be drawn). Students will need to fill in the blanks labeling the different parts of the corn plant (especially focusing on reproductive parts important for breeding)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

WORKSHEET #3 Trait Inheritance (year 2) - Observations

NAME_____________________________

 

 

Date planted: _______________________

 

 

Date crossed: _______________________

 

 

Half silk date: ________________________

 

 

Date harvested: ______________________

 

 

 

Phenotype

Phenotype crossed with

Offspring's phenotype(s)

Offspring's genotype(s)

Parent's genotype(s)

Parent #1

 

 

 

 

 

Parent #2

 

 

 

 

 

Parent #3

 

 

 

 

 

Parent #4