Download Ears of corn representing F2 generations (or test

LABORATORY:
Inheritance of Traits in Corn
Materials:
Ears of corn representing F2 generations (or test crosses)
Push Pins
Calculator
Objectives:
Hypothesis Testing of Genetic Basis for Phenotypic Traits
Understanding of Mendel’s Principle of Segregation
Understanding of Chi-Square Analysis
INTRODUCTION
Zea mays, commonly called corn in the United States, is one of the world’s most important food
crops. Because of its economic importance, the genetics of corn has been studied extensively.
The corn ear, also known as a cob, is covered with kernels. Each kernel represents a different
fertilization (fusion of egg and sperm). This is a fortunate feature for geneticists, because it
means that a single ear may be conveniently used to evaluate the hereditary basis for traits of the
offspring (kernels).
Kernel color
You may have noticed that corn chips are available
in a variety of colors, such as white, yellow, blue
(purple), and red. Many genes determine the
phenotypes of the three tissues that control the
color of a corn kernel. These tissues are the
pericarp, the aleurone (outer layer of the
endosperm), and the endosperm proper (see Figure
1). In the type of corn we will be using in our
class, the pericarp is always colorless, but the
aleurone may be colorless, purple, or red, and the
endosperm yellow or white. Keep in mind that
because the endosperm is underneath the aleurone,
a purple or red aleurone will mean that the
endosperm will not be visible (regardless of
whether it is yellow or white). The color of the
aleurone comes from the production of
anthocyanin pigments (also responsible for the
color of red onions and blueberries). Two separate
genes, C and R, play a role in the production of
anthocyanin. Genes C and R are located on
separate chromosomes and segregate.
For the aleurone to be colored, alleles C and R
must be present. The genotypes C/C R/_ and C/c
R/_ both result in a purple (Fig. 2) or red kernel.
Figure 1. The tissues of a corn kernel
involved in producing color phenotypes.
Figure 2. A corn ear showing purple,
yellow, and white kernels.
The homozygous recessive of either allele (c/c or r/r) disrupts anthocyanin production and
results in a colorless aleurone. As a result, a _/_ r/r or a c/c _/_ kernel will show the color of the
endosperm.
A red aleurone color is determined by another gene, Pr, which we will not be working with in
this laboratory. Thus, for these experiments, the presence of the C/C R/_ or C/c R/_ genotypes
will result in a purple kernel only.
In addition to C and c, the C gene has a third possible allele, C1. The CI allele is dominant over
both C and c. CI inhibits anthocyanin production, giving a colorless aleurone, regardless of the
genotype at the R locus.
If the aleurone is colorless, the kernel color will be that of the endosperm, either yellow or white
(see Fig. 2). Normal corn endosperm color (yellow) occurs when the Y gene carries the allele Y.
causes the production of carotenoid pigments in the endosperm (carotenoids are also responsible
for the color of carrots and hot or sweet peppers). In the recessive condition (y/y) carotenoids are
not produced and the endosperm is white. The Y alleles are masked by the presence of a colored
aleurone. The Y gene is independent of the C and R genes.
Kernel texture
Normal corn endosperm is high in amylose starch, a large complex carbohydate. The gene Su in
the homozygous recessive condition (su/su) produces endosperm that is high in sugars that are
much simpler in their structure than starch. As corn dries, the sugary endosperm of sweet
kernels lose water, and they wrinkle (see Fig. 3). Starchy kernels remain smooth.
Fig. 3. A corn ear showing starchy (smooth surface)
and sweet (wrinkled) kernels. Note that the kernels are
all yellow, meaning that the aleurone is colorless and
the Y genotype for each kernel must be Y/_.
Recall that the scientific method includes development of complementary hypotheses, the null
hypothesis, and the alternative hypothesis. In the scientific method, evidence is collected to
determine whether the null hypothesis should be rejected or retained. If it is rejected, the
alternative hypothesis is accepted as an explanation.
PROCEDURES
Your answers, including the following table, are (one week from today). Because this is a
group assignment, only one copy of the assignment need be turned in for the group. See
the course syllabus for more information about participating in group work.
Part A
For this part of the lab, you and your table partners should choose an ear with the number 6500
and an ear with the number 6540.
You may assume that: 1) both of the ears represent the F2 generation from an F1 generation,
produced in turn from true-breeding parents (P), and 2) the true-breeding parents (P) of the F1 are
different phenotypes for the trait you observe.
Ear #6500 is the F2 generation. The F1 generation was selfed to yield F2 ears. Two parental (P)
corn breeds, one true-breeding purple and the other true-breeding yellow, were crossed to yield
F1 progeny. Although purple color is known to be determined by more than one gene, you may
assume that a single gene is responsible in this case.
Ear #6540 is the F2 generation. The F1 generation was selfed to yield F2 ears. Two parental (P)
corn breeds, one true-breeding starchy and the other true-breeding sweet, were crossed to yield
F1 progeny.
1. Your objective is to evaluate whether the phenotypic patterns on these ears fits Mendel’s
Principle of Segregation. Propose a null and alternative hypothesis that you can evaluate
using these corn ears.
Hint: In each hypothesis, be certain that you specify the ratio of phenotypes.
Hint: You need a separate pair of hypotheses for each ear of corn.
2. Count the different types of kernels (purple and yellow in 6500; starchy and sweet in 6540)
on each ear. Count all kernels on each ear. Do not remove the ears from the plastic
wrapping or the label. To track your counting, consider inserting a tack or pin (provided)
into each row as you count. Enter your observed totals in the table at the end of this handout.
3. Complete the appropriate portions of the table at the end of this handout. Use a Chi-Square
test to evaluate your null hypothesis. (See your textbook for the Chi-Square Table of
Probabilities.)
4. On the basis of your Chi-Square analysis, what conclusions should be made regarding your
hypotheses? (Keep in mind that each corn breed should be evaluated separately.)
Part B
For this part of the lab, you and your table partners should choose an ear with the number 6600.
You may assume that: 1) the ears represent the F2 generation from an F1 generation, produced in
turn from true-breeding parents (P); 2) the true-breeding parents (P) of the F1 are different
phenotypes for the traits you observe; and 3) the F1 are double heterozygotes.
Ear #6600 is the F2 generation. The F1 generation was selfed to yield F2 ears. Two parental (P)
corn breeds, one true-breeding purple and starchy, and the other true-breeding yellow and sweet,
were crossed to yield F1 progeny. Although purple color is known to be determined by more
than one gene, you may assume that a single gene, R, is responsible for color.
5. Count the different types of kernels on each ear. For 6600, count four different types of
kernels: purple and starchy, purple and sweet, yellow and starchy, and yellow and sweet.
Count all kernels on each ear. Do not remove the ears from the plastic wrapping or the
label. To track your counting, consider inserting a tack or pin (provided) into each row as
you count. Enter your observed totals in the table at the end of this handout.
6. Your objective is to evaluate whether the phenotypic patterns on these ears fits Mendel’s
Principles of Segregation and Independent Assortment. Propose a null and alternative
hypothesis that you can evaluate using these corn ears.
Hint: In each hypothesis, be certain that you specify the ratio of phenotypes.
Hint: You need a separate pair of hypotheses for each ear of corn.
7. Use a Chi-Square test to evaluate your null hypothesis. Complete the appropriate portions of
the Chi-Square table on the following page. (See your textbook for the Chi-Square Table of
Probabilities.)
8. On the basis of your Chi-Square analysis, what conclusions should be made regarding your
hypotheses? (Keep in mind that each corn breed should be evaluated separately.)
Table: Chi-Square Analyses of Phenotypes of Corn Ears
Corn Breed 6500
Phenotypes
Observed
Number (o)
Expected
Number (e)
d
(o – e)
d2
d2 / e
Total
2 = ___________
Degrees of freedom (df) = _________
________ > P > ________
Corn Breed 6540
Phenotypes
Observed
Number (o)
Expected
Number (e)
d
(o – e)
d2
d2 / e
Total
2 = ___________
Degrees of freedom (df) = _________
________ > P > ________
Corn Breed 6600
Phenotypes
Observed
Number (o)
Expected
Number (e)
d
(o – e)
d2
d2 / e
Total
2 = ___________
Degrees of freedom (df) = _________
________ > P > ________