INVESTIGATING and APPLYING GENETICS
CORN CROP GENETICS
BACKGROUND
The corn you eat is the end product of hundreds of years of selective breeding of corn plants
to produce large ears with sweet kernels. In selective breeding, one corn plant with a
desirable trait such as good taste would be bred with another plant with a desirable trait
such as large kernels. The offspring with better taste and larger kernels would be selected for
further breeding. Through many generations of selective breeding, the taste and size of corn
kernels have been greatly improved. Even today, plant breeders constantly try to improve
the taste, disease resistance, yield and shelf life of crops such as corn.
PART I: MONOHYBRID CROSS
Each kernel on an ear of corn is an individual embryo produced by breeding parent corn
plants. This means that each kernel is a potential offspring. The color of a corn kernel is
determined by a single gene. Each kernel receives one allele, or version, of this color gene
from each parent. You will use uppercase and lowercase letters to represent different alleles
of the same gene. The two alleles are described by the plant’s genotype. A kernel with two
of the same allele has a homozygous genotype (such as SS or ss). A kernel with two
different alleles for a gene has a heterozygous genotype (such as Ss).
In corn, a purple colored kernel (P) is dominant over a yellow colored kernel (p).
 On the lab data sheet, complete a Punnett Square to predict the possible genetic
outcome between two heterozygous purple ears of corn.
 Obtain a picture of an ear of corn that is the result of crossing two heterozygous
purple corn plants (Corn Ear B)
 Count the number of purple and yellow corn kernels in the picture and then record
the numbers in the Data Table 1. Be sure to also count the total number of kernels –
this determines your OBSERVED OUTCOME.
 To calculate the EXPECTED OUTCOME, multiply the total number of corn kernels by
the fraction of predicted purple corn kernels determined in the Punnett Square.
Complete the same calculation for the yellow corn kernels.
 Perform an X2 – Analysis of your data using the formula provided in the data table.
Record the value in the Data Table 1.
 Use the X2 value to calculate the Chi Square probability to determine whether or not
your OBSERVED OUTCOME is significantly different from your EXPECTED OUTCOME
(Use Data Table 3 to help calculate the p-value. In a monohybrid cross, degrees of
freedom, (d.f.) = 1)
PART II: DIHYBRID CROSS
Now you will examine the inheritance of two traits determined by genes found on different
chromosomes in corn: the color of the corn kernels (purple vs. yellow) and the taste of the
kernels (sweet vs. starchy). Sweet kernels are wrinkled in appearance; starchy kernels are
smooth.
In corn, purple (P) is dominant over yellow (p), and starchy (S) is dominant over sweet
(s).
 On the lab data sheet, complete a Punnett Square to predict the possible genetic
outcome between two heterozygous purple, heterozygous starchy ears of corn.
 Obtain a picture of an ear of corn that is the result of crossing two heterozygous
purple corn plants (Corn Ear D)
 Count the number of purple-starchy, purple-sweet, yellow-starchy and yellow-sweet
corn kernels in the picture and then record the numbers in the Data Table 2. Be sure
to also count the total number of kernels – this determines your OBSERVED
OUTCOME.
 To calculate the EXPECTED OUTCOME, multiply the total number of corn kernels by
the fraction of predicted purple-starchy corn kernels determined in the Punnett
Square. Complete the same calculation for the other corn kernels.
 Perform an X2 – Analysis of your data using the formula provided in the data table.
Record the value in the Data Table 1.
 Use the X2 value to calculate the Chi Square probability to determine whether or not
your OBSERVED OUTCOME is significantly different from your EXPECTED OUTCOME
(Use Data Table 3 to help calculate the p-value. In a dihybrid cross, degrees of
freedom, (d.f.) = 3)
CALCULATING CHI SQUARE PROBABILITY
Table 3: 2 Values
P (probability)
.50
.30
d. f. *
.95
.90
.80
.70
1
2
.004
.103
.016
.211
.064
.446
.148
.713
.455
1.38
3
.352
.584
1.00
1.42
2.37
.10
.05
.01
1/03
2.41
2.71
4.60
3.84
5.99
6.64
9.20
3.66
6.25
7.82
11.30
 For each cross, start in the row with the appropriate df value and work your way over
until you find two numbers of which your X2 value would fall between.
 Look across the top of the chart to find out what P value your data falls under.
 You are looking for the difference to be INSIGNIFICANT, so your want to obtain a P
value ABOVE 0.05.
 This would mean that your OBSERVED OUTCOME values DO NOT differ from your
EXPECTED OUTCOME values.