Download A Corny Genetics Lesson!

IB Biology / Genetics
A Corny Genetics Lesson!
Adapted from Ken Kneidel, ABT 58: pp 420-423, Oct '96
I have corn ear number (circle one) 1 2 3 4
The character and traits featured on this ear of corn are (circle one or both)
 Kernel color (purple is dominant to yellow)

Kernel shape (smooth is dominant to wrinkled). In corn as in peas, smooth kernels are starchy while
wrinkled kernels are sweet tasting.
This ear of corn may be the result of (circle two)

a monohybrid cross between two heterozygous parents

a monohybrid cross between one heterozygous parent and one homozygous recessive parent

a dihybrid cross between two parents heterozygous at both gene loci.

a dihybrid cross between one parent heterozygous at both gene loci and one parent homozygous recessive at
both loci.
Expected results: work out the expected phenotypic ratios for the corn offspring for both possible crosses. Be
sure to define your symbols.
How to obtain observed results:
1. Create a data table for the data you are about to collect. You will staple it to this handout.
2. Trial one
o With eyes closed, point to a random kernel. Use your finger or a Q-tip to point, NOT a pen/pencil.
o Tally its phenotype.
o Tally the phenotypes of the next 15 kernels in the row that you chose. Total kernels = 16.
3. Do five more trials, each time counting a sequence of 16 kernels beginning at the point where your
previous sequence ended, for a total of 96 kernels' worth of data.
Do you understand that you generated a data set that is analogous to what a scientist would have if s/he conducted
one breeding experiment 96 times? Each kernel of corn is a seed containing one embryo. Its production is similar
to the production of one fruit fly or one pea plant in matings of those species. Each kernel is independent of the
others since each kernel results from an independent fusion of egg (connected to a silk) and sperm (pollen from a
tassel). Technically in corn – as in all higher plants – there is double fertilization: one sperm nucleus from the
pollen fertilizes the egg, while the other sperm nucleus unites with two polar nuclei near the egg to create 3n
endosperm.
Note: before corn was domesticated, all the kernels (offspring) would fall off the ear when ripe to facilitate the
dispersal of offspring. Makes sense, right? But through artificial selection, humans have bred corn that retains
its kernels. Modern corn must be therefore be propagated through human intervention!
At first glance, is one of the expected cross' results similar to your observed results?
Now perform the chi-square test to determine how closely the observed
data approximate the most likely expected data. Don't forget to start by writing
a null hypothesis. Calculate the chi-square test statistic using the expected and
observed results for N = 96 offspring, or kernels. How many degrees of
freedom are there? What probability value will you use as a cut-off point to
accept or reject your null hypothesis? Consult a chi-square table. What did you
learn from doing the chi-square test on your corn data? Show all work below!