Analysis of Monohybrid and Dihybrid Crosses Lab
Introduction to Genetic Models Using Corn, Zea mays
When Mendel crossed peas, he found phenotypic ratios remarkably close to those
predicted by his model of inheritance. However, remember that the laws of heredity (a
genetic model) predict the probabilities of certain combinations of genetic traits
occurring in offspring. In real unions of egg and sperm, we don’t expect to see ratios that
correspond exactly to the probabilities predicted by a model.
In this lab we will work with probabilities of phenotypic classes in corn. The phenotypic
classes we will examine are traits related to kernel shape and color. The color phenotype
of maize (corn) kernels usually depends on the pigmentation of the aleurone (a layer of
cells between the endosperm [central tissue] and the pericarp covering). If the aluerone is
colorless, then the kernel appears to be the color of the underlying endosperm. The shape
phenotype is related to the conversion of photosynthetic sugars products to starch and is
therefore referred to often as the starchy/sweet gene. The alleles of this gene are the T
allele that controls the conversion of sugar to starch in the maturing kernel, and the
sugary (t) allele that elevates the level of the water-soluble polysaccharide phytoglycogen
and decreases starch. Kernels are wrinkled in appearance when dried for kernels that only
have t alleles. The wild-type T allele encodes starch debranching enzyme I.
Monohybrid cross
The ears you have contain the F2 seed (kernels) of an original cross between parents that
were homozygous dominant (RR) and homozygous recessive (rr) for the aleurone color
gene.
Color gene:
Purple allele = R
Yellow allele = r
1. What are the expected phenotypic and genotypic ratios? Please show/explain how you
derived your answers.
2. Count and record the number of kernels in the different phenotypes:
Phenotypes
Number of Kernels
____________________
________________
____________________
________________
3. Using Chi-square analysis, determine whether or not your data fit the expected
phenotypic ratio. You should include: 1) null hypothesis, 2) all work in chi-square
calculations and 3) do you reject or fail to reject your null hypothesis.
Dihybrid cross
Your ear of corn for this exercise again differs in color. The second trait controls whether
the texture of the kernels is smooth or rough.
Color gene:
Purple allele = R
yellow allele = r
Shape gene: Round (starchy) allele = T
Wrinkled (sweet) allele = t
The second set of ears you have contain the F2 seed (kernels) of an original cross between
parents that were homozygous dominant for the aleurone color and sugary genes (RRTT)
and homozygous recessive for the aleurone color and sugary genes (rrtt).
1. What are the expected phenotypic and genotypic ratios? Please show/explain how you
derived your answers.
2. Count and record the number of kernels in the different phenotypes:
Phenotypes
Number of Kernels
____________________
________________
____________________
________________
____________________
________________
____________________
________________
3. Using Chi-square analysis, determine whether or not your data fit the expected
phenotypic ratio. You should include: 1) null hypothesis, 2) all work in chi-square
calculations and 3) do you reject or fail to reject your null hypothesis.