Lab: Genetics of Corn (dihybrid cross)
Question: How can the phenotypes of F2 offspring be used to determine the genotypes
and phenotypes of the F1 and P generations?
Big Idea: The genotypes of the parents and offspring can be determined by looking at
the phenotype ratios seen in the actual offspring and using a Punnett square.
Procedure: (groups of 2)
1. Assign duties: Partner 1 is the recorder. Partner 2 is the corn counter.
2. Carefully obtain one ear of corn. Do NOT remove any corn kernels! Observe the
variety of trait combinations: purple (pigmented), yellow (non-pigmented),
smooth (starchy), and wrinkled (sweet). There are four combinations possible.
3. Insert a large pin to mark the first kernel to be counted.
4. Count and tally the phenotype of each kernel on the entire corn cob.
(NOTE: This must be completed within one class period!)
5. Record the total of each phenotype, as well as the total kernel count for the cob.
Experimental Data:
Table 1
Phenotype Combinations
PURPLE SMOOTH
(starchy)
PURPLE
WRINKLED
(sweet)
TOTAL OF ALL KERNELS ON COB =
YELLOW SMOOTH
(starchy)
YELLOW
WRINKLED
(sweet)
Data Analysis:
The corn grains of the F2 generation (the actual experimental corn kernels you counted)
are a result of a cross between …
a homozygous purple/starchy parent
and a homozygous yellow/sweet parent (P)
Use the symbols: purple=P
yellow = p
P genotypes:
smooth=S
wrinkled=s
___ ___ ___ ___ x ___ ___ ___ ___
F1 genotype:
___ ___ ___ ___
Four possible genotype combinations for F1:
___ ___
___ ___
___ ___
___ ___
Table 2: Fill in the four genotype combinations on each side.
Complete the Punnett Square for the predicted F2 generation.
F1 x F1
___ ___
___ ___
___ ___
___ ___
___ ___ ___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
F2 Genotypes
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___
TOTAL =
#
___
___
___
___
___
___
___
___
___
16
F2 Phenotypes
#
purple smooth (starchy)
___
purple wrinkled (sweet)
___
yellow smooth (starchy)
___
yellow wrinkled (sweet) ___
TOTAL =
16
Comparison of Experimental to Predicted Data:
1.) Write in the “Actual Numbers” in Table 3 (see Table 1 phenotype totals).
2.) Calculate the “Predicted Numbers”:
Total Kernels on the Cob ___________ divided by the # of possible F2 genotypes (16).
_________÷16 = __________ = # per genotype box
Now, multiply the # per genotype box
x 9 (for purple smooth)
= ______
x 3 (for purple wrinkled)
= ______
x 3 (for yellow smooth)
= ______
x 1 (for yellow wrinkled)
= ______
3.) Record the predictions below, and compare to the actual numbers counted.
Table 3
Phenotype  Purple
Smooth
Actual Data
Numbers:
Predicted
Numbers:
Purple
Wrinkled
Yellow
Smooth
Yellow
Wrinkled
Do your Actual data numbers and the Predicted numbers seem close? ________
How could you explain why these numbers aren’t exactly the same? _________
_______________________________________________________________
Explanation:
In a dihybrid cross of homozygous parents (P), we see that the heterozygous F1 gametes
combine to create 16 possible genotypes resulting in four phenotypes with an
approximate 9:3:3:1 ratio in the F2 generation:
9 Purple Smooth : 3 Purple Wrinkled : 3 Yellow Smooth: 1 Yellow Wrinkled
Now, answer the Research Question:
“How can the phenotypes of F2 offspring be used to determine the genotypes and
phenotypes of the F1 and P generations?”
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