GENETIC PROPERTIES OF DOMINANT AND RECESSIVE ALLELES (from May 1992)
Objectives The main objective of this experiment is to utilize the
Chi-Square method of hypothesis testing using certain
traits found in corn. This experiment will also
demonstrate the discrete units of genetic material
discovered by Mendel.
Materials In the monohybrid experiment, a true breeding (homozygous)
and Methods white corn plant was crossed with a true breeding green corn
plant. The green and white colors are caused by the
presence and absence (respectively) of chlorophyll in the
early leaves. This first cross is called the F1 generation.
The resulting offspring are then crossed with one another
to produce the F2 generation. The statistical data that
results from the F2 generation will be used for hypothesis
testing.
In the dihybrid experiment, true breeding (homozygous)
parents have been crossed to produce F1 and F2 generations
similar to the way the monohybrid experiment is performed.
In this experiment both leaf color and the height of the
plant will be used for hypothesis testing. The heights of
the corn plants in the F2 generation fall into the same
categories (tall and short) that the original true breeding
parents were in.
Results The following results refer to the number of plants in the
F2 Generation showing the trait listed:
Monohybrid Experiment Dihybrid Experiment
F2 Generation F2 Generation
Trait Observed Trait Observed
Green 65 Green,Tall 53
White 29 Green,Short 20
White,Tall 14
White,Short 10
Discussion Monohybrid experiment
Hypothesis: The green true breeding parent has both dominant
alleles (GG), and the white true breeding parent
has both recessive alleles (gg). The result of this
is that the cross (F1 generation) will receive
exactly one of each of the alleles resulting in a
Gg genotype. The F2 Generation will result:
F1 Parent This hypothesis implies that
G g 3/4 of the offspring are green.
F1 G GG Gg This can be caused by the GG or Gg
Parent g Gg gg genotypes. The remaining 1/4
F2 Genotype of white plants are caused by
the gg allele combination.
F2 Generation By using Chi-Square,
Trait Observed Expected
2 2
Green 65 70 X = (65-70) / 70
2
White 29 24 +(29-24) / 24 = 1.3988
Total 94 94 and using 1 degree of freedom,
the probability is greater than .20 (1 in 5)
that the deviation from the expected data is due to chance.
Dihybrid Experiment
Hypothesis: The tall trait is caused by a dominant allele (T)
from one of the true breeding parents. The recessive
allele (t), is also homozygous in one of the parents.
The other trait, color of the leaves, is not a
linked trait, but follows the principle of
independent assortment. The following true breeding
parents are possible: GGTT and ggtt,
or : GGtt and ggTT.
In both cases the genotypes will be the same in
the F2 generation. The F1 generation has the
genotype: GgTt. The F2 generation will result:
F1 Parent allele
G T G t g T g t The phenotypes:
G T GGTT GGTt GgTT GgTt 9/16 Green,Tall
F1 G t GGTt GGtt GgTt Ggtt 1/4 Green,Short
Parent g T GgTT GgTt ggTT ggTt 1/4 White,Tall
g t GgTt Ggtt ggTt ggtt 1/16 White,Short
F2 Genotype Fractions of offspring
F2 Generation Using Chi-Square,
Trait Observed Expected
2 2
Green,Tall 53 55 X = (53-55) / 55
2
Green,Short 20 23 +(20-23) / 23
2
White,Tall 14 23 +(14-23) / 23
2
White,Short 10 6 +(10-6) / 6 = 6.6524
Total 97 97
Using 3 degrees of freedom, the probability that the
deviation is due to chance is approximately .10 (1 in 10).
Summary and The monohybrid experiment shows that the allele for
Conclusions white leaves is recessive. The probability that the
deviation is due to chance is greater that 20%.
The dihybrid experiment shows that the allele for short
plants is recessive. It also indicates that the two
traits are not linked. The probability that the
deviation is due to chance is approximately 10%.
Furthermore, the fact that traits that do not exist in
the F1 generation reappear in the F2 generation
contradicts the blending inheritance hypothesis.
