Genetic Observations Through The Studies of Hybrid Corn, Single Gene Human Traits,
and Fruit Flies


The basic foundation of modern genetics was led by Gregor Mendel (Corcos,
1993). Mendel was not the first to experiment with heredity, and our Lyman
Briggs biology class will not be the last to deal with genetics. Genetics is
the science of heredity. In our lab, we had three main objectives. First, we
evaluated our data on monohybrid and dihybrid corn cross seed counts against
Mendel’s theoretical expectations of independent assortment and the segregation
of alleles. Next, we used the Hardy-Weinberg Theorem to provide a theoretically
expected value for allele frequencies for single human gene traits. Lastly, we
dealt with Drosophila melanogaster and we examined red and white eye alleles to
determine if this gene is sex-linked or autosomal.
During the mid 1800’s Mendel bred garden peas to study inheritance. He
choose these plants because of their well defined characteristics and the
ability to be grown and crossed (Campbell, 1996). Mendel wanted to know the
genetic basis for variation among individuals and what accounted for the
transmission of traits from generation to generation. Mendel followed traits
for the P generation, F1 generation, and F2 generation. The P generation is the
original true-breeding parents. Their hybrid offspring is the F1 generation,
the first filial. The F2 generation is the second filial and is the self-
pollination of the F1 hybrids. It was predominantly his research on the F2
generation that led to Mendel’s Law of Segregation and Law of Independent
Assortment (Campbell, 1996).
Mendel’s Law of Segregation states that alleles sort into separate
gametes. He formed this through performing monohybrid crosses. The F2
generation will have a 3:1 phenotypic ratio. By considering more than one trait
Mendel formed his Law of Independent Assortment. He questioned whether traits
were inherited independently or dependently. By performing dihybrid crosses he
found that genes are independent and will form all possible combinations .
Crossing two different traits resulted in a 9:3:3:1 phenotypic ratio (Campbell,
1996).
Thomas Hunt Morgan also had a major contribution in the study of
inheritance. He was the first to associate a specific gene with a specific
chromosome. Morgan used Drosophila melanogaster, which are commonly known as
fruit flies. These were a good choose because they are prolific breeders, and
they only have four pairs of chromosomes (Davis, 1996). Morgan linked a fly’s
eye color to its sex. He found that females carry two copies of this gene,
while the male only carries one . Morgan’s work also led to a new, more wildly
used way for symbolizing alleles (Campbell, 1996).

Materials and Methods

Materials and methods were as per Davis (1996). For the corn cross lab,
corn was counted off of the ears of the corn, rather than through jars. For the
human characteristics between 143 to 149 students were observed. Seven
different single human gene traits were considered for this lab. The fruit fly
cross was set up on September 24, 1996. The parental (P) generation begun with
ten red-eyed males and six white-eyed females. The parent flies were removed on
October 3, 1996. Data collection was stopped on October 10, 1996.

Results

A punnett square was used for the monohybrid corn cross to find the
genotypes of the potential offspring. The gamete combinations were Su=smooth
seeds with an observed value of 497, and an expected value of 451.5; and su=
wrinkled seeds with an observed value of 105, and an expected value of 150.5.
The chi-squared value was 18.35, this value didn’t correspond with any of the
given probability values. The Null hypothesis with a 3:1 phenotypic ratio was
rejected (see figure 1). Hence, the observed number of smooth seeds versus
wrinkled seeds is not different from the expected 3:1 ratio for a monohybrid
cross (see table 1).
A dihybrid cross was used to find the genotypes of potential offspring
regarding two traits. The offspring possibilities were SuP=smooth, purple seeds
with an observed value of 577 and an expected value of 617.6; Sup=smooth, yellow
seeds with an observed value of 229 and an expected value of 205.9; suP=wrinkled,
purple seeds with an observed value of 210 and an expected value of 205.9; and
sup=wrinkled, yellow seeds with an observed value of 68.6. The chi-squared
value was 7.96, and the Null hypothesis was again rejected. The Null hypothesis
stated that the observed number of smooth, purple seeds versus smooth, yellow
seeds versus wrinkled, purple seeds versus wrinkled, yellow seeds is not
different than the expected ratio of 9:3:3:1 for a dihybrid cross (see table 2
and figure 2).
For the single gene human traits, between 143 to 149 students were
observed. The amount of data for