Genetic Observations Through The Studies Of Hybrid

Corn, Single Gene Human Traits,
And Fruit Flies Essay, Research Paper

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

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