Structural Biochemistry/Nucleic Acid/Heredity and Related Experiments

Gregor Mendel’s Experiment

Gregor Mendel was a geneticist who discovered the basic principles of heredity through breeding garden peas. He chose to work with peas because of two reason. First, they were readily available in many varieties during his time, and second, he is able to control which plants mated with which. Mendel controlled their reproductive cycle by altering the biological structure of the pea plant. The reproductive system of pea plants consist of carpels and stamens. Stamens consist of pollen, which is used to fertilize the eggs found in the carpel. Since the stamen is placed right next to the carpel, a mature flower is able to self-fertilize. In his experiment, he has two varieties of flowers: purple flowers and white flowers. Mendel wanted to achieve cross-pollination in order to see if the blending model of inheritance were correct. If it were correct, then Mendel would expect to see two two varieties produce a flower that had a pale purple color, which is a mix between purple and white. To achieve cross-pollination, Mendel removed all of the immature stamens of a purple plant before they produced pollen. Then, Mendel dusted pollen from a white plant onto the altered purple flower.

The purple and white flower, which are known as the P generation (parental generation), hybridized purple flowers, which is known as the F1 generation (first initial generation). This result shows that the blending model is incorrect. Mendel allowed the F1 plants to self-pollinate to see if the white trait had somehow been removed by the purple trait. However, he discovered that the F2 generation, the offspring of the F1 generation, produced both purple and white flowers. Mendel kept records of how many of each flower there were, and he found out that the purple flowers outnumbered the white flowers by a 3 to 1 ratio. Mendel theorized that the white trait never disappeared; it was just dominated by the purple trait. He then concluded that being purple was a dominant trait while being white was a recessive trait.

His experiments lead Mendel to formulate what is now known as the Law of Independent Assortment, which states that different traits are spread to the offspring separately. Today however, we know that this only applies to certain traits while other linked traits do not assort independently. Another important formulation from Mendel's experiments is known as the Law of Segregation, which states that alleles separate during gamete formation and join again when the offspring is created. What is amazing about Mendel's experiments and conclusions is that he had no knowledge of DNA or how the genes are actually passed on, yet he was still able to form the basis of the study of genetics with his laws.

Mendelian Inheritance Model

Important Vocabulary

Character: a specific heritable feature, e.g. hair color

Trait: a variation of a character, i.e. brown hair, or black hair, etc.

Gene: called “heritable factors” by Mendel, a gene is a piece of hereditary genetic information that corresponds to a specific character, and is represented in DNA by a specific nucleotide sequence.

Phenotype: an organism’s traits, as determined by its genes

Genotype: an organism’s genetic makeup that determines its phenotype

Alleles: alternate versions of a single gene, e.g. the different colors of Mendel’s pea-flowers

“Wild Type”: the naturally occurring phenotype in a population (i.e. red eyes and functional wings in Drosophila)

Gregor Mendel’s theory for inheritance was comprised of four principal concepts. He developed his theory by growing pea plants in an abbey garden and performing controlled crosses between these pea plants. His observations during his experiments led to the following points:

1. Differences in inherited characters are caused by different alleles for the gene that represents that character.

2. All organisms get two alleles for each inherited character, one coming from each parent organism.

3. Varying alleles are either dominant (deciding an organism’s phenotype) or recessive (not visible in an organism’s phenotype, but present in its genotype)

4. In meiosis, two alleles are segregated, or separated and are located in different gametes. (The Law of Segregation)

Thomas Morgan’s Early Experiments

Thomas Morgan was an embryologist at Columbia University who studied fruit flies (Drosophila melanogaster) due to their simple maintenance and short reproductive cycle. Additionally, fruit fly chromosomes are easily spotted with a light microscope, making the technological demand on experiments relatively low for modern standards. Also, a fruit fly only has four chromosomes in each sex cell, one of which is a sex chromosome. The female fruit fly has two “X” chromosomes, while the male fruit fly has one “X” and one “Y” chromosome. This fact played an important part in Morgan’s early experiments.

One early problem with Morgan’s choice of experimental subject was the fact that variation in fruit fly population is relatively limited; the wild type fruit fly has red eyes and functional wings. Morgan was forced to breed fruit flies for generation after generation (one fruit fly generation equals two weeks) for years before observing a variation in phenotype; this new trait was white eyes. The white-eyed fruit fly was a male. This is where Morgan’s experimentation truly began.

Morgan took a wild type female and crossed (mated) it with the white-eyed (mutant) male. This was considered the parental generation (P). The resulting offspring made up the first filial generation (F1). All of the flies in this generation were red-eyed, and there was an even distribution of male flies to female flies. From this cross, Morgan hypothesized that the white-eyed trait was recessive to the wild type. Morgan, and his students developed a notation for labeling dominant and recessive traits. The wild type allele was annotated with a (+) superscript (i.e. X+), where X is the first letter of the trait being studied (e.g. “w” for white-eyes in this first experiment). Recessive alleles were labeled only by the letter, without the (+) superscript.

Following the first cross, Morgan decided to mate a F1 male with a F1 female. The resultant offspring were 75% red-eyed, and 25% white-eyed, as predicted in Mendel’s archetypal 3:1 offspring phenotype ratio. However, the 25% white-eyed offspring were all males. Fifty percent of the offspring were females with red eyes, 25% were males with red eyes, and 25% were males displaying the recessive phenotype. This led Morgan to conclude that the trait for white eyes is a sex-linked trait (traits that are linked to an organisms gender), and that the allele for eye-color was absent on the Y chromosome, and carried exclusively on the X chromosomes. In this situation, females cannot display the recessive phenotype, but can carry the allele for it. Males that receive the X chromosome containing the recessive allele do not have a second X chromosome carrying the dominant allele to mask the recessive phenotype. This is analogous to color blindness in humans, where women can carry the recessive allele for color-blindness, but only males display the recessive phenotype. If the trait for white eyes was not sex linked, the F2 generation would have been comprised of 75% red-eyed flies (half of which are male, half of which are female) and 25% white-eyed flies (half of which are male, half of which are female). See Figures Below:

A scheme of the crosses and phenotype results of Morgan's early experiment

A scheme of the crosses with genotype and phenotype results of Morgan's early experiment

Conclusion

Morgan’s experiment led him to conclude that trait for white-eyes is sex linked. Additionally, Morgan’s experiment verified Mendel’s hypothesis that genes are carried a specific loci on specific chromosomes. Additionally, Morgan observed that sex-linked traits are found in predictable patterns in subsequent generations.

Resources

Personal Notes from A.P. Biology, Lectured by Mr. Bradley Martin, CUHS, 2005.

Reese, Campbell, Biology, 7th Ed. 2005.

Berg, et al. Biochemistry, 6th Ed. 2007.

Nelson and Cox, et al. Lehninger's Principles of Biochemistry, 5th ed. 2008.