Photosynthesis Experiment - Meiosis and Mitosis

Chapter 2: Genetics

Mendelian Genetics

When Gregor Mendel began his hybridization experiments with pea plants in 1856, knowledge of how heredity works was limited. If two organisms of different height produced offspring, it was assumed that the offspring's height would be somewhere between the height of the two parents. This notion of blending inheritance presented a significant obstacle for the acceptance of the theory of natural selection, since variation would be removed from a population by being blended into nonexistence.

However, for some characteristics -- discrete traits -- inheritance did not produce a state of being between the parents. The children of a brown-eyed father and blue-eyed mother do not end up with an intermediate eye color; rather, children inherit the eye color of a single parent. It was with these types of characteristics that Mendel performed his famous botanical experiments. After carefully selecting pea plants to breed true for particular traits, he then cross-bred strains with conflicting phenotypes (observable physical characteristics). Most importantly for those who were to follow him, he meticulously catalogued the results of these experiments.

Meiosis and Mitosis

The processes of meiosis (the production of sex cells) and mitosis (cell division) are both integral operations necessary to an organism's survival and reproduction. Mitotic divisions occur throughout the body; the results of mitotic division are two daughter cells, each containing a diploid set of chromosomes. Meiosis, however, only occurs within the sex cells of an organism. This process initially divide one cell into two, each with a diploid set of chromosomes. However, this cell generation divides once again, without a corresponding division of DNA, resulting in the final product - the egg or sperm - containing a haploid (single) set of chromosomes. Additionally, during the process of meiosis, homologous chromosomes may exchange material in a genetic recombination event called crossing over. Crossing over is one of the ways that, with each subsequent generation, genes are constantly reshuffled. The reformulation of novel combinations produces the variation necessary for natural selection to operate.

Follow these links to view animated demonstrations of meiosis and mitosis.

Mendel's Experiments

From his experimental results, Mendel inferred that inheritance must occur via discrete "particles", one from each parent, and that some of these characters are dominant -- that is, preferentially expressed -- over others. The major insights attributed to Mendel are often summarized in two "laws":

Principle of Segregation: observable traits are passed down to offspring via (discrete, non-blending) particles, one from each parent.

Principle of Independent Assortment: the particles (later dubbed genes by American geneticist T. H. Morgan) from each parent are equally likely to be passed down to offspring.

The relationship between pea plant phenotype and genotype was carefully manipulated by Mendel, who inferred the above principles through assiduous, empirical observation of trait distribution.

How could he have figured out that there were two factors involved in inheritance, and that one preferentially expressed itself over the other? If we assume that each parent contributes a single chromosome to his/her offspring for a given trait, then we can calculate the probable variants found in the first generation of offspring:

First Hybrid Generation

Yellow Parent's Traits

Y Y

Green Parent's Traits g Yg

.25 Yg

.25

g Yg

.25 Yg

.25

All of the offspring from a cross (mating) between YY and gg parents should be alike in their outward appearance (Yg). This is indeed what Mendel observed as all of his first generation pea plants produced yellow seeds; however, it is a cross between the first-generation offspring that provides some additional, critical information:

Second Hybrid Generation

First Parent's Traits

Y g

Second Parent's Traits Y YY

.25 Yg

.25

...