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
g Yg
.25
gg
.25

The final phenotypic proportions (3 yellow and 1 green) differ from the second-generation's genotype frequencies (1 : 2 : 1 for YY : Yg : gg). This disparity between genotype and phenotype provided the information necessary for Mendel to propose that some genes are dominant while others are recessive.

Pea Hybridization Generation # of Yellow Phenotype
(Genotype)
# of Green Phenotype
(Genotype)
picture of pea hybrids F0 (parents) 1 Yellow
(YY)
1 Green
(gg)
F1 All Yellow
(Yg)
0 Green
(Yg)
F2 3 Yellow
(1 YY, 2 Yg)
1 Green
(1 gg)

Besides stimulating an entirely new field of study, Mendel's work provides additional support for Darwin's theories. The tenets of Mendelian genetics would prove to be instrumental in supporting the concept of evolution by natural selection, allowing independent variation to be preserved over many generations through hidden variation; additionally, this variation could be recombined in innumerable novel combinations in future generations. Darwin's problem explaining "blending" and the preservation of variation was essentially solved. However, while Mendelian genetics provided a way to refute arguments concerning the lack of a mechanism for the preservation of variation through inheritance, Mendel's results went largely unnoticed until 1900, when the laws were independently "rediscovered" by several geneticists -- Hugo de Vries, Carl Correns, and Erich von Tschermak -- some 34 years after Mendel's original publication of his findings in 1866! (Click here to see a copy of Mendel's original paper from 1866.) It would not be until the early 1930s that a modern understanding of the relationship between genetics, morphology, and evolution became integrated into what is now called the Modern Synthesis (Ch. 3).

If you are interested in trying some experiments with Mendelian hybridization but don't have the time to catch fruit flies, give the Virtual Fly Lab a visit!

Molecular Genetics

Now that we know genes exist, how is it that genes impact living organisms? This was (and still is) a fundamental question which has no complete answer. However, what is known is that all of the processes which govern an organism's growth, development, and maintenance are ultimately based in a code located in its chromosomes.

DNA segmentAll cells that constitute an organism contain a nucleus, where the genetic material called deoxyribonucleic acid, or DNA resides. The structure of the DNA molecule resembles a twisted ladder and is often described as a "double-helix" (pictured at left). The vertical sections are composed of sugar and phosphate molecules that provide structural support for the molecule. The "rungs" of the double-helix ladder are composed of four different types of molecules called bases. These bases -- adenine, thymine, guanine, and cytosine -- have very specific bond affinities; adenine will only bond with thymine, and guanine exclusively with cytosine. So, for example, a given sequence of connected DNA base-pairs might look like the following:

...ATTACGGATAAT...
...TAATGCCTATTA...

and so forth. The information encoded in the base-pair sequence is used for functions such as protein synthesis within the cell. The actual process of protein synthesis includes two stages: transcription and translation. During transcription, the DNA molecule is unzipped and "read" by a type of RNA (ribonucleic acid) molecule called messenger RNA (mRNA). A complementary copy is then made of one of the exposed DNA segments, except that RNA substitutes the base uracil for the DNA base thymine. During translation, the mRNA molecule is decoded in triplets by transfer RNA (tRNA), which carry one of 20 amino acids. Many of the 64 possible combinations of base triplets redundantly code for a single amino acid - for example, GCT, GCC, GCA, and GCG all code for the amino acid alanine. Additionally, there are a few triplet sequences which begin and end this process of protein synthesis but do not code for any amino acid.

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