chapter two: Mendelian Genetics
Chapter Review
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 offsprings' height would be somewhere between the height of the two parents. This notion of blending inheritance, as explained in Chapter 1, presented a significant obstacle for the acceptance of the theory of natural selection, because variation would be removed from a population by being blended into nonexistence.
However, for some characteristics—discrete traits—inheritance did not produce a intermediate state 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 important for those who were to follow him, he meticulously cataloged the results of those 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, occurs only within the sex cells of an organism. This process initially divides one cell into two, each with a diploid set of chromosomes. However, the resulting cell generation divides once again, without a corresponding division of DNA, resulting in the final product—eggs or sperm—each containing a haploid (single) set of chromosomes. In addition, 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. Neither of these processes results in a blending of genetic material. Units of heredity, or genes, stay discrete.
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, nonblending) 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 then inferred these principles through empirical observations of trait distributions.
First Hybrid Generation
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| Table2_1 | |||||||||||||
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 (F0 generation) contributes a single chromosome to his or her offspring for a given trait, then we can calculate the probable variants found in the first generation (F1 generation) of offspring (Table 2.1).
Second Hybrid Generation
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| Table2_2 | |||||||||||||
All of the offspring from a cross (mating) between YY and gg parents should be alike in their outward appearance (as well as in their genotype: Yg). This is indeed what Mendel observed, as all of his first-generation pea plants produced yellow seeds; however, it was the cross between the first-generation offspring that provided some additional, critical information (Table 2.2).
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| Pea Hybridization | ||||||||||||
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Table 2-3. Illustration
of Gregor Mendel's Pea Hybridization Experiments. Image credit: W.W. Norton & Company, Inc. |
The final phenotypic proportions (3 yellow to 1 green) differ from the second-generation's genotype frequencies (1: 2:1 for YY:Yg:gg) (Table 2.3). This disparity between genotype and phenotype provided the information necessary for Mendel to propose that some genes are dominant and others are recessive.
Besides stimulating an entirely new field of study, Mendel's work provided 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; in addition, this variation could be recombined in innumerable novel combinations in future generations. Darwin's problem of how to explain the preservation of variation under the theory of blending 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. They were independently rediscovered by several geneticists—Hugo de Vries, Carl Correns, and Erich von Tschermak—some decades after Mendel's original publication of his findings in 1866! (Click here to see a copy of Mendel's original paper.) 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, which you will study in greater detail in Chapter 3.
Molecular Genetics
Now that we know genes exist, how is it that genes affect living organisms? This remains a fundamental question in genetics that has no complete answer, despite the fact that the genomes of many organisms have been mapped. However, what is known is that the processes governing an organism's growth, development, and maintenance are ultimately based in a code located in its chromosomes.
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| Figure 2.2. The DNA double helix. Image credit: W. W. Norton & Company, Inc. |
All cells that constitute an organism contain a nucleus, where the genetic material called deoxyribonucleic acid (DNA) resides. The structure of the DNA molecule resembles a twisted ladder, which is often described as a double-helix (Fig. 2-2). The vertical sections are composed of sugar and phosphate molecules, which provide the 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 (A), thymine (T), guanine (G), and cytosine (C)—have specific bond affinities: Adenine will bond only with thymine, and guanine bonds exclusively with cytosine. So, for example, a given sequence of connected DNA base pairs might look like the following:
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. . ATTACGGATAAT . . .
. . . TAATGCCTATTA . . .
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 takes place in two stages: transcription and translation. During transcription, the DNA molecule is unzipped and read by a type of ribonucleic acid (RNA) 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 (U) for the DNA base thymine. During translation, the mRNA molecule is decoded in triplets by transfer RNA (tRNA). Each triplet codes for 1 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. Furthermore, there are a few triplet sequences that mark the beginning and ending the process of protein synthesis and thus do not code for any amino acid.
Gene Expression and Complex Organisms
Not all genes that an organism possesses perform the same tasks—or even do anything at all! The redundancy built into the genome, as discussed above, is often expressed as strands of DNA that seem to have no obvious functions to an organism (introns; coding sequences are called exons). Structural and regulatory genes, however, are what scientists call the types of genes that regulate biochemistry and determine gene expression—respectively. For example, proteins with our cells called enzymes act in concert with complicated biochemical pathways to determine what sorts of chemicals will be present in our cells and in what amounts. Additionally, different proteins (as created by the process of protein synthesis described above) are present in different tissues and act in very different ways in an organism; hair, for example, is constructed primarily out of a type of protein called keratin, while bone is made principally from another protein, collagen. Another protein, hemoglobin, is responsible for transporting oxygen to all the parts of oneís body through the red blood cells. The redundancy involved in protein synthesis now takes on a new and critical importance, possibly preventing small mistakes in how proteins are made from causing large problems in how an organism is structured and functions.
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