Читать книгу Abnormal Psychology - William J. Ray - Страница 96

The study of genetics begins with the work of Gregor Mendel (1823–1884). Being curious as to how plants obtain atypical characteristics, Mendel performed a series of experiments with the garden pea plant. Peas are a self-fertilizing plant, which means that the male and female aspects needed for reproduction develop in different parts of the same flower. Therefore, successive generations of peas are similar to their parents in terms of particular traits such as their height or the color of their flowers.

Mendel found that when combining peas that have white flowers with those with purple flowers, the next generation had all purple flowers. Allowing this generation to self-fertilize brought forth plants that had purple flowers but also some that had white flowers. Mendel explained these findings by suggesting that a plant inherits information from each parent, the male and female aspects. Mendel was hypothesizing that information must be conveyed. He further suggested that one unit of information could be dominant in comparison to the other, which we now call a recessive trait. In this case, the unit of information that coded for purple would be dominant.

Mendel did not know about genes but hypothesized the existence of a specific structure he called elements. From his experiments, he determined the basic principle that there are two elements of heredity for each trait (e.g., color in the previous example). Mendel also assumed that one of these elements can dominate the other and if the dominant element is present, then the trait will also be present. In addition, Mendel suggested that these elements can be nondominant, or recessive. For the trait to appear, both of these nondominant elements must be present. These ideas are referred to as Mendel’s first law or the law of segregation.

Mendel’s first law or the law of segregation: for the dominant trait to appear, only one dominant element is needed; for the recessive trait to appear, both nondominant elements must be present

Mendel’s second law or the law of independent assortment: the inheritance of the gene of one trait is not affected by the inheritance of the gene for another trait

chromosomes: thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique

Put in today’s language, Mendel suggested that variants of a specific gene exist, which account for variations in inherited characteristics, and that an organism receives one of these from each parent. Further, one of these can be dominant or recessive, which determines which characteristics are expressed. Mendel also realized that the inheritance of the gene of one trait is not affected by the inheritance of the gene for another trait. In the previous example illustrating the inheritance of color and height, those factors influencing color do not affect height, and vice versa. That is, the probability for each occurs separately. This fact is known as Mendel’s second law or the law of independent assortment.

Since Mendel’s time, we have learned a great deal concerning the process of inheritance. What he referred to as elements or units of information, we now call genes (see Figure 2.25). We also know that genes can have alternative forms, which we call alleles. Independent researchers, Walter Sutton and Theodore Boveri, in 1903 put forth a theory we now accept as fact, that genes are carried on chromosomes. We now know that each of the approximately 20,000 human genes occurs at a specific site, called a locus, on one of our 23 different pairs of chromosomes. As genetics progressed in the twentieth century, it became clear that it was necessary to go beyond the two laws suggested by Mendel to a more complex understanding of how traits are passed from generation to generation. For example, if two genes are located close to one another on the same chromosome, then the result is different from that predicted by Mendel’s second law.

Figure 2.25 Genetic Components Found in a Drop of Blood

Source: National Human Genome Research Institute.