CHAPTER 2: THE STUDY OF HEREDITY
The basic principles of heredity were first worked out using nonhuman organisms. Through careful experimentation with the common pea plant, Gregor Mendel was the first scientist to discover the basic principles of heredity. The principle of segregation states that in the formation of sex cells, the hereditary factors separate, forming sex cells that contain either one or the other of the paired factors. The principle of independent assortment states that the inheritance patterns of differing traits are independent of one another. At the most basic level, these principles are universal among all living organisms, including the human species.
Early geneticists began to search for the physical reality of the gene. Their work led them to the cell and to small bodies within the nucleus of the cell, the chromosomes. Each chromosome consists of two strands, the chromatids, held together by the centromere. For a particular species, there is a characteristic chromosome number, which in humans is 46.
There are two basic forms of cell division. Mitosis is the division of body cells, while meiosis is the production of gametes—sperm and ova. Studies of the behavior of chromosomes during cell division have provided a physical explanation for Mendelian genetics. Deeper probing of the mechanisms of inheritance has shown that Mendel’s principles do not always apply, simply because the real hereditary mechanisms are very complex.
Scientists also began investigating the chemical nature of genetic transmission. The genetic material is a nucleic acid, DNA. DNA controls cell activities and hence determines physical characteristics. DNA, which has the ability to replicate itself, is also the mechanism through which one generation passes its characteristics on to the next.
The information contained in the DNA molecule is coded by the arrangement of base pairs. The information on the nuclear DNA molecule is transmitted by messenger RNA to the ribosome, the site of protein manufacture, where transfer RNA functions to bring the appropriate amino acids into position. On the molecular level, a gene is a segment of the DNA molecule that codes for a particular protein or segment of a protein. When random changes occur in this code they increase genotypic variation by creating “new” alleles. The various alleles of a particular gene are simply slight variants in the code itself.
