The characteristics of living organisms are determined by the hereditary factors inherited from the parents. The person who formulated many of the basic principles of heredity was the Austrian monk, Gregor Mendel (1822-1884), although the importance of his work was not realized until 1900. Working in his monastery gardens over a period of eight years and using 20 varieties of garden peas, Mendel came to some significant conclusions:
1. There are hereditary traits controlled by units (the term ‘gene’ was introduced long after Mendel’s work) that go unchanged from generation to generation.
2. Each hereditary trait is produced by two factors (genes), one from each parent. At fertilization, the two hereditary factors are brought together.
3. Hereditary factors are of two types: dominant and recessive. (The traits Mendel selected to study did not show incomplete dominance.) For his historic work, Mendel is called the “Father of Genetics.” Three other people working independently of one another rediscovered the Mendelian principles: Correns in Germany, DeVries in the Netherlands, and von Tschermak also of Austria.
As evidence accumulated about inheritance, scientists became increasingly curious about the chemical identity of the genetic material that controlled inheritance. Early experiments focused on the nucleus as the source of the hereditary traits. A German psychologist, Frederich Miescher, discovered DNA (he referred to it as nuclein) in cells as early as 1869, but Miescher did not associate it with inheritance. Miescher was not familiar with Mendel’s work.
A number of biologists in the 1880’s proposed that the transmission of hereditary traits was associated with nuclein, but it was not generally accepted for more than 50 years.
Francis H. Crick, working with James D. Watson, built a model of the DNA molecule that looked like a twisted ladder. The Watson-Crick model helped to explain the way in which DNA replicates. Watson and Crick established that the cross pieces of the “ladder” were made up of a specific order of bases, A, T, C, and G, that acted as a code during replication. The double-helix model set forth by Watson and Crick in 1953 has withstood rigorous experimentation by scientists throughout the world, and it satisfactorily explains the chemical basis of heredity. It (the double-helix DNA model) provides an adequate explanation of the duplication, mutation, and transmission of genetic material.
The determination of the sex of human offspring had been the subject of some rather vague notions up until the twentieth century. The chromosome theory (now proven) clearly explains sex determination. The critical factor is whether the sperm (male gamete) carries an X or a Y chromosome. If the sperm carries an X chromosome to the egg at fertilization, the offspring will be female (Figure 1). If, however, the sperm carries a Y chromosome, the offspring will be male (Figure 2). The female contributes only X chromosomes.
Traits other than those having to do with determination of sex are located on the X and Y chromosomes. These are known as sex-linked traits.
Finally, many characteristics are manifestations of multiple genes with incomplete dominance. There are also differences in the expected genetic expressions because of mutation in the chromosomes and genes themselves.
Figure 1. Normal female karyotype.
(figure available in print form)
Figure 2. Normal male karyotype.
(figure available in print form)