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Genetics

The orderly operation of Mendel's laws in heredity suggests that there must be a very exact mechanism which acts when the germ cells of the parents are produced and then combined in a fertilized egg. What is the actual basis of heredity? The biological research of the past century directed at answering this question is one of the great stories of the history of science. This research reached it climax in 1953 when scientists carried the understanding of heredity down to the molecular level. What did they find inside the nucleus of the cell, in the chromosomes?

That the factors governing heredity called genes are contained in the chromosomes was a long established fact. It was further well established that the genes are arranged in order along the length of the chromosomes. Furthermore, each type of organism possesses in its somatic(body) cells an even number of chromosomes which are paired, each member of a pair containing the genes for the same characteristics of the organism--for example, the gene for eye color, height, and straight or curly hair, etc.

Many genes have variant forms called alleles. The two chromosomes of a pair may, for example, both have the allele for blue eyes or for brown eyes, or the two chromosomes may have different alleles, one for blue and one for brown eyes. The gene for brown eyes is dominant over the blue gene. Therefore, if a person possesses either one or two alleles for brown eye color, the eye color expressed in the individual will be brown. Only if the eye color alleles are both the recessive blue allele will the blue eye color be expressed in the individual.

How is it that different descendants of two parents have different combinations of genes for a particular characteristic such as eye color? This is determined partly by the process called meiosis, which produces the germ cells of the parents. These germ cells contain only one of each of the pairs of chromosomes possessed by the ordinary body cells. For example, the human body cells each contain 23 pairs or a total of 46 chromosomes. The egg of the female and sperm of the male, however, contain only 23 chromosomes, one from each pair. Therefore, if the body cell chromosome pair of the parent contains both brown, both blue, or one brown and one blue allele, the germ cells which contain only one member of each chromosome pair will have, respectively, a brown, a blue, or either a brown or a blue allele with equal probability.

When the fertilization process occurs, the germ cells from the male and female parents combine, each chromosome pair of the descendant receiving one chromosome from each parent. Depending upon the gene makeup of the two parents, the descendant can have either two brown, two blue, or one of each kind of eye color allele. The eye color expressed will be determined by the dominant gene possessed by the descendant. The probabilities for the different combinations are indicated in Figure 4-4 on the previous page. These are the probabilities which Mendel observed in his breeding experiments with garden peas.

Mendel's thinking and his experiments were brilliant, far ahead of his time. He reported his work in a series of two lectures before an obscure Czechoslovakian scientific society and published it in the society's journal in 1865. He explained his work in correspondence with a leading scientist concerned with plant genetics. But nobody understood the significance of Mendel's results and theory, and his work was forgotten for thirty-five years. A major reason for this professional blindness perhaps was the fact that Charles Darwin's idea of imperceptibly slow evolutionary change had in just six years captivated the minds of most biologists. They could not accept the idea of particulate inheritance which could produce sudden changes visible in just one generation. If there had been a few Christian biologists around who were committed to the biblical record of creation and therefore rejected evolution, they might have been able to overcome the current biased thinking. They could have picked up on Mendel's revolutionary work and pushed on immediately to the development of modern genetics. So Darwinian thinking had a part in frustrating the progress of the science of genetics for a full generation. It was in 1900 that Mendel's laws of inheritance were rediscovered and genetics became a modern scientific discipline.