684 General and Applied Biology 



den of his monastery. Mendel was not a professional geneticist but his 

 training in mathematics directed him to record accurately the exact 

 numbers of the thousands of individuals of the various types produced 

 by his experimental crosses. His scientific interpretations of his recorded 

 data led to his famous laws and ratios. His work laid the foundation 

 for scientific, experimental crossing in genetics. Although his laws do 

 not explain all types of inheritance, wherever his laws do apply they are 

 as valid today as at the time of their discovery. He was fortunate in 

 having selected organisms which possessed clear-cut, alternative traits, 

 each controlled by a single pair of genes; otherwise he might not have 

 made his discoveries. If he had not discovered these phenomena in 

 heredity, they would ultimately have been formulated in 1900 by three 

 other scientists: De Vries in Holland, Correns in Germany, and von 

 Tschermak in Austria. However, Mendel died before his great contri- 

 butions were accepted and understood. He had published his results, 

 so the credit belongs to him — not to the three workers just mentioned. 

 Their contribution, which is highly important in science, was that their 

 work substantiated Mendel's earlier but unaccepted work. 



From the many traits of peas, Mendel selected the following seven 

 pairs of alternative ("diflferent") characteristics: 



By cross fertilization he experimentally crossed two pea plants, one of 

 which had one of the traits and the other plant the alternative trait. 

 The resulting hybrids, which resembled one or the other parent, were 

 then crossed with each other. In the hybrid, Mendel recognized the 

 trait which expressed itself as the dominant, while the one which was 

 latent and did not express itself he called the recessive. When he crossed 

 the hybrids, the dominant and recessive traits reappeared in a definite 

 ratio of approximately 3 dominants to 1 recessive. This ratio based on 

 outward appearance is called the phenotype ratio. The latter may be 

 resolved into a genotype ratio which is based on different genetic com- 

 positions of the various individuals (refer to later consideration in this 

 chapter) . 



