550 GENETICS AND EVOLUTION 



resulting from a given mating remained to be demonstrated experi- 

 mentally. 



In 1900, three different biologists, working independently-de Vries 

 in Holland, Correns in Germany and von Tschermak in Austria-redis- 

 covered the phenomenon of regular, predictable ratios of the types of 

 offspring produced by mating pure-bred parents. They then found Men- 

 del's published report and, realizing his priority in these discoveries, 

 gave him credit for his work by naming two of the fundamental prin- 

 ciples of heredity Mendel's Laws. 



With the genetic and cytologic facts at hand, W. S. Sutton and C. 

 E. McClung independently came to the conclusion (1902) that the 

 hereditary factors are located in the chromosomes. They also pointed out 

 that since there is a much greater number of hereditary factors than of 

 chromosomes, there must be more than one hereditary factor per chrom- 

 osome. By 1911, T. H. Morgan was able to postulate, from the regularity 

 with which certain characters tended to be inherited together, that the 

 hereditary factors (which he named "genes") were located in the chro- 

 mosomes in linear order, "like the beads on a string." 



267. Mendel's Discoveries 



Gregor Johann Mendel (1822-1884) was an Austrian abbot who 

 spent some eight years breeding peas in the garden of his monastery at 

 Briinn, now part of Czechoslovakia. He succeeded in reaching an under- 

 standing of the basic principles of heredity because (1) he studied the 

 inheritance of single contrasting characters (such as green versus yellow 

 seed color, wrinkled versus smooth seed coat), instead of attempting to 

 study the complete inheritance of each organism; (2) his studies were 

 quantitative; he counted the number of each type of offspring and kept 

 accurate records of his crosses and results; and (3), by design or by good 

 fortune, he chose a plant, and particular characters of that plant, that 

 gave him clear ratios. If he had worked with other plants, or with 

 certain other characters of peas, he would have been unable to get these 

 ratios. Now that the principles of heredity have been established, the 

 explanation for these more complicated types of inheritance is clear. 



Mendel established pure-breeding strains of peas with contrasting 

 characters— yellow seed coat vs. green seed coat, round seeds vs. wrinkled 

 ones— and then made crosses of the contrasting varieties. He found that 

 the offspring of a cross of yellow and green all had yellow seed coats; the 

 result was the same whether the male or the female parent had been 

 the yellow one. Thus, the character of one parent can "dominate" over 

 that of the other, but which of the contrasting characters is dominant 

 depends upon the specific trait involved, not upon which parent con- 

 tributes it. This observation, repeated for several different strains of 

 peas, led Mendel to the generalization, the "Law of Dominance," that 

 when two factors for the alternative expression of a character are brought 

 together in one individual, one may be expressed completely and the 

 other not at all. The character which appears in the first generation is 

 said to be dominant; the contrasting character is said to be recessive. 



