CONTINUITY OF THE RACE 



583 



Fig. 24-2. Mendel's first law— segregation— can be shown by breeding guinea pigs as outlined here. Black and white 

 animals (parents) give rise to only black pigs in the Fi generation. When these are crossed the black and white 

 hoir color segregates out so that the ratio is three black pigs to one white one in the F- generation. 



Mendel succeeded where others had 

 failed for several reasons. First he chose to 

 study single characters instead of the whole 

 individual as others had done before him. 

 He was unusually fortunate, also, in his 

 selection of material for study. Had he 

 worked with other plants or had he even 

 selected other characters, he might have 

 encountered difficulties that would have 

 precluded his clear-cut ratios. Because of 

 his training in mathematics and his rigorous 

 attention to detail, he was well suited by 

 training and aptitudes to pursue the exact- 

 ing experiments that were essential for this 

 kind of work. After eight years of exper- 

 imentation in his monastery gardens, Men- 

 del was able to give us his now famous 

 conclusions. 



It was a happy stroke of luck that Men- 

 del selected plants such as the pea in which 

 there is little difficulty in establishing pure- 

 breeding strains, that is, strains that always 

 produce a particular trait, such as flower 

 color, in all their offspring. Once he ob- 

 tained pure strains he could then cross them 

 (hybridization) and follow the trait care- 

 fully in subsequent generations. This he did 

 by keeping copious notes with actual counts, 

 no matter how numerous, recorded in tab- 

 ulated form. From these figures his ratios 

 for each experiment stood out strikingly. 

 The ratios appeared over and over again in 

 numerous experiments in which he crossed 

 a large variety of characters. This uni- 



formity of specific ratios, regardless of the 

 character, led him to formulate his final con- 

 clusions, which were later incorporated by 

 others into what are now known as Mendel's 

 Laws. 



With the rediscovery of Mendel's Laws 

 in 1900 the science of genetics took root. 

 It grew very rapidly, and continues to grow 

 today at an unprecedented rate. Research 

 workers all over the world are contributing 

 to our knowledge in this extremely produc- 

 tive field, productive not only because of its 

 practical nature but also because of the part 

 it plays in understanding the fundamental 

 processes in nature. 



MENDEL'S LAWS 



First law— segregation 



Mendel worked entirely with plants, but 

 what he learned in plants has since been 

 found to be equally true of animals. We 

 will therefore employ familiar animals to 

 describe his discoveries rather dian the 

 plants of Mendel's original experiments. 



If guinea pigs of the same color (pure 

 strains) are crossed with one another, as, 

 for example, a pure-breeding black with a 

 pure-breeding white, the offspring will all 

 be black (Fig. 24-2). From this cross one 

 outstanding fact is observed, namely, that 

 black is dominant, white recessive. This 

 means that when two such characters are 

 brought together in the production of a 



