270 GENERAL ZOOLOGY 



Mendel's experiments dealt with the heredity of what are 

 known as allelomorphic conditions of a hereditary trait or char- 

 acter. The traits studied by Mendel were such things as stature, 

 color of seed, contour of seed, etc., each of which existed in 

 allelomorphic or alternative forms. By this is meant that in the 

 case of stature, for example, some plants were tall, others dwarf, 

 but none intermediate ; seeds were either yellow or green in color, 

 smooth or wrinkled, etc. Mendel tested the heredity of these 

 allelomorphic conditions of the same character by crossbreeding 

 them and then breeding the hybrids and their progeny. In a 

 typical experiment Mendel found that when he crossed a plant 

 having yellow-colored seed coat with one having green-colored 

 seed coat, the hybrid or first filial generation (Fi) produced only 

 yellow-colored seeds. In the next generation (^2), produced by 

 the Fi, both yellow- and green-seeded plants were obtained in 

 the proportion of 3 yellows to 1 green. The green, when inbred, 

 gave only green from this point on. Of the yellows, one-third 

 proved in subsequent breeding to be pure yellow, while the 

 remainder behaved like the F h producing 3 yellows to 1 green. 

 In this experiment, yellow is said to be the dominant character 

 and green the recessive. 



Purity of Gametes. — Mendel explained the results of this experi- 

 ment by assuming that the gametes are pure with regard to the 

 genes or factors responsible for producing yellow or green color 

 in the seeds. A single gamete carries a yellow gene or green 

 gene, but never both. In the first part of the experiment when 

 yellow and green genes are brought together in the fertilized 

 egg or zygote of the F h yellow is dominant to green. In the 

 formation of the gametes of the F\, the genes for yellow and green 

 are segregated into different germ cells (law of segregation), with 

 the result that two kinds of gametes are formed, one bearing the 

 yellow gene and the other the green gene. Since this is assumed 

 to occur in both male and female gametes, the F 2 results from the 

 chance combinations of two kinds of male and female gametes, 

 as illustrated in the following diagram: 



Parents Gametes Fi Zygotes Fi Gametes F 2 Zygotes 

 Yellow YY -^Y Y Yy -> Y y 



= YY : 2Yy : yy 



Green yy >y y Yy 



