468 Orgmiisrns Are Fro ducts of Heredity and E?iviro?mief2t unit ix 



Fig. 415 Bhie A7idalusian Fowl. These ''blue'''' {slate gray) Ajidalus'ian jowl are hy- 

 brids containing a gene for blackness of feather and one for whiteness. Is this a case 

 where contrast'ing colors do or do not blend? (snyder's principles of h£:redity) 



that showed in the hybrid, dominant, and 

 the character that did not show he called 

 recessive. He experimented with other 

 contrasting characters in the garden pea 

 and in every character he found no 

 blending in the hybrids. He therefore 

 formulated his "law of dominance." 

 More recent experiments show that in 

 the majority of cases there is dominance 

 when contrasting characters are crossed. 

 There are numerous exceptions such as 

 flower color in the four-o'clock and 

 feather color in the Andalusian fowl. In 

 crosses such as these where there is no 

 dominance, there is blending inheritance 

 or ificojnplete do7ni?ia//ce. Because there 

 are exceptions we no longer speak of a 

 law of dominance. Do Exercise 7 and if 

 materials are available you will find 

 Exercises 8 and 9 very interesting. 



Mendel's experiments with the garden 

 pea. Among the seven pairs of contrast- 

 ing characters which Alcndel studied in 

 garden peas are these: tallness and short- 



ness, smoothness and wrinkledness of 

 seed coat, yellowness and greenness of 

 seed. In all of these experiments he 

 started with plants that were pure for 

 the particular character he was studying. 

 He got pure plants by permitting the 

 plants to self-pollinate for several genera- 

 tions. If the offspring always bred true, 

 that is, all had the same character as the 

 parent, he was sure the plant was pure 

 and not hybrid. When Mendel started 

 with two pure parents, no matter what 

 character he studied, the results were al- 

 ways like those diagramed in Figure 416. 

 Can you predict, by using diagrams, the 

 results of a cross between plants with 

 other contrasting characters? Try Exer- 

 cise 10. 



The law of segregation. Mendel also 

 formulated a second law, the law of seg- 

 regation. This states that, when hybrids 

 are self-pollinated or crossed, the char- 

 acter which had been hidden in die F^ 

 separates or segregates out again in some 



