288 ANIMAL BIOLOGY 



B. Mendelian Principles 



The statistical treatment of biological data as a method of 

 studying inheritance was first brought prominently to the attention 

 of biologists by the work of Galton, a cousin of Darwin, during the 

 closing decades of the last century and started the widespread 

 investigation of genetic problems. In particular, his work on the in- 

 heritance of characters in Man, such as stature and intellectual 

 capacity, is a biological classic judged by the momentous conse- 

 quences which followed from the discussion it evoked. But it 

 was reserved for Mendel to apply statistical methods to facts ob- 

 served in the progeny derived from carefully controlled experi- 

 ments in breeding. Mendel's studies that founded the modern 

 science of genetics actually were made more than a score of years 

 before Galton's, but failed to reach the attention of the biological 

 world engrossed in the evolution theory; in fact were not known 

 by Darwin to whom they would have meant so much in his work 

 to secure experimental data on heredity. We can with advantage 

 introduce the survey of genetic principles by a study of examples 

 from Mendel's own work. (Fig. 290.) 



Mendel chose seven pairs of alternative characters which 

 he found were constant in certain varieties of edible Peas, such 

 as the form and color of the seeds, whether round or wrinkled, 

 yellow or green; and the length of the stem, whether tall or dwarf, 

 and these he studied in the hyrrids. One ordinarily thinks of a 

 hybrid as a cross between two species or, at least, two characteristi- 

 cally distinct varieties of animals or plants; but as a matter of 

 fact the offspring of all sexually reproducing organisms are really 

 hybrids because two parents seldom, if ever, are exactly the same 

 in all of their germinal characters. Consequently the offspring are 

 hybrids with respect to the characters in which the parents differ; 

 but in the following exposition the terms hybrid and pure are used 

 solely in regard to the particular characters under analysis. 



1. Monohybrids 



Mendel found, in crossing pure tall and dwarf varieties of Peas, 

 that all of the progeny of this parental (P) generation, in the 

 first filial (Fi) generation, were tall like one parent, there being 

 no visible evidence of their actual hybrid character. Accordingly 

 tallness was designated a dominant (D) and dwarfness a reces- 

 sive (d) character. 



