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GENERAL PRINCIPLES OF ZOOLOGY 



Correns, Cue not, Tschermak, Castle, Davenport, DeVries, and others). Some 

 of these matters must be mentioned here. 



(1) Mendel's Law Applicable to Animals. First applied to plants the 

 law is applicable to animals as an interesting case Illustrates. There are two 

 races of the snail, Helix hortensis, which breed true. The shell of one has five 

 parallel bands, the other shell is unicolor and lacks bands. On crossing, the 

 offspring is entirely without bands; bands are therefore recessive. The de- 

 scendants of the crosses conform to the law and consist of one fourth banded 

 and three fourths of bandless individuals, a third of the latter (a fourth of the 

 whole) being homozygote and breeding true, the other two fourths being hetero- 

 zygotes, capable of being split in the succeeding generations. 



(2) It does not always occur that one character is dominant in crosses; 

 intermediate conditions may arise. Crossing of white and dark red individuals 

 of Mirabilis jalap pa produces light red offspring, their descendants being one 

 fourth dark red, three fourths light red, capable of farther splitting. 



(3) Di- and Polyhybrids. The examples given so far deal with only a 

 single character in crossing, and if the varieties used in crossing differ by only 

 a single character, the result is called a monohybrid. There may be di- and 

 polyhybrids where two or more'pairlings' are concerned. The latter condition 

 is met in peas, which differ not only by color of flower, already mentioned, but 

 by size, position of flowers, color of pod, color and form of peas. If all of these 

 characters were indissolubly connected with each other, were only the external 

 expressions of one and the same fundamental character, there would be, in 

 polyhybrids, the simple relations found in monohybrids. The different char- 

 acteristics would be inherited together and their relations to dominance would 

 be the same. But usually this is not the case. More commonly each character 

 of a polyhybrid is inherited independently of the others. In this lies the possi 

 bility of taking a single character of a variety, even those which seem to be 

 closely correlated, and by cross-breeding, to separate it and to cause it to be 

 inherited in the most diverse combinations. This brings about very complicated 

 relations in polyhybrids, which can only be made clear by extensive contrasts. 

 Hence, to illustrate the principle, a case of dihybrid is given here, a crossing of 

 two varieties of Indian corn (maize), one with only wrinkled (r), blue (b) 

 kernels, those of the other being white (w) and smooth (s). Smooth and blue 

 are the dominating characters. Taking up each character separately, there 

 would follow in the second generation: 



For color, bb. \ bw=b, \ wb=b, J ww; a total f b, \ w. 

 For form, J rr, J rs, \ sr= s, \ ss; a total f s, | r. 



If now the two characters be combined, we have the following exposition, in 

 which the recessive characters are designated by Italics, the homozygotes by 

 capitals: 



