234 



THE AMERICAN MUSEUM JOURXAL 



seed color (recessive). In order that this 

 case may be understood in its relation to 

 the zoological illustrations, it should be 

 noted that seeds are really young next- 

 generation plants. In this exhibit the 

 fact is emphasized that the extracted 

 dominants and recessives of F2 and sub- 

 sequent generations, ?'. c the pure off- 

 spring of hybrid parents, are really pure. 

 If mated, each to its kind, they carry on 

 their strain indefinitely. 



As illustrations of ^Nlendelism in verte- 

 brates, experiments with the wild gray 

 and domesticated "fancy" rats ^ are 

 exhibited in the Darwin hall. If a pure 

 gray rat be mated with a white rat the 

 offspring will all lie gray, for gray is 

 dominant while white is recessive, and 

 in the F2 generation there will be three 

 grays to one white (see page 235). 

 This white, however, will be pure. Sup- 

 pose a breeder had only one white rat, 

 l)ut wished to establish a strain. He 

 could mate it with a wild gray, and 

 although the hybrids would all be gray, 

 he could get pure white individuals 

 either by mating the original white with 

 one of its hybrid offspring, or by mating 

 hyl)rids with hybrids. In the former 

 case he would get fifty per cent hybrids 

 to fifty per cent pure white (see the 

 asparagus beetle illustration) and in 

 the latter seventy-five per cent grays 

 (one-third of them pure gray) to twenty- 

 five per cent pure white. 



Let us go a step further and consider 

 what happens if there are two independ- 

 ent pairs of characters. In this connec- 

 tion compare page 232 with page 233. 

 On page 233 it is seen that one of the 

 parents has aborted wings and dark Ixxly 

 color while the other is normal with re- 

 spect to each of these characters. Since 

 light body color and normal wing are 



1 TJiC rals shown arc lar^'fly from tlic imporlaiil 

 experiments of Professor W. K. Castle, of Harvard, 

 who kindly outlined this portion of the exhibit. The 

 rest of llie rats were ohtained from the N. Y. Zoolof;- 

 iial I'ark throiif.'h the courtesy of Mr. Ditmars. 



dominant, all of the Fi generation are 

 light and have normal wings. In the 

 Fo generation one-fourth of the offspring 

 have aborted wings, one-fourth have 

 dark body color, while three-fourths 

 have long wings and three-fourths have 

 light body color. However, there are 

 four different combinations in the ratio 

 of nine light-normal to three light- 

 aborted to three dark-normal to one 

 dark-aborted. Those acquainted with 

 the laws of chance will see that this is the 

 ratio to be expected if twelve light and 

 four dark (3:1) be independent from, and 

 combined in a random fashion with, 

 twelve long and four aborted. The 

 germinal analysis may be given as fol- 

 lows, L standing for light color, d for 

 dark color, .V for normal wing and a 

 for aborted wing. The recessive condi- 

 tion of the characters is indicated by the 

 small letters. The one parent, LLXX, 

 produces germ cells which are all L^ . 

 The germ cells of the other parent, ddaa, 

 are all da. Therefore the offspring will 

 all be LdXa. These offspring, male 

 and female, will each produce four kinds, 

 (in equal numbers) of germ cells: LX, 

 La, dX and da. Suppose the combina- 

 tions of letters just given to be eggs, and 

 combine them in a random fashion with 

 the four kinds of sperm : LX, La, dX and 

 da. LX sperm, fertilizing the various 

 kinds of eggs, would produce equal num- 

 l)ers of LLNN, LLXa, LdXX and LdXa 

 individuals. Writing out in like fashion 

 the combinations for the other kinds of 

 sperm and adding the results together, 

 we find we have 



1 LLXX +2 LLXa+-2LdXX+iLdXa = 

 9 light-normal, 

 1 LLaa+2 Ldaa = 



3 light-aborted, 

 1 ddXN-\-2ddXa = 

 3 dark-normal, 

 1 ddaa = 



1 dark-aborted. 



