CHAPTER XI 



MODIFIED MENDELIAN RATIOS; HETEROZYGOUS 

 CHARACTERS; ATAVISM OR REVERSION 



In the last chapter MendeHan ratios have been calculated on 

 the supposition that homozygous dominants and heterozy- 

 gous dominants are not distinguishable from each other, 

 which frequently is true; but if they are distinguishable from 

 each other, then a larger number of F2 classes can be recog- 

 nized and their numerical proportions are different. A case 

 of this kind was early recognized among plants by Correns. 

 (See Fig. 51.) When a white variety of four-o'clock {Mira- 

 hilis) is crossed with a red variety, Fi plants are produced 

 which bear pink flowers, and F2 consists of whites, pinks, and 

 reds in the ratio, 1:2:1. Reds and also whites breed true, but 

 pinks again produce the three sorts. This result indicates 

 that both reds and whites are homozygotes (RR and rr 

 respectively) but that pinks are regularly heterozygotes (Rr) 

 and for this reason do not breed true but are " unfixable." 

 Pink in this case may be called a heterozygous character; it 

 is for that reason unfixable. 



A similar but even better-known case among animals has 

 been described by Bateson and Punnett, that of the blue 

 Andalusian fowl. Birds of this race are of a slaty blue color 

 and are known to fanciers to be unfixable as to color. When 

 blues are mated with each other, chicks are obtained of three 

 distinct sorts as regards color, viz., blacks, blues, and 

 " splashed whites." The blacks breed true, as also do the 

 whites, but the blues invariably produce in every generation 

 the three sorts, of which blacks may be called homozygous 

 dominants (BB), whites homozygous recessives (bb), and 

 blues heterozygotes (Bb) . But it is clear that if we so desig- 

 nate them, dominance must be recognized to be imperfect. 



109 



