[Chap. XXXVIII HYBRID SEGREGATION 469 



factors may be very important as economic plants. Many of our best 

 varieties of close pollinated plants are inbred lines, namely, wheat, oats, 

 barley, peas, and beans. 



Inbreeding without selection will not entirely eliminate hybrids. This 

 fact may be discovered by starting with a plant of genotype Td and 

 noting the percentage of pure-line plants and hybrids in each successive 

 generation obtained by selfing all individuals. In the first generation the 

 hybrids constitute 1/2 of the progeny, 1/4 of the next, 1/8 in the next, 

 and so on. In the n*^'' generation the proportion of hybrids to pure lines 

 may be represented by the equation (2" — 1 ) TT : 2Td : ( 2" — 1 ) dd. 

 Thus in the 8th generation it would be 255TT : 2Td : 255dd. 



Other problems. The foregoing discussion has been limited to simple 

 cases in which the several factors that specifically influence the develop- 

 ment of the characters are in different chromosomes. If one always thinks 

 of hereditary factors in association with chromosomes, problems in which 

 two or more of the factors are in the same chromosome are no more diffi- 

 cult than the cases discussed. A more difficult type of problem is met 

 with when the development of a particular character specifically depends 

 upon the presence and interaction of several hereditary factors. Such 

 characters are referred to as multiple-factor characters. A discussion of 

 these factors and of other complications of Mendelian inheritance lies 

 beyond the scope of this book. 



Hybrid endosperms. Inheritance in the endosperm is a unique feature 

 limited to certain seed plants because the endosperm is a unique tissue 

 that develops from the triple-fusion nucleus only in the seeds of some 

 plants (Chapter XXXIII). Hybrid characters in the endosperm are 

 always evident the same season that cross-fertilization occurs, for the 

 endosperm grows to maturity during that season. We have already seen 

 that hybrid characters of the embryo, such as the green and yellow 

 cotyledons of pea seeds, are also evident the same season that cross- 

 fertilization occurs; but they are dependent upon factors inherited 

 through the fertilized egg, whereas endosperm characters are dependent 

 upon factors inherited through the triple-fusion nucleus (Fig. 214). 



To understand hybridization in endosperms it is necessary only to 

 remember (1) that the two sperms in the pollen tube have the same 

 complement of chromosomes and the same hereditary factors, and (2) 

 that the egg and the fusion nucleus in the embryo sac have the same 

 complement of chromosomes and hereditary factors, except that the 



