CYTOLOGY AND MENDELIAN HEREDITY 175 



are located in that chromosome. The expected trisomic ratios are 

 ordinaril}' obtained only when the trisomic plant is used as a female 

 parent, for it has been found that pollen grains carrying the extra chromo- 

 some usually do not function well in competition with normal pollen. 

 Moreover, the ratios may be disturbed by the tendency of the extra 

 chromosome to lag in the meiotic anaphase and thus sometimes fail to be 

 included in a spore nucleus. 



The first character to be assigned mainly to genes in a given chromo- 

 some was sex. This was possible because in many organisms there is a 

 chromosome pair that differs visibly from the other pairs and differs 

 also in males and females. As will be described in a later section, the 

 behavior of this pair is such as to yield two visibly distinct chromosome 

 complements in equal numbers in each generation, and these are found, 

 respectiveh^, in the cells of the two sexes. Nonsexual characters depend- 

 ent upon genes in the sex chromosomes are said to be sex-linked, and 

 they show a mode of inheritance differing somewhat from characters not 

 so linked. Unlike sex-limited characters, they may appear in either 

 sex, as will be explained further on (page 190). 



By these and other methods, genes have been assigned to their proper 

 chromosomes in a considerable number of organisms. In some of those 

 which have been most intensively investigated, this has been done for 

 every chromosome of the genome. 



Recombination and Crossing Over. — We have seen that when two 

 pairs of differential genes are located in one chromosome pair the 

 characters dependent upon them are linked, i.e., the character combina- 

 tions present in the two individuals originally' crossed tend to reappear 

 in the Fo generation following a testcross. In Fig. 123, right-hand 

 part, the F^ comprises individuals of only two classes, and these show 

 the original combinations. As a general rule, however, there are four 

 classes in F^: the individuals of the two additional classes show recom- 

 binations, i.e., the two pairs of characters have, as it were, exchanged 

 partners. The percentage of F^ individuals showing such recombination 

 tends to maintain a characteristic average in repeated tests involving 

 the same characters, but for different character combinations it varies 

 all the way from to 50. When the value lies near 50, it becomes 

 impossible without other indirect tests to distinguish the results from 

 those of random chromosome assortment (left half of Fig. 123). The 

 mechanism involved, however is a quite different one, since only one 

 chromosome pair is involved. The nature of this mechanism is brought 

 out in the following example (Fig. 124). 



Two well-known characters in Drosophila cultures are black body and 

 vestigial wings. Each of these is a recessive character, appearing in a 

 fly only when the gene has been received from both parents in the reces- 



