CREIGHTON AND MCCLINTOCK 



normal, knobbed chromosome (w) or 

 the longer, interchanged, knobbed 

 chromosome (/). Hence, when such a 

 plant is crossed to a plant possessing 

 the normal chromosome complement, 

 the presence of the normal chromo- 

 some in functioning gametes of the 

 former will be indicated by the ap- 

 pearance of ten bivalents in the pro- 

 phase of meiosis of the resulting 

 individuals. The presence of the inter- 

 changed chromosome in other gametes 

 will be indicated in other Fi individ- 

 uals by the appearance of eight bi- 

 valents plus a ring of four chromo- 

 somes in the late prophase of meiosis. 

 If a gamete possessing a normal 

 chromosome number 9 with no knob, 

 meets a gamete possessing an inter- 

 changed chromosome with a knob, it 

 is clear that these two chromosomes 

 which synapse along their homologous 

 parts during prophase of meiosis in the 

 resulting individual are visibly differ- 

 ent at each of their two ends. If no 

 crossing-over occurs, the gametes 

 formed by such an individual will con- 

 tain either the knobbed, interchanged 

 chromosome (a, Fig. 1) or the normal 

 chromosome without a knob (d, Fig. 

 1). Gametes containing either a 

 knobbed, normal chromosome (c. Fig. 

 1 ) or a knobless, interchanged chro- 

 mosome (b, Fig. 1) will be formed as 

 a result of crossing-over. If such an 

 individual is crossed to a plant possess- 

 ing two normal knobless chromo- 

 somes, the resulting individuals will be 

 of four kinds. The non-crossover 

 gametes would give rise to individuals 

 which show either ( 1 ) ten bivalents at 

 prophase of meiosis and no knob on 

 chromosome 9, indicating that a 

 gamete with a chromosome of type d 

 has functioned or (2) a ring of four 

 chromosomes with a single conspicu- 

 ous knob, indicating that a gamete of 

 type a has functioned. The crossover 

 types will be recognizable as individ- 

 uals which possess either (1) ten bi- 



157 



N 



Fig. 1. (above) Diagram of the chromo- 

 somes in which crossing over was studied. 



(below) Diagram of chromosome t\'pes 

 found in gametes of a plant with the 

 constitution shown above. 



a — Knobbed, interchanged chromo- 

 some 



b — Knobless, interchanged chromo- 

 some 



c — Knobbed, normal chromosome 



d — Knobless, normal chromosome 

 a and d are non-crossover types. 

 b and c are crossover types. 



valents and a single knob associated 

 with bivalent chromosome 9 or (2) a 

 ring of four chromosomes with no 

 knob, indicating that crossover gam- 

 etes of types c and b, respectively, 

 have functioned. The results of such a 

 cross are given in culture 337, table 1. 

 Similarly, if such a plant is crossed to 

 a normal plant possessing knobs at the 

 ends of both number 9 chromosomes 

 and if crossing-over occurs, the re- 

 sulting individuals should be of four 

 kinds. The non-crossover types would 

 be represented by (1) plants homo- 

 zygous for the knob and possessing 

 the interchanged chromosome and (2) 



