112 AN INTRODUCTION TO MODERN GENETICS 



will give a new ring formation and a new association of parts of one 

 complex witii parts of the other; 



(2) by segmental interchange between two chromosomes within one 

 complex. 



Darlington has shown how a process analogous to the second of these 

 would account for the formation of the rather complicated system of 

 complexes, lethals and ring-mechanism which has to be postulated. If 

 we start with an organism with the two pairs of chromosomes AxBy 

 AxB, and CD, CD, let one of the x segments be translocated to the 

 middle of the CD chromosome. We shall have AxB, AS and CxD, CD, 

 both pairs being terminally associated. Then a crossing-over in the 

 two x segments (near one end of the segment) may occasionally occur 

 and give AxD, CxB, and it will be possible to obtain an organism 

 AxD.DC.CxB. BA. in which there are two complexes, one with and 

 one without the x segment, and also a balanced lethal mechanism 

 since the AxD, AxD, CxB, CxB type with reduplicated x and the AB, 

 AB, CD, CD type with deficient x might be expected to be in viable. 

 Definite evidence that the Oenothera complexes have originated in 

 some way similar to this has been found. Chiasma formation can be 

 occasionally observed between the middle sections of the chromosomes 

 in one complex, which shows that these middle sections are homo- 

 logous, corresponding to the x segment in the example above. 



The detailed working out of the hypothesis of segmental interchange 

 has been undertaken by various workers, and the hypothesis has sur- 

 vived the important test of prediction. Cleland^ was able to deduce, 

 from the pairing relations in parent species and some of their hybrids, 

 what should be the pairing in other hybrids which had not yet been 

 examined, and subsequent observation showed the correctness of his 

 deductions. 



^ Cleland 1931. 



