82 The Source of Variability 



combination of alleles of different genes situated on the same 

 chromosome. Thus two exchanging chromatids starting out with 

 alleles A'B'C'D' and A"B"C"D", respectively, may end up after 

 crossing-over as A'B"C"D' and A"B'C'D", respectively. The se- 

 quence of the genes ABCD is unchanged, but the combination of 

 alleles is different. If, in one meiotic division, crossing-over occurs 

 between a certain pair of chromatids, then the chances are that 

 at the next such division crossing-over will occur between the other 

 pair. In this fashion an almost complete recombination of alleles 

 may occur within the same chromosome. 



Several circumstances inhibit crossing-over. It does not occur in 

 areas of chromosomes bearing inversions or in the chromosomes of 

 male Diptera. White (1954) cites other examples but points out 

 that in at least some cases genie mixing lost by this inhibition of 

 crossing-over is partially regained by an increase in number of 

 chromosomes. 



CHROMOSOME SEGREGATION 



A mixing of the chromosomes also occurs at meiosis in the following 

 fashion. If the gametic chromosomes of one parent are A and B 

 and those of the other are A' and B', then the fertilized individual 

 will have the chromosome pairs AA' and BB'. When these chromo- 

 somes divide at reduction division they do so independently of their 

 parental origin and strictly by chance. In other words, if A goes into 

 one gamete, it is pure chance whether B or B' goes with it. Hence 

 a large number of gametes will have all the possible recombina- 

 tions, in this case AB, A'B', and A'B, AB'. 



CYTOPLASMIC SEGREGATION 



The various alleles associated with cytoplasmic inheritance exhibit 

 either no segregation or uniparental segregation. If no segregation 

 is manifest, then all the progeny of a given pair transmit their 

 cytoplasmic alleles to all their offspring. Under these circumstances, 

 in the absence of selective factors, cytoplasmic genes would spread 

 gradually and completely through the entire interbreeding system. 

 If, as is usually the case, the inheritance were uniparental, the cyto- 

 plasmic alleles would become distributed continuously through 

 lines arising from one parental type, such as ova or plus mating 

 cells, but these alleles would not pass to the next generation in 

 unions in which the character occurred only in the other parental 

 type, such as sperm, pollen, or a minus mating cell. 



