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CHAPTER 15 



and one normal (w +) from one parent, and 

 one normal and one mutant (+ /) from the 

 other parent. When this dihybrid is back- 

 crossed, the crossovers (w t or -\- +) and non- 

 crossovers (u' + or + /) also occur in the pro- 

 portion 1 : 63. Apparently crossing over is 

 a genetic phenomenon which occurs with the 

 same frequency whether the two mutant genes 

 enter the dihybrid from the same or from dif- 

 ferent parents. Crossovers, therefore, occur 

 in the gametes of an individual with a fre- 

 quency that is constant and independent of 

 the specific way the alleles were carried by the 

 gametes that fused to initiate that individual. 

 If this is so, then it must follow that even 

 in -f +/+ + and w t/w t individuals, one 

 gamete in each 64 produced is a crossover 

 for these genes, but is undetected because it 

 carries no new combination of alleles. We 

 see then that when two linked mutants enter 

 a zygote together, because they are located 

 on the same chromosome, they tend to stay 

 together when transmitted to the next gen- 

 eration {coupling), but if they enter the zygote 

 separately, being located on different mem- 

 bers of a pair of homologous chromosomes, 

 they tend to be transmitted separately to the 

 next generation {repulsion). 



In another species, the sweet pea, it is found 

 that the trait purple flowers is due to a single 

 gene (+) whose recessive allele (/•) produces 

 red flowers. Long pollen (+) is dominant to 

 round pollen {ro). A pure line of purple long 

 (+ +/ + +) crossed to red round {r ro/r ro) 

 produces Fi that are all purple long 

 (+ -\-/rro). Since self-fertilization of the 

 Fi produces too many Pi phenotypes and too 

 few, new, recombinational types (purple 

 round and red long) for these two pairs of 

 genes to be segregating independently, these 

 genes also must be linked. But, as before, 

 linkage is not complete. In this case, the 

 crossovers obtained can be accounted for if 

 the P2 (Fi) dihybrid forms gametes in the 

 relative proportions 



10-f -I- : lOrro : 1 +ro : 1 r+. 



This rate of crossing over is obtained no 

 matter how the genes enter the dihybrid. 

 Notice, however, that this constant rate (Ki) 

 diff"ers from the rate observed in the garden 

 pea example previously discussed O^). 



Consider also the rate of crossing over in 

 two other cases. You recall, in Drosophila, 

 that the mutant gene, w, for white eye is 

 X-linked. So also is a different mutant gene 

 which produces miniature wings {m). Using 

 pure lines, a white-eyed, long-winged fly is 

 crossed to a dull red-eyed, miniature-winged 

 one. The Fi female carries two X's and is, 

 therefore, w +/+ m. This female is then 

 mated to any male and the sons are scored 

 phenotypically. Any male can be used as 

 parent, since any male will transmit to his 

 sons a Y chromosome that is devoid of the 

 genes under consideration. In fact, it can 

 be shown that the Y is lacking most of the 

 genes known to be present on the X, the gene 

 bobbed bristles, bb, being a notable excep- 

 tion. (Besides a bb allele, the Y contains 

 several genes for male fertility which have 

 no alleles on the X.) Since the sons will re- 

 ceive their X from their mother, they will 

 show by their phenotype directly which of 

 these alleles each received in the gamete she 

 provided. It is found among the sons of this 

 mating that about one crossover type appears 

 for every two that are noncrossovers. 



Finally, in man, color-blindness (c) and 

 hemophilia type A {h) are recessive X-linked 

 mutant genes absent on the Y chromosome. 

 Though rare, there are some women with 

 the genotype + h/c +, i.e., having one of 

 these mutants on each X. Available data 

 indicate that crossover {c h or -\ — [-) and non- 

 crossover {-\- h or c +) sons occur in the 

 approximate ratio of 1 : 9. 



These results show that when linkage be- 

 tween genes fails to be complete, the per- 

 centages of crossovers formed between any 

 two given pairs of genes is constant, but that 

 this frequency can be quite different in cases 

 studied in different organisms. 



