CYTOLOGY AND MENDELIAN HEREDITY 



177 



be no linkage: the two character.s, gray and long, and likewise the two 

 characters, black and vestigial, would then be exhibited together in the 

 next generation by about 50 per cent of the flies, the chance frequency 

 based on random assortment, rather than 83 per cent. 



We have next to inquire into the origin of the new combination appear- 

 ing in 17 per cent of the flies after the backcross. In the original female 

 both chromosomes carry B V; hence every egg has this combination. 

 The male has b v in both chromosomes of the pair; hence every sperm has 

 b V. All flies in Fi will therefore have B V in one chromosome of the pair 

 and 6 y in the other; they are heterozygous for both pairs of genes. 

 When the females of the Fi generation mature their eggs, the two chro- 

 mosomes disjoin in meiosis so that half of the eggs carry one and half 

 the other. If the chromosomes are passed along unaltered, no new 

 combinations appear in the next generation. 



■b 



Fig. 125. 



-Diagram of crossing over between Bb and Vv responsible for lecoinbinaticns in 

 case illustrated in Fig. 124. 



Now let it be supposed that in some of the ooc.ytes two nonsister 

 chromatids exchange portions at some point between the two pairs of 

 genes in question (Fig. 125). Some eggs will then carry unaltered 

 chromosomes {B V) {b v), while others will carry altered ones (B v) 

 (b V). Fertilization of these four classes of eggs bj^ sperms carrying 

 b V will obviously result in flies of four classes, two of which are of new 

 kinds. The percentage of recombinations appearing depends upon the 

 proportion of the oocytes in which chromatid exchange (crossing over) 

 occurs between the two pairs of genes. If it occurred in every oocyte, 

 only 50 per cent of the resulting flies would show the recombinations, 

 since two normal as well as two altered chromatids would result in each 

 oocyte. From this it can readily be seen that the frequency (17 per cent) 

 of recombination in the present example is due to the fact that the proper 

 chromatid exchange occurred in 34 per cent of the oocytes. Such crossing 

 over between pairs of linked genes is a phenomenon occurring generally 

 in plants and animals, although in some cases, notably in the males of 

 Drosophila, it is absent. 



Position of Genes in the Chromosome. — One of the theories of 

 inheritance propounded late in the nineteenth century stated that 

 "ancestral germ plasms" are arranged in a linear series in the chromo- 



