CHROMOSOMES AND MEN DELI AN HEREDITY 295 



also carried in the same chromosome. Hence in the Fi fly one chromo- 

 some of a certain homologous pair carries B V, while the other carries 6 v, 

 and they tend strongly to continue into the next generation in these 

 conditions. Were the two pairs of genes in question, B b and V v, carried 

 by different pairs of chromosomes instead of in the same pair, there would 

 be no linkage: the two characters, 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, 

 rather than 83 per cent. 



We have next to inquire into the origin of the new combinations 

 appearing in 17 per cent of the flies after the back-cross. In the original 

 female both chromosomes carry B V; hence every egg has this combina- 

 tion. The male has & t; in both chromosomes of the pair; hence every 

 sperm has 6 v. All flies in Fi will therefore have B V in one chromosome 

 of the pair and 6 ?; in the other; they are heterozygous for both pairs of 

 genes. When the females of the Fi generation mature their eggs, the two 

 chromosomes 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. 



Now let it be supposed that in some of the oocytes'' two non-sister 

 chromatids exchange portions at some point between the two pairs of 

 genes in question. This will mean that some eggs will carry unaltered 

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

 Fertilization of these four classes of eggs by sperms carrying b v will 

 obviously result in flies of four classes, two of which are of new kinds. 

 This mutual exchange of corresponding portions of homologous chromo- 

 somes, which may result in such recombinations of linked characters, 

 is known as crossing-over. The percentage of recombinations appearing 

 depends upon the proportion of the oocytes in which chromatid exchange 

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

 50 per cent of the resulting flies should show the recombination (other 

 things being equal), since two normal and two altered chromatids result 

 in any one cell. From this it can readily be seen that the frequency 

 (17 per cent) of recombination in the above example is due to the fact 

 that the proper chromatid exchange occurred in but 34 per cent of the 

 oocytes. Such crossing-over between pairs of genes linked in various 

 degrees is a phenomenon occurring generally in plants and animals, 

 although in some cases, notably in the males in Drosophila, it is absent.^ 



* That crossing-over occurs in the primary oocytes rather than in earUer or later 

 cells is shown by the work of Plough (1917) and Gowen (1929a, 1933). 



^ Huettner (1930) reports that in Drosophila males synapsis is incomplete and of 

 brief duration, but that the meiotic mitoses are otherwise normal. Other works on 

 meiosis in Drosophila are those of Huettner (1924), Metz (19266), League (1929), 

 Guyenot and Naville (1929), and Woskressensky and Scheremetzewa (1930). 



