304 INTRODUCTION TO CYTOLOGY 



Among the offspring were four types. In the first type the maternal 

 member of the chromosome pair in question had neither knob nor trans- 

 located piece, and the characters of the plants showed that it carried c 

 and Wx. In the second type the chromosome had both knob and trans- 

 located piece, and it carried C and wx. The two remaining types had 

 morphologically altered maternal chromosomes, one of them showing the 

 piece but no knob while the other had a knob but no piece. Hence the 

 female parent had furnished gametes of four kinds with respect to the 

 chromosome in question, crossing-over having resulted in the two altered 

 chromosomes. The characters of the plants showed that the genes 

 concerned as well as portions of the chromosomes had been exchanged in 

 the crossover. 



Almost at the same time there appeared Stern's (1931a) report of an 

 essentially similar situation in Drosophila (Fig. 174). In this case two 

 special strains were used, one having a portion of the F-chromosome 

 attached to an X-chromosome, and the other having one of its X-chromo- 

 somes broken in two. By suitable crossing there was produced a female 

 with one X-chromosome fragmented and the other carrying the attached 

 piece of a F; hence this chromosome pair was heteromorphic at two points: 

 the attachment and the break. The fragmented X carried the genes cr 

 (for recessive carnation eye-color) and B (for dominant bar eye), while 

 the other X carried their respective normal allelomorphs. When this 

 fly was crossed to a normal male, it was observed that among the offspring 

 some of the male individuals (which receive their X-chromosome from 

 their mother; see p. 378) carried unaltered X-chromosomes and the 

 maternal factor combinations, whereas others indicated in their chromo- 

 some morphology and genetic behavior that an exchange somewhere 

 between Cr cr and B b had taken place in the maternal X-chromosome 

 pair. The females also gave similar evidence that an exchange of cor- 

 responding regions by homologous chromosomes is associated with a 

 recombination of linked characters. 



Among other observations suggesting an exchange of portions of 

 homologous chromosomes may be mentioned the following. In the 

 diplonema tetrads of Stethophyma grossum, Janssens (1924) described 

 breaks or thin regions in two of the four chromatids at the chiasma, 

 and in some cases two of the four chromatids in anaphase / appeared to be 

 composite in structure. Darlington (1930c) reported configurations in 

 the multivalent chromosomes of polyploid Hyaciyiihus which, on the 

 assumption that more than two homologous chromosomes never associate 

 closely enough for chromatid exchange in one small region, he could only 

 interpret on the basis of chromatid exchange. Further evidence was 

 seen in the peculiar "figure-of-8" configurations and the interlocking 

 bivalent rings in (Enothera (Darlington, 193 le; Catcheside, 19316). In 



