68 THE CHROMOSOMES 



consequently unaltered ; on this h3rpothesis a 

 paternal and a maternal chromatid actually ' cross 

 over ' (in the literal sense) in such a way that on one 

 side of the chiasma a paternal chromatid is paired 

 with a paternal and a maternal with a maternal while 

 on the other side a paternal is paired with a maternal, 

 and a maternal with a paternal. 



On the second hypothesis two of the four chromatids 

 have broken at the end of pachytene at exactly the 

 same level and re-joined diagonally in such a way as 

 to produce an X (Fig. 136). On the first hypothesis 

 a chiasma may give rise (by subsequent breaking) to 

 a genetical cross-over ; on the second hypothesis a 

 genetical cross-over (breakage and reciprocal re-fusion) 

 has preceded the appearance of the chiasma and 

 given rise to it. On the second hypothesis a paternal 

 chromatid is associated with another paternal one 

 and a maternal with another maternal one on either 

 side of the chiasma. It is now known with complete 

 certainty that the second hypothesis is the correct 

 one. There are a number of proofs but the simplest 

 one is given in Fig. 13e and/. It sometimes happens 

 that there is an unequal pair of chromosomes (i.e. a 

 piece has been broken off the end of either the paternal 

 or the maternal one, so that the genes that they 

 contain are, e.g. abcdefghijklmn and abode fghi). In 

 this case if a single chiasma is formed in the a . . . i 

 region the result will be an ' unequal bivalent ' as 

 in Fig. 13e and not as in Fig. 13/.^^^ This is quite a 

 conclusive proof of the second hypothesis ; there are 

 others which are even more conclusive but more 

 difficult to understand. 2®' ^^' ^^^ 



Each chiasma is thus a visible sign that a single 

 genetical cross -over has taken place. We have now 

 to ask what is it that causes the chromatids to break, 

 why do a paternal one and a maternal one always 

 break at the same level, why do never more nor less 

 than two break at that level and, finally, why does 

 reciprocal re -fusion always take place ? Entirely 



