. The physical basis of heredity . ssovers betweenwhite and miniature; there will also be 20 per cent, ofcrossing over between miniature and forked. In otherwords, there will be both red miniature and white longflies, and there will also be crossovers between white andminiature, , miniature wings straight spines, and lon^wings forked spines. It follows that there may also becases in which crossing over has taken place between bothof the above combinations at the same time (Fig. 38),that is, there will be some flies that are white long-wingedand forked and others that are red miniature and
. The physical basis of heredity . ssovers betweenwhite and miniature; there will also be 20 per cent, ofcrossing over between miniature and forked. In otherwords, there will be both red miniature and white longflies, and there will also be crossovers between white andminiature, , miniature wings straight spines, and lon^wings forked spines. It follows that there may also becases in which crossing over has taken place between bothof the above combinations at the same time (Fig. 38),that is, there will be some flies that are white long-wingedand forked and others that are red miniature and straight CROSSING OVER 93 spines. A list of these classes with the expectation basedon the results from a single experiment is given below. Double crossoverin both ( «- ^/ | ^ ^ /^ Inasmuch as this subject and certain peculiarities inthe results can be better understood after the evidence for ^on-crossover Single crossover ( w mf in 1st (-^^ \w M ^^e^^ ]w mf in 2nd f^^ region \w j^j w m /. —Scheme to illustrate double crossing over between white and forked. The genefor miniature standing between furnishes the evidence. the linear order of the genes has been discussed, I shallnot press further the discussion here. It should be pomtedout, however, that the question of crossing over involvesmore than the independent action of the pairs in the casesso far considered; for, as will be shown later, when cross-ing over takes place not single genes but great groups ofgenes are involved. This block effect, as it may be called, 94 PHYSICAL BASIS OF HEEEDITY is not in evidence unless a larger number of genes thantwo is studied in the same experiment. These questionswill be discussed in Chapter IX. For the purpose of clearer exposition I spoke of link-age, in the preceding chapter, as though the term shouldbe limited to cases where all the genes of a group holdtogether, and have used the term crossing over to meanthe breaking of the group
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