. The physical basis of heredity . n 50 per cent., althougli the actual num-bers given for distances between two genes may be asmuch as 107 based on summation of short distances. Thelatter method of calculation is the accurate way of statingthe result, and whenever possible it is adhered to, ,the percentage numbers for crossing over are sum totalsbased on results obtained with genes so near together thatdouble crossing over is practically excluded. Another illustration where the difference between thedirect calculation between two factors (scute and forked)and the piece-by-piece estimate i
. The physical basis of heredity . n 50 per cent., althougli the actual num-bers given for distances between two genes may be asmuch as 107 based on summation of short distances. Thelatter method of calculation is the accurate way of statingthe result, and whenever possible it is adhered to, ,the percentage numbers for crossing over are sum totalsbased on results obtained with genes so near together thatdouble crossing over is practically excluded. Another illustration where the difference between thedirect calculation between two factors (scute and forked)and the piece-by-piece estimate is greater than 50, is asfollows: At one end of the series of sex-linked genes is afactor scute (zero) and near the other end forked. Thedirect data for crossing over between them gave a cross-over value of Between them three other loci werepresent in the same experiment, and crossing over betweenthem could also be detected. As shown in the table below,the sum of these crossover values gave units betweenscute and ^C_ <Jut ^ The presence of the intermediate factors makes it pos-sible to pick up most of the double crossing over thatoccurred between scute and forked. When a correctionis made for these the difference between and disappears. Another and still more extremeexample will help to make this more evident. Near oneend of the second chromosome is the gene for star (eyes), THE OEDER OF THE GENES 123 near the other end is the gene for speck. Bridges fur-nishes the following data in regard to crossing overbetween these loci. When only these distant loci areused the crossover value is When the sum of thecrossover values between the following seven genes istaken as the value for star and speck it amounts to star
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