. The physical basis of heredity . Heredity. 262 PHYSICAL BASIS OF HEEEDITY one pure for dichete and the one pure for lethal. The sur- viving classes continue to produce the aame kind of offspring since they are, like the parents, heterozygous for the two lethal factors. But the factors are not near enough together to prevent crossing over, which occurs in about 5 per cent, of cases between the lethal and peach genes. The next diagram, Fig. 114, shows how when crossing over takes place in the female, there result four Crossover eggs o«»v,™ C5f. of total) Sperm N-- P-- It U N \ P. 95+95+5 Diche
. The physical basis of heredity . Heredity. 262 PHYSICAL BASIS OF HEEEDITY one pure for dichete and the one pure for lethal. The sur- viving classes continue to produce the aame kind of offspring since they are, like the parents, heterozygous for the two lethal factors. But the factors are not near enough together to prevent crossing over, which occurs in about 5 per cent, of cases between the lethal and peach genes. The next diagram, Fig. 114, shows how when crossing over takes place in the female, there result four Crossover eggs o«»v,™ C5f. of total) Sperm N-- P-- It U N \ P. 95+95+5 Dichete 5 Dichete Peach P 97 Va IXl 0^ P Fig. 114.—Diagram illustrating crossing over of factors shown in Fig. 113. classes (see squares), of which two die (as before), and of the two that survive one is dichete peach. Taking both non-crossover and crossover results together, the expec- tation is 95 -f- 95;+ 5 dichete to 5 dichete peach or 97^^ to 2%. This stock then breeds true for dichete without showing the gene it carries for peach eye-color except in a small percentage of cases, and if the peach-eyed fly should be unable to establish itself in nature, like some of the (Enothera mutants, the stock would not be changed by it, but continue to throw off a few "mutants" with peach-colored Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Morgan, Thomas Hunt, 1866-1945. Philadelphia : Lippincott
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