MENDEL'S LAW OF SEGREGATION. Although the ratio of 3 to 1 in which contrasted characters reappear in the second or F 2 generation is sometimes referred to as Mendel's Law of Heredity, the really significant discovery of Mendel was not the 3 to 1 ratio, but the segregation of the characters (or rather, of the germinal representatives of the characters) which is the underlying cause of the appearance of the ratio. Mendel saw that the characters with which he worked must be represented in the germ-

WHITE. ( Plate II , figure 11.) The recessive character white eye-color, which appeared in May 1910, was the first sex-linked mutation in Drosophila (Morgan, 1910 a , 1910 b ). Soon afterwards (June 1910) rudimentary appeared, and the two types were crossed (Morgan, 1910 c ). Under the conditions of culture the viability of rudimentary was extremely poor, but the data demonstrated the occurrence of recombination of the factors in the ovogenesis so that white and rudimentary, though both sex-link

Bridges, Calvin B. 1913. Non-disjunction of the sex-chromosomes of Drosophila . Jour. Exp. Zool., 15, p. 587, Nov. 1913. 1914. Direct proof through non-disjunction that the sex-linked gens of Drosophila are borne by the X chromosome. Science, 40, p. 107, July 17, 1914. 1915. A linkage variation in Drosophila . Jour. Exp. Zool., 19, p. 1. July 1915. 1916. Non-disjunction as proof of the chromosome theory of heredity. First instalment, Genetics I, p. 1-52; second instalment, Genetics I, No. 2, 107

Plate I. Fig. 1. Normal ♀. Fig. 2. Sable ♀. Fig. 3. Lemon ♂. Fig. 4. Abnormal abdomen ♀. Fig. 5. Abnormal abdomen ♀. Fig. 6. Yellow ♀. Plate II. Fig. 7. Eosin, miniature, black ♂. Fig. 8. Eosin, miniature, black ♀. Fig. 9. Cherry. Fig. 10. Vermilion. Fig. 11. White. Fig. 12. Bar (from above). Fig. 13. Bar (from side). Fig. 14. Spot ♀ (abdomen from above). Fig. 15. Spot ♀ (abdomen from side). Fig. 16. Spot ♂ (abdomen from above). Fig. 17. Spot ♂ (abdomen from side). Fig. 4. Abnormal abdomen ♀. Fi

[1] For a fuller discussion see "The Mechanism of Mendelian Heredity" by Morgan, Sturtevant, Muller, and Bridges. Henry Holt & Co., 1915. [2] B. C. here and throughout stands for back-cross. [3] The first dark body-color mutation "black" (see plate II , figs. 7, 8) had appeared much earlier (Morgan 1911 b , 1912 c ). It is an autosomal character, a member of the second group of linked gens. Still another dark mutant, "ebony," had also appeared, which was found to be a member of the third