that these factors will remain together, even linked in 

 groups, in the inheritance. Such linkages have been dis- 

 covered. • Bateson and Punnett in 190G found that when a 

 sweet pea with factors for purple flowers and long pollen 

 grains was crossed with a sweet pea with factors /or red 

 flowers and round pollen grains, the two factors that came 

 from the same parent tended to be inherited together . 

 Later, Emerson found coupling of determiners and depart- 

 ures from normal Mendelian ratios when corn with red 

 grains and white cobs was crossed with corn with white 

 grains and red cobs. To Morgan' and his associates, 

 however, we are indebted for exact, elaborate, and sug- 

 gestive investigations with Drosophila, a fruit fly, in which 

 linkage relations were determined. 



J Morgan' found that in Drosophila. which was normally 



red-eyed, there appeared in the course of breeding experi- 

 ments as many as 25 distinct mutations in this eye-color. 

 He supposes, therefore, that at least 25 factors are concern- 

 ed in the production of this red eye. and that when a single 

 one changes a different color is obtained. This one factor, 

 however, may be called the unit factor for this particular 

 color, so it may be treated as a simple Mendelian factor in 

 segregation. 



Again, the idea of sex-linked and sex-limited characters 



j has been developed, with the result that many problems, 

 formerly too intricate for solution, are now yielding • to 

 analysis. 



Linkage has been shown to occur in sweet peas, prim- 

 rose, snapdragon, groundsel, corn, tomatoes, wheat, oats, 

 evening primrose, grouse-locust, bat, silk-worm, and poult- 

 try (Morgan). 



Cases of Sex-Linked Inheritance (See also page 142). 

 Certain characters are sex-linked in that they are trans- 

 mitted by individuals of one sex almost exclusively to off- 

 spring of the other sex. Color-blindness in man is an ex- 

 ample. Men cannot hand on the character without having 

 it, whilst women can. "A woman is only color-blind when 

 she is homozygous for the character; a man is color-bHnd 

 both when he is homozygous and heterozygous for the 

 character. The normal women who transmit it are heter- 

 ozygous for it" (Darbishire). 



(1)— Prof. T. H. Morgan was born in 1866, and educated at Kentucky 

 State College and Johns Hopkins Univ. He is now Professor of 

 Experimental Zoology, Columbia University. His publications are 

 Evolution and Adaptation." "Heredity and Sex." "The Mechanism 

 of Mendohan Heredity," and "A Critique of the Theory of Evo- 

 lution. 



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