The production of such gametes is expla'ned by Mor- 

 gan on the theory of crossing-over. At synapsis the two 

 chromosomes, one containing bv, the other GL, sometimes 

 become sUghtly twisted so that they do not separate com- 

 pletely but an exchange or crossing over of factors takes 

 place with the formation of two mixed chromosomes (Fig. 

 21). Morgan states that this crossing-over in the Fi fe- 

 males occurs in 17% of black vestigial and gray long flies. 

 The effect of this 17% of crossing-over on the Fa is to pro- 

 duce about 21% black vestigial, 70% gray long, 4% black 

 long, and 4% gray vestigial. (Fig. 21). 



Students who may wish to pursue this phase of inheri- 

 tance further are referred to Morgan's Mechanism of Men- 

 delian Heredity, and Physical Basis of Heredity. 



(e) — Endosperm Inheritance 



Reference has been made to the phenomenon of Xenia 

 in corn (See page 30). We know that the endosperm is not 

 a part of the embryo. It is the product of the fusion of a 

 male cell with a fusion nucleus which is formed by two 

 nuclei. It is, therefore, a hybrid of three nuclei (a triple 

 fusion), and appears the same year the seed is planted. 



The crosses of sweet and starchy corn, and red and 

 white corn, show typical xenia as it was first understood, 

 i.e. the direct effect of foreign pollen on the endosperm. 



An interesting fesult occurs, however, when the pollen 

 of a white corn falls on the silks of a red corn, for a red corn 

 is produced. In this case the color comes not from the 

 pollen, but from the fusion nucleus of the red corn. The 

 results of these reciprocal crosses may be explained as fol- 

 lows: — R and W are the two factors for color, R dominant; 

 In the first case the endosperm has the genetic female of R 

 WW, in the reciprocal WRR. Both will, therefore, be red. 



The above illustration and the one given on page 30 

 are sometimes cited as cases of maternal inheritance. 



(f)— Breeding for Quality and Quantity of Milk 



Improvement of Quality of Cow's Milk. 



We must look to Denmark for an example of such im- 

 provement, for there milk records have been kept for nearly 

 fifty years. The improvement is to be attributed ( 1 ) to the 

 use of sires who were the sons of cows giving high quality 

 milk and who also produced daughters giving high quality 

 milk; and (2) to the breeding from dams giving high qual- 

 ity milk. 



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