THE MENDELIAN METHOD 



to cross-over with the frequency required by the breeding data. 

 And we shall fmd later that they can be seen to undergo other 

 changes whose results can be recognized by breeding. 



We can now turn back and look at the facts of heredity from the 

 chromosome point of view. Our starting point must be the parental 

 chromosome which can be passed on unchanged to the offspring. 

 Recombination between linked genes is the result of a change in 

 their chromosome, the change always produced by crossing-over. 

 If there is no crossing-over the different chromosomes are recom- 

 bined as wholes and the individual chromosome passes on from 

 generation to generation as a unit of heredity. All its genes will then 

 go in one block and appear like one gene, a super-gene, having 

 many different effects. 



Strange to say this failure of crossing-over actually happens in the 

 male of Drosophila and other flies. At meiosis in the sperm mother 

 cell, the chromosomes pair but no chiasmata are formed and no 

 crossing-over takes place. The chromosomes passed from father to 

 offspring pass as unbroken units. Thus a chromosome can remain 

 unbroken for any number of generations. It is not, however, likely 

 to do so since the chance of being passed to a female, where crossing- 

 over will occur, is a half in each generation, for half the flies are 

 females. Of all the chromosomes in flies only i in 1,024 will have 

 been free from crossing-over for ten generations or more. 



To this rule there is one exception. One chromosome is passed 

 down only from father to son. This is known as the Y chromosome 

 and its smaller partner, with which it pairs at meiosis, is known as 

 the X. In the female there are two X's and these correspond to the 

 middle-sized linkage group (Fig. 9). The male, XY, is thus hetero- 

 zygous for this pair of whole chromosomes, the female homozygous. 

 The male at meiosis produces four sperm from each mother cell; 

 two have an X and two a Y. At the same time the female produces 

 eggs all with one X. 



Thus the cross between male and female is a back-cross for the 

 X-Y pair of chromosomes or, if you like, the X-Y super-gene, and 

 half the offspring are of each sex. Further, we must notice, the 

 daughter receives an X from each parent, the son an X from his 

 mother and a Y from his father. It would be easy on this evidence 

 to say that the difference between X and Y determines sex. It would 



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