300 



EVOLUTION AND GENETICS 



The inheritance of sex in Drosophila in a case involving non-dis- 

 junction of the X chromosomes in the female parent illustrates 

 the effect of abnormal allosome number. Females of Drosophila 

 normally produce only one kind of egg; non-disjunction results in 



kinds I I kinda 

 of \X\ of 

 egga 



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Fig. 173. — "Non-disjunction and its results in Drosophila. The two large 

 circles in the first row represent male and female flies producing sperms and 

 eggs respectively. Non-disjunction in the female gives two kinds of eggs, 

 with XX and no sex chromosomes, instead of the normal single kind with one 

 X. At fertilization there are possible four combinations rather, than two, 

 as shown in the large circles of the second row. Owing to the several ways 

 in which her three sex chromosomes may be distributed at maturation, the 

 female represented by the third circle produces four kinds of eggs. When 

 mated to a normal male (below the horizontal line) with two kinds of sperms, 

 eight combinations are possible (last row). Numbers 1, 4 and 5 are normal 

 flies and give the usual type of progeny. Numbers 2, 6 and 7, owing to the 

 presence of three sex chromosomes, give exceptional results when bred. 

 Types numbers 3 and 8 do not appear in the cultures, probably because 

 they die very early. The original male has red eyes and the original 

 female white eyes. Red eyes (represented by the dots) appear in every fly 

 bearing the x-chromosome of the original male." (From Walter, diagram 

 by Sharp based on data from Bridges and Morgan.) 



the production of two kinds, one with no x chromosome and one 

 with two. In combination with normal spermatozoa they give 

 the results shown in Figure 173. 



Evidence for the Chromosome Theory 



The Contributions of the Sexes. Nuclear Components. In 



the phenomena of Mendelian inheritance it is evident that the 

 male and female parents contribute equally to the heritage of 



