GENETICS AND EUGENICS 



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When the two genes are for the same trait, the resulting individual 

 is said to be homozygous for that trait and, of course, shows it. When 

 each of the two genes is for a different member of the allelomorph, 

 in which case we use the term Jieterozygous, one gene takes precedence 

 over the other, but the recessive gene is in no way destroyed; it sim- 

 ply lies dormant and bides its time. How these genes are arranged 

 into a limited number of packages called chromosomes and how 

 chromosomes are distributed in the process of sperm and egg forma- 

 tion have been explained in a previous chapter. 



Plotting Crosses 



The Monohybrid Cross. — When the genetic constitution of paren- 

 tal stocks is known, it is possible to plot out the results of various 

 kinds of crosses. The checkerboard is the simplest device for such 



T d 



Fig. 74. — The outcome of a monohybrid cross between two heterozygous (Td) 

 individuals is according to the ratio 1:2:1, i.e., (TT), (Td), (Td). (dd). 



T d 



g 



Fig. 75. — The checkerboard may be used to determine the possible gene combina- 

 tions in the ova and sperm of a possible dihybrid cross. A tall green-podded pea 

 (Tg) is crossed with a dwarf yellow-podded pea (dy). 



computations. The genetic constitution of the various kinds of male 

 gametes is set down along the ordinate and that of the various kinds 

 of female gametes along the abscissa. These values are then copied 

 into the squares horizontally and vertically and their sums give the 

 values of the various kinds of gametes that will result from the cross. 

 In the Fg (second filial generation) cross of Mendel's experiment 



