GENETICS AND EUGENICS 



825 



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 heterozygous, 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 

 genes and chromosomes are distributed in the process of sperm and 

 egg formation was explained in Chapter VIII. 



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 



g 



Fig. 427. 



Fig. 428. 



Fig. 427. — The outcome of a monohybrid cross between two heterozygous in- 

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



Fig. 428. — The checkerboard may be used to determine the possible gene com- 

 binations in the ova. and sperm of a dihybrid cross. 



computations. The genetic constitution of the various kinds of male 

 gametes are 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 Fa cross of Mendel's experiment with tall and dwarf peas, 

 half of the male gametes and half of the female gametes contained a 

 gene for taUness (T) and half in each case contained a gene for 

 dwarf ness (d). The outcome, therefore, is shown in Fig. 427. 



The Polyhybrid Cross. — It is often desirable to know the outcome 

 of a cross in which two or more allelomorphs are considered together 

 as in the case of crossing a taU, green-podded pea (Tg)* with a dwarf, 



•It is customary to indicate the dominant trait by using a capital letter and 

 the recessive trait by using a lower-case letter. 



