GENETICS 247 



Thorough famiharity with the principles involved in the guinea-pig mat- 

 ings described above should insure this ability. , 



Accuracy of the Ratios. — In describing the offspring of various matings 

 it has seemed advisable to start from the formulas in mary cases and 

 state the result that is expected on theoretical grounds. When an F-i 

 was described as being composed of 75 per cent, of black individuals and 

 25 per cent, of albinos, no particular experiment was in mind. The pro- 

 portion stated is the one theoretically expected. In a large number of 

 individuals, if there are no disturbing circumstances, such as a greater 

 death rate in one of the classes than in the other, the actual observed num- 

 bers are rather close to the theoretical ones. One actual experiment 

 which should theoretically have resulted in an F<2. of two classes, of 75 

 per cent, and 25 per cent, respectively, did actually result in 72 per cent, 

 and 28 per cent, respectively. How nearlj^ observed results approach the 

 theoretical may be determined by the student, without the labor of a 

 breeding experiment, by tossing pennies. If two coins of regular form, 

 with edges perpendicular to the flat surfaces and with sharp corners, be 

 tossed upon a hard smooth surface, heads and tails should turn up in 

 the following proportions: in one-fourth of the throws both heads, in 

 one-fourth both tails, in one-half one head and one tail. The mathe- 

 matician can compute how far observed results may diverge from the 

 theoretical without indicating any defect in the theory, but for most 

 persons the trial by pennies is more convincing. 



Simultaneous Inheritance of Two Pairs of Characters — In many cases 

 it is important to trace the inheritance of two pairs of contrasted charac- 

 ters at the same time. This is not a needless complication of what is 

 otherwise a simple matter, for some of the most importart advances 

 in our knowledge of heredity have followed from such simultaneous tests. 

 An example in guinea-pigs will make clear the usual relation between two 

 independent pairs of characters. 



In most guinea-pigs the hair on the back all slopes in one general 

 direction, producing a smooth coat. The animals in Figs. 190 and 191 

 both have smooth coats. In some, however, the hair is arranged in whorls, 

 sloping away from a central point in each whorl. The occurrence of 

 several such whorls on the back gives an animal a very unkempt appear- 

 ance, and it is said to have a rough coat (Fig. 196). Rough coat is found 

 to be dominant over smooth, and all of the numerical results obtained 

 with black and albino colors, in 7^2 and F^, can readily be duplicated' with 

 rough and smooth coat. 



When these two pairs of characters are studied in the same animals, 

 they behave independently of one another. If a smooth black pig, 

 homozygous foi- both these characters, be mated with a homozygous rough 

 albino animal, all the offspring will be black and rough (see Fig. 197, Fx). 

 The one dominant character is contributed by the one parent, the other 



