584 



CONTINUITY OF LIFE 



CO ^3 



Fig. 24-3. Mendel's second law— independent assortment— can also be demonstrated with guinea pigs. By crossing a 

 black rough pig with a white smooth one the F, generation consists of only black rough pigs. If any two of these 

 pigs are crossed both characters (coat color and texture) reappear, but in different combinations. There will be 9 

 black rough, 3 black smooth, 3 white rough, and only one white smooth. This shows that the characters are 

 independent units. 



hybrid, one character will completely mask 

 the other. In the above case, the factor for 

 black hair completely dominates the factor 

 for no color, or white. Consequently, all of 

 the offspring are black. 



The hybrid is known as the first filial 

 or Fi generation. In order to determine 

 whether or not the white is completely lost 

 in the Fi pigs, they can be bred together, 

 producing another generation, the F2 

 (second filial). When many crosses are 

 made, so there are large numbers of off- 

 spring, about three-fourths will be black 

 and one-fourth white. This is called the 3:1 

 ratio which Mendel was able to obtain so 

 many times with his plants. From this ex- 

 periment it is obvious that the character 

 for white was not really lost in the Fi gen- 

 eration. It was merely temporarily hidden 

 or latent, because when the hybrids were 

 crossed the character for white reappeared 

 unchanged and in a definite ratio. This 

 means that the character was a unit and 

 remained as a unit, even though it was un- 

 able to express itself when in the presence 

 of the dominant character. This is Mendel's 

 Law of Segregation. 



Second law— independent assortment 



The above crosses involve only one char- 

 acter, namely, color of the skin coat. What 

 would happen if two characters are fol- 

 lowed through two generations? Will the 



characters again be lost, will new combina- 

 tions be formed, or what will happen? 



There are thousands of characters in any 

 animal, but by selecting any two, preferably 

 those that demonstrate a striking dominant 

 and recessive condition, it should be pos- 

 sible to determine the fate of the charac- 

 ters as they pass from one generation to the 

 next. Employing guinea pigs again we may 

 select another pair of characters in addi- 

 tion to black and white color, such as rough 

 and smooth coats (Fig. 24-3). Ordinarily 

 the coats are smooth, with all of the hairs 

 pointed in one direction. However, there is 

 a breed in which the hair grows in whorls 

 in various places on the body, giving tlie 

 animal a roughened appearance. The char- 

 acter is easily seen and is dominant over 

 smooth. When a rough black guinea pig is 

 crossed with a smooth white one, all of the 

 hybrids ( Fi ) are rough black. When these 

 are inbred new combinations are seen. 

 There are nine rough black, three rough 

 white, three smooth black, and one smooth 

 white. This is called tlie 9:3:3:1 ratio. The 

 essential fact obtained from this experiment 

 is that each of the two characters retains its 

 identity absolutely independent of the other. 

 They are all combined in the Fi, though 

 only the dominant ones, namely, rough and 

 black, were visible. In the Fo each charac- 

 ter went its own way independent of the 

 other and showed up in the offspring in 



