10 SIZE INHERITANCE IN RABBITS. 



of character differences). Since any pair of these combinations may- 

 meet in forming an individual of the next generation there will be 2,187 

 possible zygotic combinations . (Mendel's formula = 3 n ) . Lang (1910), 

 using a method first introduced by Punnett, gives a table of the zygotic 

 combinations in F 2 of a cross involving three character-differences. In 

 the present case each dominant character is found to give in F 2 a simple 

 3 : 1 ratio to its absence. The formulae of their frequencies one by one 

 would be, AA+2Aa+aa; BB+2B6+66; CC+2Cc+cc, etc. The com- 

 bined product of all these frequencies will give the frequencies of all 

 the combinations found in the second generation. Their sum is 16,384 

 (Mendel's formula = 4 M ). It is readily seen that there will be only one 

 term that lacks all the large letters, as did the " recessive" parent. 

 This is the product of the last terms of the individual frequencies. In 

 every 16,384 F 2 individuals there will, in the long run, be only one plant 

 looking like the recessive parent. There will be many apparently like 

 the dominant parent, but individuals with identical breeding possibili- 

 ties will occur no more frequently than forms resembling the recessive 

 parent. The product of the first terms of the individual frequencies 

 is the only homozygous dominant combination possible. The great 

 number of individuals will have about half the dominant characters 

 and half the recessive characters. Although over 16,000 plants would 

 be required to find the parental combinations, 100 would clearly show 

 segregation through the appearance of new associations of characters. 



In the case of these seven characters of peas, dominance is practically 

 complete. If a unit is present once — that is, if a character has come 

 from only one parent — it is visible. But suppose there is no dominance 

 and that a character received from one parent is only half as strong as 

 when it is received from both. Smooth seeds by wrinkled would give 

 slightly wrinkled seed; yellow by green seed would give pale yellow- 

 green, etc. Thus since one " dose " could be distinguished from a double 

 "dose," every zygotic combination would produce a different visible 

 condition and the number of classes distinguishable would be 2,187. 



Besides lacking dominance, imagine that these seven units all influ- 

 ence one character of one part. Let us assume, then, that the seed is 

 given its characteristic yellow color by seven independent units or 

 factors, each of which produces a shade of yellow. The presence of 

 six of these units would produce a paler yellow and five a yellow paler 

 still. And since dominance is lacking, intergrades would be formed 

 due to the different effects of single and double "doses." In crossing a 

 yellow with a green seeded plant, one that lacked all the units for form- 

 ing yellow, the hybrid seed would have a uniform pale yellow color. 

 Each unit would be present once, so that half as many units as in the 

 yellow parent would be forming yellow, and the color produced would 

 be half as strong. The same number of combinations would be found 

 in the second generation as in the first case of the pea cross, but as each 



