HEREDITY 



273 



B from b in the reduction division of the chromosomes carrying 

 them. But since the synaptic mates, Aa and Bb, may arrange 

 themselves on the spindle independently of one another, there is 

 the possibility of four kinds of cells resulting from the division, 

 depending on whether or not chromosome carrying A goes to the 

 same pole of the spindle as the chromosome carrying B. Thus 

 one result of the reduction division would be a separation of A a, 

 Bb combination into A, B and a, b; the only other into A, b and 

 a, B; yielding in all four possible gene combinations. If three 

 pairs of characters are hybridized and if they are independently 



Homozygous parents 



Reduction division (only one class of 

 gametes from each parent) 



t) (Dif) (D 



Fi zygotes (all alike) 



Reduction division (two classes of gam- 

 etes from each F 1 zygote) 



F2 zygotes (one homozygous dominant: 

 two heterozygous: one homozygous 

 recessive) 



Fig. 164. — The behavior of a single pair of chromosomes in a Mendelian 

 cross. Only the reduction division of maturation is shown. A, dominant; 

 a, recessive. 



assorted, three pairs of chromosomes are required to explain the 

 breeding results. In such a case, if the third pair of characters 

 be represented by C, c, there would be the possibility of eight 

 different kinds of Fi gametes, as follows: ABC, abc, AbC, aBc, 

 ABc, abC, Abc, and aBC. From this it should not be inferred 

 that a chromosome carries only one gene. A chromosome may 

 carry many genes but only one of an allelomorphic pair. 



Dominance. — In Mendel's experiments one character of an 

 allelomorphic pair is dominant and is always expressed in 

 heterozygous individuals to the complete exclusion of the reces- 

 sive character. In later work and with other material it has 

 been found that dominance is often incomplete, and the question 

 as to whether or not one character is dominant to its allelomorph 

 is a matter of no theoretical importance to the principle of 



