The essential feature is the reduction phase whereby 

 the number of chromosomes is reduced by one-half. The 

 modified mitotic division, called synapsis, consists in the 

 union of the chromosomes in pairs (paternal and maternal) 

 mstead of splitting lengthwise, and then one member pass- 

 mg mto a new daughter-cell, and the other member passing 

 into the other daughter-cell. 



(d) — The Chromosome Theory of Heredity 



Sutton in 1904 put forward the following theory of the 

 mechanism of heredity, based on the behavior of chromo- 

 somes. 



All the cells of an individual have two sets of chromo- 

 somes obtained from the sperm and ovum respectively at 

 fertilization. These are known as the maternal and pater- 

 nal chromosomes, and they retain their separate individ- 

 uality in every cell of the zygote. In some zygotes the ma- 

 ternal chromosomes may dominate, thus giving different 

 physical characteristics to the developing individual. 



Frorn the following diagram (Fig. 28), where, for the 

 sake of simplicity, the sperm and ovum have each two 

 chromosomes, AA and BB respectively, "it can be seen that 

 there are three possible combinations in such a domination . 

 The two paternal chromosomes may dominate, and in that 

 case the new individual will resemble the male parent. 

 The two maternal chromosomes may dominate, and in that 

 case the individual will resemble the female parent. There 

 are two chances out of the four that the dominating chromo- 

 somes will be paternal and maternal, and in these cases the 

 new individual will blend the characters of the two parents. 

 It will be observed that this is exactly the ratio of Mendel's 

 law" (Coulter). 



With a form having three pairs of chromosomes there 

 would be eight possible combinations, four pairs sixteen 

 combinations, etc. In general ,the number of possible com- 

 binations would be represented by 2n, where n is the num- 

 ber of pairs of chromosomes in the somatic cells. 



Exercise. — Try to account for the fact that no two 

 persons are exactly alike, not even the members of one fam- 

 ily. 



In the crossing of a tall pea with a dwarf pea the chro- 

 mosome explanation of the results is as follows : The form- 

 er has all the chromosomes bearing the factor tall (T), the 



137 



