Soo 



ZOOLOGY 



Later researches support this principle in the main, but show 

 that these unit characters do influence one another. Some- 

 times they are linked or coupled in such a way that one cannot 

 be inherited without another; sometimes they repel one another 

 so that they cannot be inherited together; and sometimes a 

 quality that appears to us as one in the body can be shown by 

 breeding to be made up of two or more cooperating unit char- 

 acters in the germ cells. 



Fig. 254. 

 Possible Kinds of Male Gametes 



Fig. 254. Diagram showing the possible kinds of crosses of the hybrids of the first filial genera- 

 tion in Mendel's peas. Each hybrid parent may produce eggs and sperm carrying, when they 

 segregate, either tallness, T, or dwarfness, D, but not both. These in the long run will be equal in 

 numbers. Each kind of sperm will have equal chances of uniting with either kind of egg. 



Questions on the figure. — Why are these the only possible gametes? Follow 

 out the details of the diagram and see just why the lettering in the squares is as it 

 is, and determine the quality of the resulting offspring in each case. Is it clear 

 why the resulting proportions are as they are ? Mendelians now think of dwarfness 

 as an absence of the quality tallness, and would letter it t instead of D. Develop 

 diagrams of your own, on this basis, in lieu of Figs. 254 and 255. 



b. TPie principle of dominance. There are genes, or deter- 

 miners, in the germ plasm which stand for these unit characters 

 in the body. When these are brought together by the union 

 of two germ cells, one (e.g., that for tallness) will dominate that 

 for dwarfness (in peas). The gene for dwarfness, though present 

 in all the cells, both body cells and primordial germ cells (see 

 §§49, 60), has no part in developing the body. The recessive 

 characteristic cannot develop in the presence of the dominant 

 determiner. Whenever the dominant determiner is absent, 



