4Q2 



ZOOLOGY 



This parent and its quality are said to be dominant. When the 

 individuals of this first hybrid generation are self-fertilized, or 

 crossed with each other, 25 per cent, will be pure and like the 

 recessive grandparent; 25 per cent, will be pure and like the 

 dominant grandparent; and 50 per cent, will be hybrids like the 

 immediate parents, and having the bodily appearance of the 

 dominant grandparent. 



The diagram (Fig. 253) will aid the student in following the 

 general results. 



493 . Mendel's Laws. Three laws or principles were deduced 

 by Mendel from his experiments, and have been perfected by 

 later workers. 



PIG. 254. 

 Possible Kinds of Male Gametes 



T 



D 



TT 

 Pure tall 



T(D) 



Hybrid tall 



T(D) 



DD 



1 Hybrid tall Pure dwarf 



FlG. 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 or dwarfness, but not both. These in the long run will be equal in num- 

 bers. 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? 



a. The principle of unit characters. This suggests that 

 organisms do not inherit the whole parental nature as a unit, 

 but that each parental quality acts as a unit in inheritance. 

 Each individual, from the point of view of inheritance, is made 

 up of many independent unit characters which may be inherited 

 in any combination. 



