96 GENETICS IN RELATION TO AGRICULTURE 
fore, greatly matter what methods are adopted so long as they represent 
clearly and adequately the operation of these principles. It is necessary 
for the student to familiarize himself with at least some of the more 
widely employed methods, remembering that with the basic principles 
clearly in mind, it should be possible to interpret very easily the methods 
employed in the presentation of any body of Mendelian data. 
For ordinary work it is well to have a definite system of interpreting 
problems. In the following treatment only cases involving the category of 
independently segregating pairs of factors are considered, but it should be 
possible to extend the system without great 
difficulty to other categories as will be pointed 
out in those sections dealing with such 
categories. 
When an individual is heterozygous for 
one pair of factors two types of gametes are 
possible. If the factors involved are repre- 
sented by A anda, the genotypic constitution 
of such an individual is Aa, and the two 
types of gametes are A and a. If an in- 
dividual is heterozygous for two pairs of 
factors, its genotypic formula will be AaBb, 
and there are then possible four different kinds 
of gametes, namely AB, Ab, aB and ab. As 
the number of heterozygous factors increases, 
therefore, the number of possible combinations 
of factors increases geometrically so that it is 
Fic. 47.—Method of writing necessary to adopt a method of writing 
acacia Ele an eae down these possible combinations. The 
method of dichotomy may be used in such 
cases, and the diagram, Fig. 47, explains its operation without further 
comment. 
With as few as five pairs of factors, therefore, there are possible no 
less than thirty-two different kinds of gametes as follows: 
h 
ve 
/ 
A} 
AAAAAL HOhohohoy 
i 
han 
ABCDE AbCDE aBCDE abCDE 
ABCDe AbC De aBC De abC De 
ABCdE AbCdE aBCdE abCdE 
ABCde AbCde aBCde ; abCde 
ABcDE AbcDE aBcDE abcDE 
ABcDe AbcDe aBcDe abc De 
ABcdE AbcdE aBcdE abcdE 
ABcde Abcde aBcde abcde 
By following this method consistently it is possible to write out readily 
the possible combinations of any series of factors. Fortunately or un- 
Digitized by Microsoft® 
