Heredity - 503 



HOW TRIPLETS MAY BE PRODUCED 



(fy Single sperm fertiliies 

 — single egg 



Fertilized eg^ 



(embryo 



divides 



as in twins 



RESULT: IDENTICAL TRIPLETS 

 (Always of same sex) 



/q\ Two separate sperms fertilize 

 ^-^ two separate eggs 



RESULT: Triplets, two of whom are 

 really identical twins, the other a fra- 

 ternal twin of theirs, of the same or 

 opposite sex. 



(q) A third type, of "unmatched" triplets, can result from the 

 ~ three separate sperms and three separate eggs. 



he union of 

 Fig. 26-25. (From Scheinfeld, You and Hered/'/y. Permission of J. B. LippincoM Co.) 



MULTIPLICITY OF GENIC EFFECTS 



Each part and characteristic of an organ- 

 ism is usually affected not by a single pair of 

 genes, but by several or many pairs. There 

 are, for example, at least 50 different genes 

 that are known to have an influence upon 

 the eye color of Drosophila; and under natu- 

 ral breeding conditions, all these different 

 genes cooperate in determining the eye color 

 of the individual offspring. 



A cross between the red-eyed and white- 

 eyed flies can be called a "one-factor cross" 

 only when the original P t stocks are homo- 

 zygous and similar as to all other gene pairs 

 having any influence on the eye color. Delib- 

 erately, the stocks selected for most genetic 



experiments differ from each other by only 

 one or two hereditary factors; and a long 

 preliminary process of inbreeding is neces- 

 sary to secure such stocks. In a sense, there- 

 fore, the breeding experiments of the labora- 

 tory geneticist, which yield such simple ratios 

 among the offspring, are somewhat artificial 

 and arbitrary. Wild flies, obtained directly 

 from their natural habitat, tend to be hetero- 

 zygous for many genes. Consequently many 

 different recombinations appear among the 

 offspring. Under these conditions, it is not 

 possible to predict the results of a given mat- 

 ing very accurately. Without the inbreeding 

 technique, therefore, geneticists might not 

 have been able to ascertain the mechanism 

 of heredity, except among a minority of 



