Human stature 

 100- 



90 



80 

 70 

 60 

 50 

 40 

 30 

 20 

 10 

 



d- 





B 



D 



E 



MULTIPLE FACTORS IN INHERITANCE 



A person's stature is represented, on a percentage scale, as the sum of four seg- 

 ments, the length of each being determined by independently inherited factors. The 

 average proportions of ^the Jour segments are shown at A. The extremes for the 

 head-neck segment are shown'at B, those for the trunk at C, and so on. One does 

 not inherit six-footed ness, or even tallness or shortness, as a simple trait. One 

 inherits several independent factors which, acting together, result in a man's being 

 5 feet 6 inches or 6 feet 1 inch. (Based on data from C. B. Davenport) 



When Mendel made his experiments, little was known about the nucleus 

 of cells, and nothing about the behavior of chromosomes during cell-division. 

 To explain his findings, however, Mendel made certain suppositions, or guesses, 

 about what probably goes on as eggs and sperms are being formed. And his 

 suppositions turned out to agree in some ways remarkably well with the facts 

 (see illustration opposite). 



Nuclear Division We have already seen that when germ cells (eggs and 

 sperms) are being formed, the number of chromosomes becomes reduced to 

 half the number present in each body cell. When a sperm cell unites with an 

 egg cell in fertilization, the resulting zygote contains the full number of 

 chromosomes. Half of these came from the male parent and half from the 

 female parent (see illustration, p. 376). If we suppose that the chromosomes 

 bear Mendel's assumed "determiner", the behavior of the chromosomes fits 

 in astonishingly with the facts found by Mendel and other experimenters. 

 This was pointed out by an American biologist, W. S. Sutton. The facts of 



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