THE BEARERS OF HEREDITY 353 



tions 2 and 3 both produce red x white hybrids or pink, 

 and constitute half the offspring. Combination 4 produces 

 pure white and occurs one-fourth of the time. This may- 

 be formulated in general terms by saying that the Fz 

 recombinations will result in 1 first grand-parental type 

 to 2 hybrid Fi types to 1 second grand-parental type. It 

 is commonly referred to as the 1:2:1 ratio. 



The Bearers of Heredity are Chromosomes. — In 

 the preceding paragraph it has been assumed that the 

 germ cells or gametes receive some sort of determiners now 

 known as factors or genes which they pass on to the off- 

 spring. It is now desirable to inquire whether there is 

 any mechanism suitable to perform this function. The 

 reader should now recall that it has been shown in chap- 

 ters XXV-XXVII that (1) The gametes are the sole con- 

 tribution of the parents to the offspring, (2) That the 

 egg is often a thousand times larger than the sperm, but 

 that their nuclei are approximately equal, (3) That the 

 chromosomes are the only parts of the nucleus which 

 maintain a continuous identity. The large size of eggs is 

 due to their having much more cytoplasm than the sperm 

 and, usually, to this being stored with abundant food for 

 the young organism. The sperm lacks this almost wholly, 

 but is yet equalty potent in heredity. It seems reasonable 

 to think, therefore, that at least the chief part of the 

 hereditary units called genes are contained in the nucleus. 

 If that is so they must also be contained in the permanent 

 part of it, the chromosomes. In fact this is known to be 

 true, but its demonstration involves a knowledge of facts 

 with which the student is not yet familiar. 



The Chromosomes contain the Genes. — The stu- 

 dent is now asked to recall what he has already learned 

 in Chapter XXV about the behavior of chromosomes. 

 There it was shown that in ordinary cell division each and 

 every chromosome splits lengthwise into two exactly simi- 

 lar halves, one of which goes to each of the new nuclei. 

 This involves a similar splitting of every gene in the 



