GENES AND MANKIND BLIVEN 295 



Under the microscope the giant salivary chromosomes of Droso- 

 phila (100 to 170 times larger than those in other tissues) look some- 

 what like tiny snakes with alternate transverse markings of light 

 and dark. You must not think, however, that each dark area is one 

 gene or even that the genes are concentrated in them. All that the 

 scientists are willing to say is that certain genes are associated with 

 certain dark areas and that the bands in some sense mark the limits 

 within which certain genes are known to lie. There are of course 

 many genes in each part of each chromosome. 



There is one important exception to the rule that there are 48 

 chromosomes in every human cell. The male sperm and the female 

 ovum which meet to start a new life have only 24 chromosomes each. 

 They meet and join at the moment of fertilization and the chromo- 

 somes which appear in the cells of the offspring come from both 

 parents. This meeting of the chromosomes and genes is not a 

 matter of mere blind chance. Genes for the same characteristic of 

 the individual (color of hair or eyes, pigmentation, etc.) are located 

 always in the same chromosomes. (Any one character, such as eye 

 color, is not, however, controlled only by genes from a single chromo- 

 some.) The genes from both father and mother jointly dictate the 

 characteristics of the child. 



As science discovered more than a hundred years ago, there are 

 dominant and recessive characters. When they meet, we now know, 

 the dominant will win out, as the word itself suggests. In Mendel's 

 famous experiment, he crossed a true-breeding form of red peas 

 with white ones. Red is dominant; white is recessive. In the next 

 generation all the plants had red flowers, but they were carrying 

 the white genes none the less. The self-fertilized grandchildren of 

 these peas were 25-percent pure red and 25-percent pure white. The 

 other 50 percent were red in color, but they, too, carried white genes, 

 so that their descendants would not breed true to the dominant red. 



Sometimes characteristics are controlled according to whether 

 the organism possesses one gene of a certain type, or a pair of them. 

 For example, there are two genes which, when they appear in pairs, 

 cause a man to sing bass and a woman to sing soprano. Two other 

 genes, in pairs, produce tenors and altos. One gene from each pair, 

 together cause baritones and mezzo-sopranos. As Dr. Herluf 

 Strandskov has pointed out, the children of a basso and a soprano 

 can only sing bass or soprano; a tenor and an alto can have only 

 children of those voices. A baritone and a mezzo-soprano is the 

 only mating that can hope to produce a quartet ! 



How does a child grow from an almost invisible, microscopic fer- 

 tilized Qgg into a 200-pound football player? He grows, and so 

 does everything else in nature, by cell division. Did you ever put 



