GENE MUTATION 



Figure 73. Sex De- 

 termination IN 

 Drosophila. Note 

 that all chromo- 

 somes of the fe- 

 male are perfectly 

 paired, while the X 

 chromosome of the 

 male has an unlike 

 mate, the Y chro- 

 mosome, which is 

 hooked in this spe- 

 cies. ( From Dod- 

 son, "Genetics," 

 W. B. Saunders 

 Co., 1956.) 



Female 



gametes 



xygotes 



llA + X and llA -h O (no sex diflFerential chromosome). When these two 

 kinds of sperm fertilize the eggs, the parental conditions are re-estab- 

 lished, that is, half of the zygotes have 22A + 2X and become females, 

 while the other half have 22A + X + O and become males. More com- 

 monly, as in Drosophila and man, the X chromosome of the male has an 

 unlike mate, the Y chromosome. Thus in Drosophila (Figure 73), females 

 have 6A + 2X, while males have 6A + X -f Y. Meiosis results in eggs with 

 3A + X and sperm of two types, 3A -^ X and 3A + Y. Fertilization then 

 leads to two equal classes of progeny, 6A + 2X ( females ) and 6A + X + Y 

 (males). 



In general, very few mutant genes are known on Y chromosomes, but 

 the X chromosomes have many. The pattern of inheritance of such sex- 

 linked genes necessarily follows that of the X chromosomes, and so is 

 readily identified. Because such genes are unpaired in males, sex-linked 

 recessive genes inherited from the mother show up in male progeny. 



Quantitative Inheritance. Inheritance of quantitative traits also de- 

 serves special mention. These include all of those individual differences 

 which must be defined by measurement rather than qualitatively. Ex- 

 amples include size, proportions of parts, intensity of color, rate of pro- 

 duction of a vitamin, concentration of a protein, and many others. The 

 genes which influence these traits are inherited in the usual Mendelian 

 fashion, but swarms of pairs of genes collaborate in the determination of 

 each character. The effect of each such gene is small, so that variation in 

 a heterozygous population appears to be continuous. Also, dominance is 

 often lacking, so that there is a simple additive effect of the genes present. 

 Because of these characteristics, quantitative inheritance can only be 

 studied with the tools of statistics. 



All of the basic phenomena of inheritance can be understood in terms 

 of the above principles. They should therefore be thoroughly reviewed. 



205 



