474 Orgau'mns Are Products of 



Fig. 428 A drawhii>; of the \;hvit chrovwsowes 

 ill the salivary ^land cells of fruit fly larvae. 

 The lines, of course, arc not ;j,enes. The letters 

 mark the location of some ^leiies. ( r. s. painter) 



^ The offspring m;n' have a character 

 that is not found in the parents or grand- 

 parents. A recessive gene may be car- 

 ried for man\^ generations without meet- 

 ing its mate; it may, therefore, be many 

 generations before it makes itself notice- 

 able in any of the offspring. Such an in- 

 dividual may be called a "throw-back." 



Heredity and Environiitent unit ix 



4. Genetics explains further why the 

 many offspring of the same parents are 

 all different from one another. When you 

 studied the inheritance of two characters 

 at the same time you saw that there were 

 nine different genetic make-ups possible 

 in the F.. If you study three genes which 

 are not linked in one chromosome you 

 liet twenty-seven different possible off- 

 spring. Four pairs of genes will produce 

 81 different genetic make-ups. 



5. Mathematicians have calculated 

 that in man, \\'\x\\ 24 pairs of chromo- 

 somes, there are many millions of pos- 

 sibilities for different combinations. No 

 t^^'o human beinos will have the same 

 crene combination, except for identical 

 twins about which you \\\\\ read later. 



6. By independent assortment and fer- 

 tilization, new combinations and recom- 

 binations are continually occurring. In- 

 dividuals of the same species are not 

 exactly alike; they are different from one 

 another, or as it is more often stated, they 

 show variations. Some variations are 

 caused by the fact that in each reduction 

 division the chromosomes (and genes) 

 assort independently and at each fertili- 

 zation new combinations are made. 



Questions 



1. List as many wavs as \-ou can think of in which you arc different 

 from cither of your parents. 



2. Explain how artificial cross-pollination is done. What color of flow er 

 do you get by crossing a red four-o'clock with a white? What is the 

 result of crossing two pink four-o'clock plants? 



3. If 7? represents the gene for redness why must a red four-o'clock be 

 represented by RR? How would you represent a white plant? A pink 

 plant? Which of these are pure, which hybrid? Explain. 



4. What two kinds of gametes might be formed by primary sex cells 

 which have a pair of genes, RW? Show by diagram why you might 



