degree of homozygosity increases. Unfavor- 

 able recessives are apt to be widely spread in 

 any mixed population, since heterozygous in- 

 dividuals, which suffer none of the injurious 

 effects, continue to act as carriers. But in- 

 breeding tends to increase homozygosity and 

 there is always a good chance that unfavor- 

 able recessives may become homozygous in 

 the selected stock. On the other hand, it is 

 quite possible, by selecting only the most 

 vigorous and fertile individuals of each gen- 

 eration, to produce a highly inbred line that 

 is superior to the original mixed population 

 as to general health, vigor, and fecundity. 



Heredity - 517 



The results of artificial selection can be 

 most startling. Among dogs, for example, 

 one can scarcely believe that the "pocket 

 Mexican hairless" belongs to the same spe- 

 cies as the gigantic Great Dane. But always 

 a limit is reached as to how far a certain 

 quality may be carried in a given direction; 

 and barring mutations and aberrations, this 

 limit cannot be exceeded by selection. Arti- 

 ficial methods of increasing the frequency of 

 mutations are too recent to have yielded 

 many practical results, but perhaps these 

 methods can eventually be adapted to utili- 

 tarian purposes. 



1. Explain how and why the known processes 

 of cell division and sexual reproduction con- 

 vinced biologists, even before they knew any- 

 thing about genes, that the mechanisms of 

 hereditary transmission must reside in the 

 nuclei of the gametes (and other germ cells). 



2. Explain the meaning of the phrase "con- 

 tinuity of the germ cells," exemplifying the 

 discussion by describing the "germ cell cycle" 

 of the frog. 



3. How does the prophase of the first matura- 

 tion (meiotic) division, as it occurs in sper- 

 matogenesis (and oogenesis), differ from the 

 prophase of an ordinary (mitotic) division? 

 (At least three differences should be men- 

 tioned.) 



4. Answer the foregoing question with reference 

 to (a) the metaphase of the first maturation 

 division and (b) the prophase of the second 

 maturation division. 



5. Discuss the similarities and differences be- 

 tween spermatogenesis and oogenesis, em- 

 phasizing the functional importance of these 

 points of comparison. 



6. What is synapsis? When does it occur? What 

 are its genetic consequences? 



7. Define and localize: (a) spermatogonia (and 

 oogonia); (b) tetrads; (c) secondary spermato- 

 cytes (and oocytes); (d) spermatids; (e) polar 

 bodies, and (f) ripe ovum. 



8. Explain the meaning of the phrase "free 

 assortment of the chromosomes during meio- 

 sis," using the sperm produced by a sperma- 

 tocyte with two pairs of chromosomes (Aa 

 and Bb) to exemplify the discussion. 



TEST QUESTIONS 



9. Make a series of labeled diagrams to show all 



the stages of maturation in a spermatocyte 

 having two pairs of chromosomes {Aa and 

 lib). 



10. Specify all the possible types of gametes that 

 can be formed by each of the following types 

 of spermatocyte: 



a. AA bb cc 



b. AA Bb Cc 



c. Aa Bb Cc 



11. All the offspring produced by crossing red 

 four-o'clocks with white ones have pink 

 flowers. What proportions of what flower 

 colors will be yielded among the direct off- 

 spring of each of the following crosses: 



a. pink X pink 



b. pink X red 



c. pink x white 



d. red X white 



12. Among the various offspring from the fore- 

 going crosses, indicate those that are (a) 

 homozygous and (b) heterozygous. 



13. In summer squashes, white fruit (Y) is domi- 

 nant over yellow fruit (y). If a squash plant 

 homozygous for white is crossed with one 

 homozygous for yellow, what will the geno- 

 type and phenotype ratios be: 



a. if two of the Fj individuals are inbred 



b. if one of the Fj is mated with its white 

 parent 



c. if one of the Fj is mated with its yellow 

 parent 



14. In the fruit fly, long wing (F) is dominant 

 over vestigial wing (v). What results would 

 be obtained when a heterozygous long- 



