Questions and Problems 431 



mitted as n + 1 types through the pollen, and three have re- 

 sulted in tetrasomics when a trisomic was selfed. When a tetra- 

 somic was selfed, it produced normal 2n plants, tetrasomics 

 like itself, the corresponding primary trisomies, and also sec- 

 ondary trisomies. When crossed as both a male and a female 

 with normal diploid plants, it produced diploids and trisomies 

 but no tetrasomics. Apparently n + 2 gametophytes are in- 

 viable either as males or females. 



QUESTIONS AND PROBLEMS 



1. Assume that A is dominant over a and that a plant is trisomic for 

 the chromosome bearing the a locus. Assume that n + 1 pollen is not 

 viable. What would be the ratio of A to a in the 2n and 2n + 1 offspring 

 from a self-fertilization of plants whose genotypes are: (1) AAA; (2) 

 AAa; (3) Aaa; (4) aaa? 



2. What would be the offspring in question 1 if all the pollen and eggs 

 were viable? 



3. What would be the offspring in question 1 if only 30 per cent of 

 the n+ 1 eggs an^ none of the n + 1 pollen were viable ? 



4. In Datura, the genes for purple (P) and white (p) are in the "18" 

 end of the 17-18 (Poinsettia) chromosome. This chromosome is rarely 

 transmitted through the pollen and to only 27.7 per cent of the eggs in 

 primary trisomies. What would be the ratio of purple to white from self- 

 pollinations of PPp and Ppp plants, and what would be the results of 

 backcrosses in both directions to pp plants? (Neglect the fact that in 

 Datura, n -f- 1 gametes are transmitted through the female in greater 

 proportion when the male is another plant than when the trisomic is self- 

 pollinated.) 



5. In plants of Prime Type 1 of Datura, the following extra chromo- 

 somes are present producing trisomies: (a) 1*2; (6) 7*7; (c) 19*20; 

 {d) 1*18; (e) 17*17. Which types (primary, secondary, or tertiary) 

 would they produce, and what configurations would they produce in a 

 Prime Type 1 plant? 



6. If the extra chromosomes in question 5 were present in Prime 

 Type 2 plants, which types do they produce (primary, secondary, or 

 tertiary), and what configurations would they produce? 



7. How, by means of trisomies, could a newly discovered gene be 

 placed in its particular chromosome? 



8. Arrange the following in order from the greatest to least genie 

 unbalance which they produce, assuming that the same chromosome is 

 involved in each case: (a) primary trisomic; (b) secondary trisomic; 

 (c) tetrasomic; (d) monosomic. Explain. 



