514 The Determination of Sex 



tion, endosperm formation, or both, and in that it produces a 

 new individual with the characteristics of a seedling whereas 

 the individual arising by vegetative apomixis has the charac- 

 teristics of an adult. In the third type a diploid gametophyte 

 gives rise to a diploid sporophyte. In this type there is a regu- 

 lar alternation of generations, but, although the two generations 

 differ morphologically, they do not differ with respect to their 

 chromosome number. There are several methods by which this 

 development of the two generations occurs which naturally does 

 not involve either a fiormal meiosis or fertilization. Plants that 

 arise by apomixis are usually very similar phenotypically, but 

 occasionally show genetic variation. 



QUESTIONS AND PROBLEMS 



1. In some dioecious plants, occasional bisexual flowers are formed. 

 If a bisexual flower on a normally female plant whose sex chromosomes 

 are XX is self-fertilized, what would be the sex of the offspring? 



2. In what fundamental ways do the balance theories of Bridges, 

 Goldschmidt, and Correns agree and in what ways do they differ? 



3. If no polyploid types of Drosophila melanogaster had ever arisen, 

 what evidence would there be that sex was determined by the X chromo- 

 somes rather than by the Y chromosome? 



4. In Lychnis (Melandrium) gene B determines broad leaves and b 

 narrow leaves. These genes are on the X chromosome. All pollen with 

 the b gene is lethal. What offspring would be expected from the follow- 

 ing crosses? 



XB /XB xXb /Y 

 XB/Xb XXb /Y 

 XB/Xb XXB/Y 



5. In Silette otites, the male appears to be heterogametic. In a 

 colchicine-induced tetraploid, the female is XXXX and the male XXYY. 

 If these are crossed, what should be the ratio of males to females if any 

 tetraploid with a Y chromosome is a male? 



6. If the female i^x problem 5 were the heterogametic sex, what would 

 be the ratio in the offspring if two Y chromosomes are necessary to make 

 a female? If a cross between two tetraploids produced a ratio of 5 

 males : 1 female, which theory w^ould be correct, that the male is hetero- 

 gametic as in problem 5 or that the female is the heterogametic sex? 

 Explain. 



