~>l CHAPTER 4 



4.3. What types and frequencies of gametes are formed by the following genotypes, 

 all gene pans segregating independently? 



(a) An lib C< (C) A a lib Cc 



(b) AABBCcDD (d) MmNnOoPp 



4.4. How mam different diploid genotypes are possible in offspring from crosses in 

 which both parents are undergoing independent segregation for the following 

 numbers o\ pairs of heterozygous genes — 1, 2, 3, 4. n? 



4.5. What conclusions could you reach about the parents if the offspring had pheno- 

 types in the following proportions? 



(a) 3 : 1 (c) 9 : 3 : 3 : 1 



( b ) I : I ( d ) 1 : 1 : 1 : 1 



4.6. Would you be justified in concluding that a pair of chromosomes can contain 

 only a single pair of genes? Explain. 



4.7. Suppose a particular garden pea plant is a septahybrid. What proportion of its 

 gametes will carry all seven recessive nonalleles? All seven dominant nonalleles? 

 Some dominant and some recessive nonalleles? 



4.8. What proportion of the offspring of the following crosses, involving independent 

 segregation, will be completely homozygous? 



(a) AaBbXAaBb (c) Aa BB Cc X A A Bb cc 



(b) AA BBCCX AA bbec (d) AA' X A" A'" 



4.9. Following independent segregation, why would you expect that gametes fertilize 

 at random with respect to their genotypes? 



4.10. Discuss the particulate nature of the genetic material. 



4.11. Does the discovery of independent segregation of nonalleles affect your concept 

 of gene size? Explain. 



4.12. Discuss your current understanding of the term "genetic recombination." 



4.13. Discuss the factors that can modify the expected phenotypic ratio. 



4.14. In snapdragons, red flowers ( R ) are incompletely dominant to white (/), the 

 hybrid being pink; narrow leaves (N) are incompletely dominant to broad leaves 

 (/;), the hybrid being intermediate in width ("medium"). Assuming the gene 

 pairs recombine independently, give the genotypic and phenotypic ratios ex- 

 pected among the progeny of a cross between the following: 



(a) a red medium and a pink medium plant 



(b) a pink medium and a white narrow 



(c) two identical dihybrids 



4.15. Suppose an albino child also suffers from thalassemia minor. Give the most 

 likely genotypes of the parents. 



4.16. How can you explain that a certain kind of baldness, due to a single gene pair, 

 is dominant in men and recessive in women? 



4.17. Though blue-eyed couples ordinarily have only blue-eyed progeny, brown-eyed 

 couples may also have blue-eyed children. Select and define gene symbols so 

 you can give the complete genotypes of all individuals mentioned in each of 

 the families listed: 



(a) One member of a pair of twin boys has brown eyes, the other has blue eyes. 



(b) A blue-eyed, woolly-haired child resembles his father in one of these respects 

 and his mother in the other respect. 



(c) A brown-eyed, nonthalassemic child is like the grandmother but unlike the 

 mother in both of these respects. 



