56 CHAPTER 4 



4.2 l >. When one crosses pure White 1 eghorn poultry with pure White Silkies, all the 

 I are while. In the 1 ■'■_.. however, large numbers of progeny occur in a ratio 

 approaching 13 white:3 colored. Choosing and defining your own gene symbols, 

 explain these results genetically. 



4.30. (a) In the yellow daisy the Bowers typically have purple centers. A yellow- 

 centered mutant was discovered which when crossed to the purple-centered type 

 gave all purple-centered I,, and among the F 2 47 purple and 13 yellow. Explain 

 these results genetically . 



(b) later, another yellow-centered mutant occurred which also gave all purple 

 F, from crosses with purple-centered daisies. When these F, were crossed to- 

 gether, however, there were 97 purples and 68 yellows. Explain these results 

 genetically. 



(c) How can you explain that a cross between the two yellow-centered mutants 

 produced all purple-centered F,? 



4.31. Give a single genetic explanation that applies to all the following facts regard- 

 ing human beings: 



(a) One particular deaf couple has only normal progeny. 



( b ) One particular deaf couple has only deaf progeny. 



(c) One particular normal couple has many children, about % are normal and 

 Y 4 deaf. 



(d) One particular normal couple has all normal children. 



(e) Normal, identical twins marry normal, identical twins and have a total of 

 9 normal and 9 deaf children. 



4.32. How can you explain the observations with regard to lint color of cotton that 

 brown X green gives green F,, which when mated together produce F-, which 

 contain mostly brown, some greens, and a few whites? 



4.33. Suppose two unrelated albinos married and had 8 children, 4 albino and 4 non- 

 albino. How could you explain these results? 



4.34. When, during the life cycle, can dominance and/or epistasis occur or not occur 

 in maize? Neurospora? 



4.35. When two plants are crossed it is found that °%4 of the progeny are phenotypically 

 like the parents, and 1 <u of the progeny are different from either parent but re- 

 semble each other. Give a genetic explanation for this. 



4.36. Would you expect to find epistasis in man in marriages involving genetic alterna- 

 tives for both woolly hair and baldness? Brown eyes and albinism? Baldness 

 and brown eyes? 



4.37. Assume, in man, that the difference in skin color is due primarily to two pairs 

 of genes which segregate independently: BB CC is black, bb cc is white, any three 

 of the genes in black produce dark skin, any two medium skin, and any one 

 produces light skin color. Give the genotypes of parents who are: 



(a) Both medium, but have one black and one white child. 



(b) Both black but have an albino child. 



(c) Both medium and can have only medium children. 



(d) Medium and light and have a large number of children: % medium, : }s light, 

 's dark, \s white. 



