IGA 8/e Chapter 2

Chapter Two 39Answer:a. This problem involves X-inactivation. Let B = black and b = orange.FemalesMalesX B /X B = black X B /Y = blackX b /X b = orange X b /Y = orangeX B /X b = calicob. P X b /X b (orange) X B /Y (black)F 1 X B /X b (calico female)X b /Y (orange male)c. Because the males are black or orange, the mother had to have been calico.Half the daughters are black, indicating that their father was black.d. Males were orange or black, indicating that the mothers were calico. Orangefemales indicate that the father was orange.68. The pedigree below concerns a certain rare disease that is incapacitating but notfatal.a. Determine the most likely mode of inheritance of this disease.b. Write the genotype of each family member according to your proposedmode of inheritance.c. If you were this family’s doctor, how would you advise the three couples inthe third generation about the likelihood of having an affected child?Answer:a. Recessive (unaffected parents have affected progeny) and X-linked (onlyassumption is that the grandmother, I-2, is a carrier). If autosomal, then I-1,I-2, and II-6 would all have to be carriers.b. Generation I: X A /Y, X A /X aGeneration II: X A /X A , X a /Y, X A Y, X A /X – , X A /X a , X A /YGeneration III:X A /X A , X A /Y, X A /X a , X A /X a , X A /Y, X A X A , X a /Y,X A /Y, X A /X –c. Because it is stated that the trait is rare, the assumption is that no onemarrying into the pedigree carries the recessive allele. Therefore, the firstcouple has no chance of an affected child because the son received a Ychromosome from his father. The second couple has a 50 percent chance ofhaving affected sons and no chance of having affected daughters. The third