WORKED PROBLEMS

Problem 1

Joanna has “short fingers” (brachydactyly). She has two older brothers who are identical twins; both have short fingers. Joanna’s two younger sisters have normal fingers. Joanna’s mother has normal fingers, and her father has short fingers. Joanna’s paternal grandmother (her father’s mother) has short fingers; her paternal grandfather (her father’s father), who is now deceased, had normal fingers. Both of Joanna’s maternal grandparents (her mother’s parents) have normal fingers. Joanna marries Tom, who has normal fingers; they adopt a son named Bill who has normal fingers. Bill’s biological parents both have normal fingers. After adopting Bill, Joanna and Tom produce two children: an older daughter with short fingers and a younger son with normal fingers.

• a. Using standard symbols and labels, draw a pedigree illustrating the inheritance of short fingers in Joanna’s family.
• b. What is the most likely mode of inheritance for short fingers in this family?
• c. If Joanna and Tom have another biological child, what is the probability (based on your answer to part b) that this child will have short fingers?

Solution Strategy

What information is required in your answer to the problem?

• a. A pedigree to represent the family, drawn with correct symbols and labeling.
• b. The most likely mode of inheritance for short fingers.
• c. The probability that Joanna and Tom’s next child will have short fingers.

What information is provided to solve the problem?

The phenotypes of Joanna and Tom and their family members.

For help with this problem, review:

Information on pedigrees in Section 6.2.

Solution Steps

• a. In the pedigree for the family, identify persons with the trait (short fingers) by filled circles (females) and filled squares (males). Connect Joanna’s identical twin brothers to the line above by drawing diagonal lines that have a horizontal line between them. Enclose the adopted child of Joanna and Tom in brackets; connect him to his biological parents by drawing a diagonal line and to his adopted parents by a dashed line.

  Hint: See Figure 6.2 for a review of symbols used in a pedigree.

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  Hint: See Table 6.2 for a review of the characteristics of different modes of inheritance.

• b. The most likely mode of inheritance for short fingers in this family is autosomal dominant. The trait appears equally in males and females and does not skip generations. When one parent has the trait, it appears in approximately half of that parent’s sons and daughters, although the number of children in the families is small. We can eliminate Y-linked inheritance because the trait is found in females as well as males. If short fingers were X-linked recessive, females with the trait would be expected to pass the trait to all their sons, but Joanna (III-6), who has short fingers, produced a son with normal fingers. For X-linked dominant traits, affected men should pass the trait to all their daughters; because male II-1 has short fingers and produced two daughters without short fingers (III-7 and III-8), we know that the trait cannot be X-linked dominant. The trait is unlikely to be autosomal recessive, because it does not skip generations and approximately half the children of affected parents have the trait.
• c. If having short fingers is autosomal dominant, Tom must be homozygous (bb) because he has normal fingers. Joanna must be heterozygous (Bb) because she and Tom have produced both short- and normal-fingered offspring. In a cross between a heterozygote and homozygote, half the progeny are expected to be heterozygous and the other half homozygous (Bb × bb → Bb, bb); so the probability that Joanna’s and Tom’s next biological child will have short fingers is .

Problem 2

Concordance values for a series of traits were measured in monozygotic twins and dizygotic twins; the results are shown in the following table. For each trait, indicate whether the rates of concordance suggest genetic influences, environmental influences, or both. Explain your reasoning.

Solution Strategy

What information is required in your answer to the problem?

For each trait, whether it is influenced by genetic factors, environmental factors, or both.

What information is provided to solve the problem?

Concordance values of monozygotic and dizygotic twins for each trait.

For help with this problem, review:

Concordance in Twins in Section 6.3.

Solution Steps

• a. The concordance for ABO blood type in the monozygotic twins is 100%. This high concordance in monozygotic twins does not, by itself, indicate a genetic basis for the trait. Because concordance for ABO blood type is substantially lower in the dizygotic twins, we would be safe in concluding that genes play a role in determining differences in ABO blood types.

  Recall: The influence of genetic factors on the trait is indicated by higher concordance in monozygotic twins than dizygotic twins.

• b. The concordance for diabetes is substantially higher in monozygotic twins than in dizygotic twins; therefore, we can conclude that genetic factors play some role in susceptibility to diabetes. The fact that monozygotic twins show a concordance less than 100% suggests that environmental factors also play a role.

  Recall: Less than 100% concordance in monozygotic twins indicates that environmental factors play a role in the trait.

• c. Both monozygotic and dizygotic twins exhibit the same high concordance for coffee drinking; so we can conclude that there is little genetic influence on coffee drinking. The fact that monozygotic twins show a concordance less than 100% suggests that environmental factors play a role.
• d. The concordance for smoking is lower in dizygotic twins than in monozygotic twins; so genetic factors appear to influence the tendency to smoke. The fact that monozygotic twins show a concordance less than 100% suggests that environmental factors also play a role.
• e. Monozygotic twins exhibit substantially higher concordance for schizophrenia than do dizygotic twins; so we can conclude that genetic factors influence this psychiatric disorder. Because the concordance of monozygotic twins is substantially less than 100%, we can also conclude that environmental factors play a role in the disorder as well.