Question

1. Use the information provided below to draw a pedigree of the Alex’s family and to answer the questions stated below:

A) Alex’s family pedigree consists of three generations (I-1: Alex; I-2: Cynthia; II-1: James; II-2: Mary; II-3: Bobby; II-4: Joe; II-5: Linda; II-6: Evelyn; II-7: Kate, II-8: John; III-1: Mike; III-2: Tina; III-3: Gina; III-4: Angel; III-5; Dave.

B) Alex and Cynthia are the grandparents of Mike, Tina, Gina, Angel and Dave. C) Mike, Tina and Gina are the children of James and Mary.
D) Joe and Linda are the parents of Angel and Dave.
E) James and Linda married into the Alex’s family.

F) All individuals in the 2nd generation are offspring of Alex and Cynthia, except James and Linda.

G) The Alex’s family segregates a genetic disease, Huntington’s disease. Alex is homozygote for the disease, and Cynthia does not carry the disease alleles.

H) James and Mary have two affected offspring, a male and a female. I) All the children of Joe and Linda are affected.

J) Determine the mode (i.e. pattern) of inheritance of Huntington’s disease in the Alex’s family.

K) Show (i.e. label) the genotypes of all the individuals in the pedigree by showing how the disease alleles segregate (show every person alleles) in the family.

L) Label both affected and unaffected individual in the pedigree correctly.

M) Applied genetic question: If one or two offspring of Alex and Cynthia failed to express the Huntington’s disease, what genetic explanation can be used to support the lack of Huntington’s disease in those offspring?

Answer 1

Alex is homozygote for the disease-causing gene and Cynthia is not. Huntington's disease is a rare neurodegenerative disease and so it can be assumed that people marrying into second or subsequent generations do not have the disease, either in homozygote or heterozygote form. It is not specified in the question as well, so the natural assumption is that they are not affected.

With this assumption in place, it can be found that the disease is an autosomal dominant one. because, if James did not have any disease alleles ( as is the assumption in case of rare diseases), and Mary is a heterozygote (the only possibility for all of Alex and Cynthia's children as Alex and Cynthia are both homozygotes, their children can be affected only in such a case. Also, since Joe's and Mary's children are affected, i.e the children of a son as well as a daughter of Alex and Cynthia are affected, and both sexes are affected in the subsequent generation as well it is not an X linked disorder.

Therefore, all of Alex and Cynthia's children are affected, while the people who married into the family, i.e., James and Linda are not affected.

Both of Linda and Joe's children, i.e. Angel and Dave are affected,

Only two, one male and one female offspring of James and Mary are affected, i.e. either Tina or Gina, as well as Mike is affected.

So, the pedigree chart would be as the image given below