Shown below is the pedigree of a rare neurodegenerative disease in humans. 13 14

Question

Shown below is the pedigree of a rare neurodegenerative disease in humans. 13 14 89 101 12 13 1 a. Based on the pedigree, characterize each of the following modes of inheritance as likely, unlikely, or impossible: autosomal dominant, autosomal recessive, Y linked, X-linked dominant, and X-linked recessive. Justify your reasoning. Further scrutiny of members of this family reveals the following: 1) The discase's degree of expression varies among individuals, with some mildly affected, others moderately affected and stll others severely affected; and 2) Individual III-14 was previously married. Her ex-husband is phenotypically normal, with no history of the disease in his family going back ten generations. He has custody of the couple's two children, a girl and a boy, both of whom are affected with the discase. The partial pedigree is shown below (the "slash" represents separation or divorce) 14 15 15 16 Do these findings eliminate any of the remaining "likely or unlikely" modes of inheritance listed in part (a)? Are any of these modes of inheritance still possible? If so, which one(s) and why? b. Is mitochondrial inheritance a likely, unlikely or an impossible mode of inheritance for this discase? Does it provide an explanation for the discase's variable expressivity? c. d. How can mitochondrial inheritance explain individual III-14?

Explanation / Answer

a) Autosomal dominant inheritence - unlikely - because mating between affected and normal individual produces normal offspring.

Autosomal recessive inheritance- possible - can express only in homozygous states, both the parents should be affected or both should be carriers for disease to occur in children.

Y linked - impossible - females are also affected

X linked dominant - possible - mating between affected female and normal male produces offsprings who are all affected by the disease.

X linked recessive - unlikely - In X linked recessive inheritance, males are affected more and females are less likely to be affected.

b) The results of the cross between III14 and 15 eliminates autosomal dominant, autosomal recessive and X linked recessive modes of inheritance. The remaining is autosomal X linked dominant mode of inheritance. Since in some individuals, the disease is not expressed fully III14 has not been affected.

c) Mitochondrial inheritance is possible. Mitochondrial inheritance is totally maternal. All offsprings produced by the affected females will develop disease. This phenomenon can be seen in pedigree chart.

d) Since it is already said that in few individuals, disease is not expressed fully III14 could be suffering from the disease and transmits it to both her offsprings.

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