Based on the pedigree, characterize each of the following modes of inheritance a

Question

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 disease’s degree of expression varies among individuals, with some mildly affected, others moderately affected and still 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 disease. The partial pedigree is shown below (the “slash” represents separation or divorce)

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?

Explanation / Answer

If the condition is autosomal dominant, all the offsprings of affected parent should develop the diaease. This condition is not found and therefore autosomal dominant mode of inheritance is unlikely.

Autosomal recessive condition expresses itself only in homozygous condition. Cross between affected person and normal person will always produce normal offspring with heterozygous state. So, this mode of inheritance is also unlikely.

In X linked dominant inheritance, all the affected females will transmit the disease to their sons. So, this is possible in the given pedigree.

In X linked recessive, the female offsprings will always be a carrier. This condition is not found. So, this mode of inheritance is impossible.

In Y linked inheritance, the affected males will transmit the condition to all their sons which is not observed in the given pedigree. So, this mode of inheritance is impossible.

The marriage between III 14 and 15 produced two children, one male and one female both affected. III15 is normal and therefore the disease must have expresded in III14 in milder form. This condition confirms that the condition is transmitted in X linked dominant mode of inheritance.

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