26. You are studying a rare skin condition and are trying to determine if it is
ID: 187127 • Letter: 2
Question
26. You are studying a rare skin condition and are trying to determine if it is recessive or dominant and if it is autosomal or X-linked. You are looking through patient records and come across a family in which both parents have the condition, their daughter has the condition, but their son does not. (a) Which best describes the condition? iv. autosomal recessive v. autosomal dominant vi. X-linked recessive vii. X-linked dominant (b) For each of the possible answers to part a, explain why they are or are not consistent with the data on this family.Explanation / Answer
Dominant : Presence of one copy of the gene or one allele is sufficient to express the trait.
Recessive : Presence of two copies of the gene or both alleles are necessary to express the trait.
X-linked recessive : Males need only one affected X to express the trait as males have one X and one Y. And, females need two affected XX to express the trait.
X-linked dominant : Only one X can express the trait in female.
Affected Father XY x Mother XX
Affected daughter XX and unaffected son XY
It cannot be recessive because both parents will have all affected alleles and hence all the children would have affected. So, it cannot be autosomal or X-linked recessive.
If it is X-linked dominant, the following is the result
If it is autosomal dominant, the following is the result.
It cannot be autosomal dominant because 3/4 of the children are affected.
Answer : Therefore, it is X-linked dominant. Because half of the son can be unaffected and half of the daughters can be unaffected.
X Y X XX - affected daughter XY - affected son X XX - daughter XY - sonRelated Questions
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