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Sunday, January 06, 2008

An affected male infant born to normal parents

005. An affected male infant born to normal parents could be an example of all of the following, except:

1. An Autosomal dominant disorder.

2. An Autosomal recessive disorder.

3. A polygenic disorder.

4. A vertically transmitted disorder.

Answer

1. An Autosomal dominant disorder.

4. A vertically transmitted disorder.

Reference:

Harrison 16th Edition Page 374

Robbins 7th Edition Page 150

Quality

Thinker, Trap

Status

Repeat, but with different choices and a trap

QTDF

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Discussion

Autosomal dominant disorders assume particular relevance because mutations in a single allele are sufficient to cause the disease. In contrast to recessive disorders, in which disease pathogenesis is relatively straightforward because there is loss of gene function, in dominant disorders there are various disease mechanisms, many of which are unique to the function of the genetic pathway involved.

Explanation

In autosomal dominant disorders, individuals are affected in successive generations; the disease does not occur in the offspring of unaffected individuals. (Harrison)

Comments

In autosomal dominant disorders, Males and females are affected with equal frequency because the defective gene resides on one of the 22 autosomes. Autosomal dominant mutations alter one of the two alleles at a given locus. Because the alleles segregate randomly at meiosis, the probability that an offspring will be affected is 50%. Unless there is a new germline mutation, an affected individual has an affected parent. Children with a normal genotype do not transmit the disorder. Due to differences in penetrance or expressivity, the clinical manifestations of autosomal dominant disorders may be variable. Because of these variations, it is sometimes challenging to determine the pattern of inheritance.

Tips

Ä Children with normal genotype do not transmit an Autosomal Dominant disease. But there are conditions in which an affected child can be born to the normal parents in case of Autosomal Dominant Inheritance. (Am I confusing you??!!) These conditions are

o Mutations, in with an affected individual has a normal parent

o Reduced Penetrance – where the individual inherits the mutant gene, but can be phenotypically normal

Ä On the other hand, you may think that if a male (or female) child is affected by a disease transmitted vertically, he CANNOT have a mother who is normal (not affected). If the baby has HIV transmitted vertically, the mother should have had that infection. So what is the answer for this question ???

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