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Chapter 17

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“[Minors] should not be tested except under the following specific circumstances:

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  1. there are immediate medical benefits, such as institution of measures that can prevent the disease, delay its onset, limit its severity, or prevent secondary disabilities, or

  2. there is a benefit to another family member and no anticipated harm to the minor.

    When the results of genetic testing will be used solely for future reproductive decisions or when parents request it and there are no benefits to the child, in most circumstances it should be deferred until the child can request such testing as an autonomous individual who is able to appreciate the emotional and social consequences, as well as the genetic facts, of the results.”

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Classification

Definition

Examples

Malformation

An abnormally formed structure caused by an intrinsic defect in tissue formation

Bilateral renal agenesis caused by the failure of the ureteric buds to form

Cleft lip caused by hypoplasia of the embryonic nasal swellings

Deformation

Normal tissue that is subsequently distorted by abnormal mechanical forces

Oligohydramnios caused by early amniotic rupture, resulting in fetal compression and arthrogryposis

Congenital hip dislocation caused by breech presentation

Disruption

Normal tissue that is subsequently subject to loss of vascular supply caused by ischemia or hemorrhage

A newborn with absent pectoralis major muscle caused by an intrauterine vascular event

A newborn with finger amputations caused by extrinsic compression and resultant ischemia from amniotic bands

Dysplasia

A generalized abnormality of certain tissues

A patient with short stature and rhizomelic shortening of the long bones caused by skeletal dysplasia

Severe anemia, thrombocytopenia, and neutropenia caused by myelodysplastic syndrome

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Syndrome, Association, and Sequence

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  • Syndrome: A recognizable pattern of signs or symptoms that “run together.” Typically differs from association in that a molecular cause has been elucidated. Examples include Turner, Marfan, and CHARGE syndromes (previously charge was an association prior to discovery of a causative gene).
  • Sequence: A pattern of deformations and malformations which is a consequence of a single malformation. Examples include Pierre Robin and Potter sequences.
  • Association: A group of malformations that occur together more than would be expected by chance alone. Examples include VACTERL and MURCS associations.

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Inheritance

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Mendelian Patterns of Inheritance

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  • Autosomal dominant
    • Classic description: Carriers of one mutant allele exhibit the trait or disease and pass it to their offspring with 50% probability regardless of gender (eg, male-to-male inheritance is possible).
    • Allele characteristics: Point mutations (leading to a gain of function), nonsense mutations, insertions, or deletions (in genes that exhibit haploinsufficiency). Protein products are often structural proteins or regulatory signaling proteins.
    • Pitfalls in interpretation: Many individuals with autosomal dominant diseases appear without family history because of a de novo mutation. Incomplete penetrance and variable expressivity may lead to difficulty in determining who in a family is affected. Gonadal mosaicism in one parent may ...

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