Patterns of Inheritance

Question 1

autosomal dominant

Answer 1

one copy of abnormal/mutated gene leads to disorder
*usually one affected and one unaffected parent (child has a 50% chance of having the disorder)

Question 2

common trick in autosomal dominant

Answer 2

oftentimes, being homozygous for the mutation is LETHAL; so if 2 affected parents and they ask for the chance of child being affected, they would have a 2/3 chance of having a liveborn child

Question 3

achondroplasia

Answer 3

*autosomal dominant
*most common cause of disproportionate short stature
*clinical features: disproportionate short stature, rhizomelic shortening, relative macrocephaly, trident hands; normal intelligence and life expectancy

Question 4

autosomal recessive

Answer 4

two copies of the mutation/abnormal gene required to manifest the disorder; if both parents are carriers, there is a 1/4 chance of affected child

Question 5

trick with autosomal recessive questions

Answer 5

if they tell you that the child is NOT affected and ask the chance that they are a carrier, then it is 2/3 (because you eliminate the chance that they are affected)

Question 6

phenylketonuria (PKU)

Answer 6

*autosomal recessive
*elevated Phe and low tyrosine
*clinical features: mousy/musty odor, lighter pigmentation, seizures/microcephaly; severe to profound intellectual disability

Question 7

X-linked dominant

Answer 7

only need 1 abnormal X chromosome to be affected
-affected mother: can pass it to sons and daughters
-affected father: can ONLY pass it to daughters (must give their son the Y chromosome)
*oftentimes these are embryonic lethal in males

Question 8

Aicardi syndrome

Answer 8

*X-lined dominant
*cardinal features:
1) agenesis of corpus callosum
2) chorioretinal lacunae
3) infantile spasms
*occurs exclusively in females

Question 9

X-linked recessive

Answer 9

*most commonly affects sons, and daughters are carriers
-carrier mother: affected vs unaffected son = 50:50; carrier daughter vs. unaffected daughter = 50:50
-affected father: NO affected sons (they get Y chromosome); ALL daughters are carriers

Question 10

OTC deficiency

Answer 10

*X-linked recessive
*buildup of ammonia can lead to catastrophic neonatal presentation
*s/s can be variable

Question 11

mitochondrial DNA mutations

Answer 11

**only inherited from mother

Question 12

MERRF (myoclonic epilepsy with ragged red fibers)

Answer 12

*mtDNA disorder
*clinical features: myoclonus, epilepsy, ataxia, weakness
**ragged red fibers (blue picture with red-staining mitochondria)