Read Mendelian Disorders from Cause to Inheritance
Mendelian disorders often follow single-gene inheritance, while the molecular defect predicts characteristic symptoms.
Mendelian Disorders is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Infer carrier parents
For an autosomal recessive disorder, unaffected carrier parents are heterozygous and can have an affected homozygous child. β-thalassaemia with unaffected parents is consistent with Bb × Bb.
Connect gene defect to phenotype
Phenylketonuria results from impaired phenylalanine metabolism and can cause phenylpyruvic acid accumulation, cognitive impairment and reduced pigmentation. Thalassaemias reduce globin-chain production; α-thalassaemia commonly follows deletion of α-globin genes on chromosome 16.
Classify inheritance
Sickle-cell anaemia is autosomal recessive; haemophilia and red-green colour blindness are X-linked examples.
Fragile X repeat expansion
Fragile X syndrome is associated with CGG-repeat expansion in FMR1. Larger expansions are associated with affected individuals; transition to a full mutation is especially likely through maternal transmission. Paternal transmission of a premutation is comparatively stable.
OlympIQ exam lens
- ✓Unaffected parents with affected recessive child are often carriers.
- ✓PKU: phenylalanine metabolites and reduced pigmentation.
- ✓α-thalassaemia: reduced α-globin synthesis.
- ✓Sickle-cell disease is autosomal recessive.
- ✓Fragile X: CGG repeat expansion in FMR1.
- ✓Maternal transmission is important for full-mutation expansion.
A globin-chain deficiency causes imbalance; do not attribute the disorder to excess of the deficient chain.
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