Pedigrees and Human Genetic Disorders
Reading Pedigrees
Pedigree Clues
Pedigrees summarize how a trait appears across generations. Analysts use them to infer whether a condition is autosomal dominant, autosomal recessive, X-linked, mitochondrial, or influenced by other factors.
Autosomal Dominant vs Recessive
Autosomal dominant traits often appear in every generation, while autosomal recessive traits may skip generations and appear among siblings with unaffected parents.
A Strong Clue
An affected father cannot pass an X-linked trait to his sons, because sons receive his Y chromosome, not his X chromosome.
Which pattern often suggests an autosomal recessive disorder?
Carriers can have unaffected phenotypes, so recessive traits may appear unexpectedly in siblings.
Correct answer: Affected children born to unaffected parents
Why are mitochondrial disorders typically inherited from the mother?
Mitochondria are usually maternally inherited.
Correct answer: Because the zygote receives most of its mitochondria from the egg cytoplasm.
If a trait appears in every generation and affected individuals usually have an affected parent, it is most consistent with
Vertical transmission is typical of dominant inheritance.
Correct answer: Autosomal dominant
Common Disorders
Examples include cystic fibrosis, sickle cell disease, Huntington disease, and Duchenne muscular dystrophy. Each illustrates a different inheritance pattern and molecular mechanism.
What is a carrier in genetics?
Carriers are common in recessive disorders.
Correct answer: An individual who has one disease-causing allele but typically does not show the full phenotype.
Which disorder is classically X-linked recessive?
Duchenne muscular dystrophy is a classic X-linked recessive disorder.
Correct answer: Duchenne muscular dystrophy