Genetic Inheritance Patterns
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Direct answer
Genetic inheritance patterns describe how a gene variant travels from parents to children, and the pattern fixes the recurrence risk and the look of the family tree. The main modes are autosomal dominant, autosomal recessive, X-linked recessive, X-linked dominant, mitochondrial and multifactorial. Reading a pedigree correctly, especially noticing whether a trait skips generations or affects only males, is the skill most often tested.
What you must remember
- Autosomal dominant: one altered copy causes disease, both sexes are affected equally, and each child of an affected parent has a 50 per cent risk. Examples include achondroplasia, Marfan syndrome and neurofibromatosis type 1.
- Autosomal recessive: both copies must be altered, carriers are usually unaffected, and the recurrence risk is 25 per cent. Consanguinity raises the risk. Examples include cystic fibrosis, phenylketonuria and beta thalassaemia major.
- X-linked recessive: males are affected, carrier mothers pass the variant to half of their sons, and there is no male-to-male transmission. Examples include Duchenne muscular dystrophy and haemophilia A.
- X-linked dominant: affected females are commoner than affected males, and an affected father passes the trait to all daughters but no sons. Example: X-linked hypophosphataemic rickets.
- Mitochondrial: the trait comes only from the mother, because sperm mitochondria are not passed on, and severity varies through heteroplasmy. Examples include Leber hereditary optic neuropathy and MELAS.
- Multifactorial: several genes interact with environmental factors, recurrence risk is low but rises with the number of affected relatives. Examples include cleft lip and palate, pyloric stenosis and neural tube defects.
- Pedigree symbols: squares are males, circles are females, filled shapes are affected, and a double horizontal line between partners marks consanguinity.
A stepwise method for reading a pedigree
A family presents with a child who has a disorder affecting several male relatives across two generations. The pattern is worked out from the chart before any test is ordered.
- Map the sexes. Note how males and females are affected, since sex distribution is the first clue that separates autosomal from X-linked inheritance.
- Check vertical transmission. A trait seen in every generation suggests dominance, whereas a trait that skips a generation suggests recessiveness.
- Look for father-to-son transmission. Its presence excludes X-linked recessive inheritance, because a father passes his only X chromosome to his daughters.
- Note consanguinity. A double line between parents of an affected child points to autosomal recessive inheritance.
- Trace the maternal line. Affected people of both sexes with no father-to-child transmission suggest mitochondrial inheritance.
- Assign the recurrence risk. Use 50 per cent for each child in autosomal dominant disease, 25 per cent in autosomal recessive disease, and a 50 per cent chance of an affected son for a carrier mother in X-linked recessive disease.
Where the exam tries to catch you
Four errors recur on this topic.
- Trap: a 50 per cent risk for every autosomal disorder Correct: autosomal recessive disease carries a 25 per cent risk for each child of two carriers, while autosomal dominant disease carries 50 per cent.
- Trap: father-to-son transmission in X-linked recessive disease Correct: this is impossible, so its presence points to an autosomal pattern.
- Trap: an affected father passing mitochondrial disease to his children Correct: mitochondrial DNA is maternally inherited, so only an affected mother transmits it.
- Trap: assuming every family tree with a skipped generation is recessive Correct: reduced penetrance and new mutations can also cause skipping, so the whole pedigree must be read together.
Frequently asked questions
Can autosomal dominant disease pass from father to son?
Yes, autosomal dominant disease can pass from father to son, because the gene lies on an autosome and not on the X chromosome. Achondroplasia and Marfan syndrome both show this pattern. Father-to-son transmission therefore excludes X-linked recessive inheritance but does not rule out autosomal dominant inheritance.
What is the recurrence risk for siblings of a child with autosomal recessive disease?
When both parents are carriers, each sibling of an affected child has a 25 per cent risk of being affected in every pregnancy. Each pregnancy carries the same risk independently. Two-thirds of the unaffected siblings are carriers, and consanguinity raises the chance that both parents carry the same variant.
Why does mitochondrial disease come only from the mother?
Mitochondrial disease comes only from the mother because the egg supplies almost all the cytoplasm and its mitochondria, while sperm mitochondria are not passed on. All children of an affected mother can inherit the variant, but children of an affected father are unaffected. Heteroplasmy explains why severity varies within a family.
Which disorders show anticipation, and why does it matter?
Anticipation means that a disorder appears earlier or more severely in successive generations, and it is seen in Huntington disease and myotonic dystrophy. Both are autosomal dominant disorders caused by expanding trinucleotide repeats. Anticipation matters because a family history of earlier onset should prompt genetic counselling and testing.