Patterns of inheritance


When looking at the inheritance of disease in humans, it can be a powerful tool of prediction to know the pattern of inheritance adopted by the disease-causing allele(s). Some such patterns are described below.

Autosomal dominant: a pattern of inheritance involving a dominant allele that is carried on an autosome. Autosomal dominant diseases include Huntingdon’s Disease. Sex is irrelevant in the transmission of the disease. If an affected parent mates with an unaffected parent, then 50% of the offspring would be expected to inherit the disease. Not all autosomal dominant diseases show full penetrance and expressivity, although Huntingdon’s Disease invariably does show 100% penetrance.

Autosomal recessive: a pattern of inheritance involving a recessive allele that is carried on an autosome. Autosomal recessive diseases include cystic fibrosis. Sex is irrelevant in the transmission of the disease. If a person is heterozygous for an autosomal recessive disease then they are said to be carriers; when two carriers mate 25% of the offspring are expected to have the disease.

X-linked dominant: a pattern of inheritance involving a dominant allele that is carried on an X chromosome. X-linked dominant diseases include Charcot-Marie Tooth Disease (CMT). More females are affected than males, due to the presence of an additional X chromosome in females. An affected mother’s offspring, whether a boy or a girl, has a 50% chance of being affected depending on which of the mother’s X chromosomes is inherited. A father who is affected will affect all of his daughters and none of his sons, since the daughter must inherit her father’s X chromosome and the son must inherit his father’s Y chromosome.

X-linked recessive: a pattern of inheritance involving a recessive allele that is carried on an X chromosome. X-linked recessive diseases include Duchenne muscular dystrophy, haemophilia and Red-Green colour blindness. Such disorders mainly affect males, since males need inherit only one copy of the allele (on their single X chromosome), while females need two copies. Male-to-male transmission is impossible because only the Y chromosome is passed down the paternal-son line. Affected males are often born to unaffected parents. Affected females require an affected father and a mother who carries the allele on one of her X chromosomes.