What is polydactyly, and how does inheriting a dominant allele cause this condition?
What is polydactyly, and how does inheriting a dominant allele cause this condition?
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Polydactyly is a congenital condition characterized by the presence of one or more extra fingers or toes, resulting in a total count greater than the normal five digits on each hand or foot. The term “polydactyly” comes from the Greek words poly (meaning “many”) and dactylos (meaning “finger”). This condition can manifest in various forms, including preaxial (extra digits on the thumb or big toe), central (extra digits between existing fingers or toes), and postaxial (extra digits on the little finger or toe) polydactyly.
Inheritance of Polydactyly
Polydactyly is primarily inherited in an autosomal dominant manner. This means that only one copy of the mutated gene from either parent is sufficient to cause the condition. Here’s how this inheritance pattern works:
- Dominant Allele: In autosomal dominant disorders, the presence of a dominant allele leads to the expression of the trait. For polydactyly, if an individual inherits the dominant allele responsible for the condition from one parent, they will typically exhibit polydactyly, regardless of whether the other parent has the condition.
- Penetrance and Expressivity: The penetrance of polydactyly can vary, meaning that not all individuals with the dominant allele may express the condition (some may have a mild form or none at all). Additionally, expressivity refers to how severely the condition manifests in different individuals; some may have more pronounced extra digits than others.
- Genetic Variants: Several genes have been implicated in polydactyly, including GLI3, SHH (sonic hedgehog), and others involved in limb development. Mutations in these genes can disrupt normal limb patterning during embryonic development, leading to the formation of extra digits.
- Family History: Polydactyly often runs in families due to its genetic basis. If a parent has polydactyly, there is a significant chance that their children may inherit the condition, typically with a 50% probability for each child if one parent carries the dominant allele.
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