• Question: Polydactyly is an inherited condition, due to a dominant allele, causing extra fingers or toes. What is the chance of a child having polydactyly if their parents are Pp and pp?

    Asked by Esha to Barbara, Matt, Ravinder, Sophie, Tristan on 7 Mar 2015.
    • Photo: Barbara Shih

      Barbara Shih answered on 7 Mar 2015:

      I sure hope this is not a trick question since I don’t know anything about polydactyly. One parent would either pass on a P or p, while the other would pass on a p or a p.

      Parent 1: P or p
      Parent 2: p or p

      Since any combination with P would lead to polydactyly (i.e. both Pp and PP would lead to polydactyly), I find it easier to work out the probability of not having polydactyly (i.e. probability of having pp) because you can then use the same formula for different scenarios.

      Probability of Parent1 to pass p = 50%
      Probability of Parent2 to pass p = 100%

      50% x 100% = 50% (probability of child having pp)
      100%-50% = 50% (probability of child not having pp)

      Therefore there is 50% chance of the child having polydactyly.

    • Photo: Matthew Moore

      Matthew Moore answered on 9 Mar 2015:

      Polydactyly isn’t actually fully understood! During animal development (that includes us) a number of different genes can go wrong to produce too many fingers or toes, or to combine fingers and toes which should be separate!

      This may be dominant as you mentioned, or may be associated with some disorder or many are simply not well understood. Their ‘molecular’ basis is relatively well understood, which proteins involved in development of fingers and toes that are responsible, but not always the exact genes.

      Barbara is right about dominant genes but also a parent could be dominant-dominant (PP), then even if the other parent was recessive-recessive (pp) then there would be a 100% chance of getting it because one parent is always going to donate a dominant allele which is all that’s needed!

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