P Gene:
Parental Cross |
Normal x Normal |
---|---|
|
|
Progeny | Some children with extra digits |
Why is this unusual result seen? Because both parents are normal we expect that they homozygous recessive and would not produce offspring with extra digits.
Explanation: One parent must carry at least one dominant allele (P allele) and its genotype is probably Pp. This parent with the Pp genotype exhibits reduced penetrance for the P allele.
Penetrance - the frequency of expression of an allele when it is present in the genotype of the organism (if 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant)
For polydactyly, an extra digit may occur on one or more appendages, and the digit can be full size or just a stub. Therefore, when the P allele is present it expresses variable expressivity.
Expressivity - variation in allelic expression when the allele is penetrant