PLSC 431 - Intermediate Genetics

Mendelian Genetics

Variation in Gene Expression

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

First Page Previous Page Next Page Last Page