Marvolo Gaunt has the homozygous trait for speaking parseltongue (being able to speak to snakes) and his wife also had the homozygous trait. If parseltongue is dominant, what is the likelihood that their daughter, Merope, will speak parseltongue?
If parseltongue is a dominant trait, and both Marvolo Gaunt and his wife have the homozygous dominant genotype for speaking Parseltongue, it means that both of them possess two copies of the parseltongue allele (we can represent this dominant allele as "P"). Their genotypes would both be PP.
When two individuals with homozygous dominant genotypes have a child, the potential genetic combinations for their offspring would only result in the dominant trait being expressed. Each parent would pass on one of their "P" alleles to their child, since each parent provides one allele for each gene.
Therefore, their child would also be homozygous dominant for speaking Parseltongue with a genotype PP. The likelihood of their daughter Merope speaking Parseltongue is 100%, provided that parseltongue is entirely determined by this genetic factor and there are no other factors that could influence the ability to speak to snakes.