Assuming Merope Gaunt has the homozygous dominant trait for speaking parseltongue and her husband Tom Riddle has the homozygous recessive trait for not being able to speak parseltongue, what is the likelihood that their son, Tom Riddle, Jr. (later to become Lord Voldemort), will speak parseltongue?
If Merope Gaunt has the homozygous dominant trait for speaking parseltongue (let's represent it as PP) and Tom Riddle has the homozygous recessive trait for not being able to speak parseltongue (pp), their son's genotype would be Pp (heterozygous).
According to Mendelian inheritance, a heterozygous individual (Pp) has a 50% chance of inheriting the dominant trait (P) and a 50% chance of inheriting the recessive trait (p). So, there is a 50% likelihood that Tom Riddle, Jr. will inherit the ability to speak parseltongue.