Question

the figure above represents a pedigree for a family in which dark - shaded symbols represent individuals with one of the two major types of colon cancer. numbers under the symbols are the individuals age at the time of diagnosis. a slash through the symbol indicates that the individual is deceased. in each generation of this family after generation i, the age at diagnosis is significantly lower than would be found in nonfamilial (sporadic) cases of this cancer (~63 years). what is the most likely reason? this is pure chance: it would not be expected if you were to look at a different family. this cancer requires mutations in more than this one gene. members of this family know to be checked for colon cancer early in life. hereditary (or familial) cases of this cancer typically occur at earlier ages than do nonfamilial forms.

Explanation:

In the pedigree, multiple family members have colon cancer at younger ages. Hereditary (familial) cancers often show a pattern of earlier - onset compared to sporadic (non - familial) cases. Pure chance would not explain the consistent pattern in this family. The fact that it's a family - related pattern rather than a need for multiple mutations (which is more about the biological process of cancer development in general, not specifically the age - of - onset pattern in a family) and the idea that members know to be checked early (while possible, it doesn't fully explain the genetic - like pattern of younger ages across generations as well as the hereditary cancer concept) are less likely. Hereditary cancers are known to have earlier - age diagnoses in families due to inherited genetic predispositions.

Answer:

Hereditary (or familial) cases of this cancer typically occur at earlier ages than do nonfamilial forms.