CBSE Class 10 Science 2017 · Paper 31/1 · Question 14

Mendel crossed pure tall pea plants with pure short (dwarf) ones. All the $F_1$ plants were tall, yet when they were self-pollinated, short plants reappeared in $F_2$, in the ratio of about $3$ tall : $1$ short. So the $F_1$ plants had inherited both traits but expressed only tallness. He explained this with factors (genes): each plant carries two copies of the factor for a trait, one from each parent. The $F_1$ plants are $\text{Tt}$; one copy of the dominant factor $\text{T}$ is enough to make the plant tall and masks the recessive factor $\text{t}$, which is expressed only when both copies are $\text{t}$ ($\text{tt}$, short). Thus a recessive trait can be inherited without being expressed.