Final answer:
The trait that is masked in the F1 generation and reappears in the F2 generation is known as a recessive trait. This is explained by Mendel's laws of inheritance and is observed with a 3:1 phenotypic ratio in the F2 generation.
Step-by-step explanation:
The trait and its unit factor that doesn't actually disappear in the F1 generation but are merely hidden or masked, only to reappear in 1/4 of the F2 offspring, are known as recessive traits and their associated unit factors. In the context of genetics, these unit factors are more commonly referred to as genes. Mendel’s laws of inheritance explain that traits can be dominant or recessive. When crossing two parents that are homozygous for contrasting traits (e.g., tall and short), all F1 offspring will display the dominant trait. These F1 offspring are heterozygous, carrying both a dominant (T) and a recessive (t) gene. However, when the F1 generation is self-pollinated or crossed with each other, the F2 generation will have a 3:1 phenotypic ratio, with 1/4 being homozygous recessive (tt) and displaying the recessive trait. This demonstrates that the recessive trait was not lost, but simply masked by the presence of the dominant trait in the F1 generation.