Answer: Mendel considered traits hidden in the first generation to be recessive.
Explanation: Recessive traits, in order to be expressed in an organism's phenotype (physical characteristics) must be homozygous. This means that in a monohybrid cross, which means a trait with only one gene, (E.g. Aa, aa, AA) in order for a recessive gene to be expressed it must be combined with itself. (Examples include aa, bb, cc,)