In hereditary cross only one character is expressed outwardly in the individual of 1st filial generation because all individuals of 1st filial genrations are hybrid. It means all of them contains both dominant and recessive factor of a contrasting character. 

According to Law of dominance or, 1st law of heredity dominant character is expressed outwardly even in presence of its allele. That’s why only dominant character or, only one character is expressed outwardly in 1st filial generation.