explain why when Mendel crossed a purebred short pea plant with a purebred tall pea plant, the resulting F1 generation had only tall plants.
When Mendel crossed a purebred short pea plant with a purebred tall pea plant, the resulting F1 generation had only tall plants because of the principles of dominant and recessive traits in genetics.
In this case, the gene for tallness is dominant (T) and the gene for shortness is recessive (t). When a purebred tall plant (TT) is crossed with a purebred short plant (tt), all the offspring in the F1 generation will inherit one dominant allele for tallness from the tall parent, making them all heterozygous (Tt) for the tall trait. Since the dominant allele masks the expression of the recessive allele, all the plants in the F1 generation will exhibit the tall phenotype, even though they carry the allele for shortness.
This is known as the principle of dominance, where one allele in a pair of alleles will be expressed over the other allele in a heterozygous individual. This is why the F1 generation of Mendel's pea plants all appeared tall, even though they carried the allele for shortness.