Question

why did the f1 offspring of mendels classic pea cross always look like one of the two parental varieties?
no genes interacted to produce the parental phenotype.
each allele affected phenotypic expression.
the traits blended together during fertilization.
one allele was dominant.

Explanation:

Mendel's experiments showed that when he crossed pure - breeding (homozygous) pea plants for traits like tall and short, the F1 generation all showed the trait of one parent. This is because of the principle of dominance. In a pair of alleles (for a given trait), one allele can mask the expression of the other. The dominant allele is the one whose trait is expressed in the heterozygous (F1) condition. For example, if we consider the allele for tallness (T) and shortness (t), in a cross between TT (tall) and tt (short) parents, the F1 is Tt. The T (tall) allele is dominant over t (short), so all F1 plants are tall.

• The first option “No genes interacted to produce the parental phenotype” is incorrect. Genes do interact in inheritance.
• The second option “Each allele affected phenotypic expression” is not the reason for the F1 looking like one parent. In the case of simple dominance (as in Mendel's first experiments), only the dominant allele affects the phenotypic expression in the F1.
• The third option “The traits blended together during fertilization” is against Mendel's findings. Mendel proposed the particulate theory of inheritance (genes are discrete units), not a blending theory.

Answer:

One allele was dominant.