When Mendel crossed pure breeding peas having yellow seed color (YY) with pure breeding peas having green seed color (yy), all of the offspring had yellow seed color. These results mean that yellow seed color is the:

dominant color over green seed color.

Explanation:

The results mean that yellow seed color is the dominant color over the green seed color.

In other word, the allele for yellow color (Y) is dominant over the allele for green (y) color as far as the peas plant is concerned.

A cross between YY and yy will produce all Yy genotype offspring. These offspring appear yellow phenotypically because the dominant allele prevent the recessive allele from expressing itself when they are together (heterozygous).

Recessive alleles can only be expressive when they are in homozygous condition while dominant alleles are usually expressed both in homozygous and heterozygous conditions.

The Yellow seed color is the dominant trait.

Explanation:

In genetics, traits or characteristics are carried in genes , and variant forms of genes located in the same place on a chromosome are known as alleles. Hence in the traits in this question, the alleles for yellow seed color are Y and Y (YY) while that for green color are y and y (yy). Dominance is said to occur in a trait when it is expressed phenotypically despite being heterogeneous in its genetic composition. For example The yellow seed color is said to be dominant because if a seed has the genetic composition "Yy", the phenotype of the seed will be yellow. On the other hand, the recessive trait is one that is masked phenotypically when occuring heterogeneously in genetic composition.

In the example described here, crossing seed YY and yy gives the result below:

               y          y

Y             Yy        Yy

Y             Yy        Yy

In the F1 generation, every offspring is heterogeneous, but we are told that they all have a yellow color (phenotype), hence since yellow color is expressed in a heterogeneous state, yellow color is dominant, and green color is recessive.