In pea plants yellow colour is the dominant trait for the colour of the seeds and green colour is recessive.

We use ‘YY’ symbol for the yellow coloured seeds and ‘yy’ for green seeds.

Following is the hybridisation experiment using symbols:

Here in case of yellow seeds we had taken heterozygous combination because in the given question recessive trait is appearing in First generation and it only happens when we have heterozygous dominant as our parental generation. If we had chosen homozygous dominant as our parental generation we can’t have recessive trait in first generation. So, in order to satisfy the question’s condition we had done so.