Answer:
Both parents have the genes for yellow peas and green peas
Explanation:
If a plant has inherited one yellow peas gene and one green peas gene, the plant will produce yellow peas because (as you mentioned) yellow peas are dominant. that means the parents are heterozygous (Cc). If both parents have the gene versions Cc, then there is a 25% chance the offspring would get the gene version cc, coding for green peas.
One reason would be the increased chance of Down syndrome
