When a red flower is crossed with a red flower you always get red flowers. When a yellow flower is crossed with a yellow flower you get all yellow flowers or sometimes you get yellow flowers and red flowers. What is the simplest explanation for this observation

Red is recessive, yellow is dominant.

Explanation:

Think about it in terms of genotype, with Y representing yellow and y representing red. Recessive means that both alleles must be expressed to get the phenotype, whereas dominant alleles only require one, because they cover up the recessive allele.

If you have two red flowers, their genotypes are yy and yy, from which you can only get yy offspring.

If you have two yellow flowers, their genotypes could be YY or Yy, from which you can get all yellow flowers, or red flowers if the parents are Yy and Yy.