Question

why can't we make YY gamete or RR gametes as oppose to YR and yr?

Answer 1

This is because the gametes are Haploid. That means it have only one set of chromsome. However in diploid cells 2 sets of chromosomes. One set comes from mother and other from father.

Now, these sets have 1 copy of a gene each. These are called alleles. These alleles are different forms of genes. Thus, Y and y are two forms of a gene present at 2 sets of chromosomes. And they cannot be present on the same set. Thus, same stands for R and r alsom There forebdiploid will have RR or rr or Rr, and YY , Yy, Yy. Therefore, haploid gametes will have only one of Y or y. And R or r