In this case, we have alleles with codominance, red (R) and white (W), when one individual is red their alleles are (RR, homozygous) when is white is (WW, homozygous) meanwhile is roan is (RW, heterozygous). In the present cross, we have an offspring of roan (RW) and red (RR), which means there are no homozygous for white (WW), which led us to the conclusion that the parental cross is red (RR) x roan (RW). This can be easily confirmed with a punnet square.
R R
R RR RR
W RW RW
