In a certain plant species, individual plants have either yellow, red or orange flowers - HSC - SSCE Biology - Question 15 - 2015 - Paper 1

To determine the genotypes of the original parent plants (P generation), we analyze the F2 generation results of 6 red, 11 orange, and 5 yellow flowered plants.

1. Understanding the Flower Colors:

   • Let R represent the allele for red flower color.
   • Let Y represent the allele for yellow flower color.
   • The orange flowers will be a result of the combination of one red and one yellow allele (RY).

2. Analyzing the Ratios:

   • The ratio of red to orange to yellow in the offspring is 6:11:5. This suggests that red appears in lower frequency compared to orange and yellow, which is indicative of a dominant/recessive relationship.

3. Applying Mendelian Genetics:

   • The parental cross could be between a homozygous red (RR) and a homozygous yellow (YY) or between a heterozygous plant (Rr) with the homozygous counterpart. If the parents were Rr and RY, we could observe this ratio in the second generation because Rr can produce R (red) or Y (yellow), resulting in various combinations.

4. Determining Genotypes:

   • Given the expected ratios and the presence of orange among the F2 progeny, we conclude that both parent plants must contribute an allele that results in the presence of orange flowers. Thus, the most plausible genotypes for the original parents are RR and rr (Option C).