In pea plants, round peas (R) are dominant to wrinkled peas (are). You do a test cross between a pea plant with wrinkled peas (genotype rr) and a plant of unknown genotype that has round peas. You end up with three plants, all which have round peas. From this data, can you tell if the parent plant is homozygous dominant or heterozygous? Figure:cannot be explained in words

No we can not tell if the parent plant is homozygous dominant or heterozygous

Explanation:

Test cross is done to fine the genotype of the dominant phenotype.

If genotype is homozygous, the entire resulting progeny will have dominant phenotype:

R R

r Rr Rr

r Rr Rr

If genotype is heterozygous, half of the progeny will have dominant phenotype and other half will have recessive phenotype:

R r

r Rr Rr

r Rr Rr

However, to evaluate the results of a test cross, sufficient data should be available. The result should be statistically significant. Here, only three plants are obtained in next generation. Although all of them had round peas we still can not say that the parent was homozygous. Parent might have been heterozygous but by chance event all the plants might have got dominant allele. The data set is too small to come to a conclusion.