In peas, the allele for tall plants is completely dominant over the allele for short plants. The quickest way to determine whether a tall pea plant is homozygous or heterozygous is to cross it with another plant which is:.

homozygous recessive for shortness

Explanation:

Since the tallness allele is dominant over the allele for shortness, the quickest way to determine the zygosity of a tall pea plant is to test cross with a plant that is homozygous recessive for the allele.

When this is done, analysis of the phenotypes of the offspring of the test cross will give an indication of the zygosity of the parent. If it is homozygous tall, all the offspring are expected to be tall while if it is heterozygous tall, 50% of the offspring are expected to be tall and 50% short.

helicase, i think thats it