A sickly plant with yellowish leaves and a low photosynthesis rate is crossed with the pollen of a normal plant with green leaves. 60% of the progeny survive and all of them have yellowish leaves and a sickly appearance.
a) Since the rate of photosynthesis is low this results in yellowish leaves. The reason might be defective chloroplasts in the leaves. Hence, the chloroplasts need to be checked for any abnormalities and if the DNA (deoxyribonucleic acid) is abnormal, then it is the prime cause for the progeny to be sickly with yellowish leaves. As this trait is inherited only from the mother, the normal chloroplasts from the donor plant are not inherited. This explains why all the surviving progeny have the characteristics of the mother plant.
b) A simple test would be to take a normal green plant and cross it with the pollen of the sickly plant. If the defect is due to abnormal chloroplast DNA then the progeny will show normal characteristics of the green plant, since this disease is inherited from the mother only.
c) The symptoms of reduced photosynthesis, sickliness and yellowish appearance are all attributed to the absence of chlorophyll. As chlorophyll is the site of photosynthesis, which produces sugars in the plant, an absence of chlorophyll will reduce sugar levels, which give the plant a sickly appearance. The yellowish appearance is due to reduced photosynthesis, which is due to abnormal chloroplasts.