Explain the changes that can be observed in the characteristics of river water when sewage is discharged into it and a few weeks after the discharge with respect to:

(i) level of dissolved oxygen

(ii) population of fresh water organisms.

What is chance mutation? Explain this phenomenon using application of DDT as an example.

(a) State what happens in the human body when malarial parasites infected RBCs burst to release the parasites in the blood.

(b) Mention the specific sites in the host body where production of

(i) sporozoites and

(ii) gemetocytes take place in the life cycle of the malarial parasites.

Which chromosome carries the mutated gene causing β thalassaemia? What are the problems caused by the mutation?

(a) Draw a labelled diagram of the sectional view of microsporangium of an angiosperm.

(b) Explain the development of male gametophyte in the microsporangium.

OR

(a) Name the hormone that initiates spermatogenesis in humans. Describe the process of spermatogenesis in sequence mentioning the ploidy of the cells at each step.

(b) Draw the diagram of a mature human sperm and label the parts that

(i) helps it reaching to the ovum.

(ii) providing energy for it to reach the ovum.

(iii) helping it to gain entry into the ovum.