The correct answer is  Halving of chromosomes during gamete formation.

Key Points

• During gamete formation, number of chromosomes in each gamete is halved due to meiosis. In meiosis, diploid cells form four haploid cells having half number of chromosomes.
• During gamete formation, re is meiosis in which mor cell divides into 4 cells having a half number of chromosomes.
• In fertilization, male and female gametes fuse to form a zygote having same number of chromosomes as parents. Hence, number of chromosomes remains constant.
• The gametes are a special type of cells that contain only half amount of DNA as compared to normal cells of an organism. So, when a male gamete combines with a female gamete during sexual reproduction, n new cell 'zygote' will have normal amount of DNA.
• The original chromosome number (as in parent) is restored after fertilization in sexual reproduction.

The correct answer is Western blot and ELISA.

Key Points

• ELISA Test, ELISA, which stands for enzyme-linked immunosorbent assay, is used to detect HIV infection.
• If an ELISA test is positive, Western blot test is usually administered to confirm diagnosis.
• HIV is virus that causes AIDS.
• This virus has a long incubation period before it starts to attack immune system (10 years).
• The immune system is human body’s natural defence mechanism, hence HIV can make it harder for patient to fight off infections and diseases.
• HIV demolishes a particular type of WBC (White Blood Cells) and T-helper cells. This virus also makes copies of itself inside se cells. T-helper cells are also known as CD4 cells.

Additional Information

• The HIV infection spreads through following ways
  • Unprotected sexual interaction with an already infected person.
  • Reusing needles used by an infected person.
  • From an infected mor to baby through placenta.
  • Blood transfusion from an infected person.