Human ReproductionNEET Zoology · Class 12 · NCERT Chapter 2

You can expect 2 to 3 questions from Human Reproduction in NEET 2027. It is one of the highest-yield zoology chapters. The most tested topics are spermatogenesis vs oogenesis differences, menstrual cycle hormones (especially FSH and LH peaks), placental hormones (hCG), and the structure of mature graafian follicle.

Three big differences. (1) Number of gametes: one spermatogonium gives 4 sperms; one oogonium gives 1 ovum plus 3 polar bodies. (2) Timing: spermatogenesis is continuous from puberty; oogenesis begins in the female fetus, pauses at prophase I, resumes at puberty for one egg per cycle, and completes meiosis II only after fertilization. (3) Cytoplasm distribution: sperm has minimal cytoplasm; ovum keeps almost all cytoplasm so it can support the zygote.

Ovulation happens on day 14 of a 28-day cycle. It is triggered by a sudden surge of LH (luteinizing hormone) that peaks around day 13 to 14. The LH surge causes the mature Graafian follicle to rupture and release the secondary oocyte into the fallopian tube.

Fertilization happens in the ampullary-isthmic junction of the fallopian tube (also called the uterine tube or oviduct). The sperm travels from the vagina through the cervix and uterus to reach this site. The ovum, released from the ovary, is picked up by the fimbriae and moved into the ampulla. Only one sperm penetrates the egg; the cortical reaction immediately blocks all other sperms.

Human chorionic gonadotropin (hCG) is secreted by the trophoblast cells of the developing blastocyst (and later by the placenta). Its job is to keep the corpus luteum alive past day 14 so it continues secreting progesterone. Progesterone maintains the uterine lining and prevents menstruation. hCG is the hormone detected in home pregnancy tests (urine) about 10 to 14 days after fertilization.

Both human sperm and ovum are haploid (n = 23). The sperm carries 22 autosomes plus either an X or a Y sex chromosome. The ovum always carries 22 autosomes plus an X chromosome. When they fuse, the zygote is diploid (2n = 46). The sex of the baby is decided by the sperm: X-sperm gives a girl (XX), Y-sperm gives a boy (XY).

Identical (monozygotic) twins come from one zygote that splits into two embryos after fertilization. They share 100% of their genes and are always the same sex. Fraternal (dizygotic) twins come from two separate eggs fertilized by two separate sperms. They share only about 50% of their genes (like normal siblings) and can be the same or different sex.