Sec 4 Combined Science (Biology) Reproduction in Humans notes

Sexual reproduction is the fusion of gamete nuclei to form a zygote, producing offspring that are genetically dissimilar from both parents. Unpack it: gametes are the special sex cells — sperm from the male, eggs from the female — each carrying half the usual genetic material. When a sperm's nucleus fuses with an egg's nucleus, the full set is restored in one new cell, the zygote. Because that zygote draws its instructions from two different individuals, the offspring it becomes is genetically dissimilar — a unique shuffle, not a copy.

The testes (two) make the sperm — the male gametes. They hang outside the body in the scrotum, a pouch of skin, because sperm production needs a temperature a little cooler than the body's core. From each testis, a sperm duct carries sperm upwards towards the urethra; on the way, the prostate gland adds fluid that nourishes and transports the sperm, so they can swim. The urethra is the shared exit tube — it runs through the penis, which delivers the sperm during reproduction.

The scrotum's outside position is a favourite "explain why": sperm production is temperature-sensitive, and the body's core is too warm — a couple of degrees cooler outside does the job.

The ovaries (two) make the eggs — the female gametes — and release one at a time in a monthly rhythm. Each oviduct is the tube beside an ovary: it collects the released egg and carries it towards the uterus; it is also where fertilisation happens. The uterus is the muscular bag where a fertilised egg implants and develops. The cervix is the narrow neck at the uterus)s entrance, and the vagina is the passage that receives sperm during reproduction and serves as the birth canal.

The menstrual cycle is the monthly preparation of the female body for a possible pregnancy. It runs about 28 days — with natural variation between women and between months. Day 1 is the first day of menstruation (a period): the uterus)s lining, no longer needed, is shed. Over the next days the lining rebuilds under the influence of oestrogen. Around day 14 of a 28-day cycle, ovulation occurs — an egg is released from an ovary into an oviduct. Progesterone then maintains the thickened lining in case a fertilised egg implants. With no pregnancy, the lining breaks down and the cycle starts again.

Because cycles vary naturally in length (typically around three to five weeks), ovulation day varies too — it sits roughly two weeks before the next period. The fertile phase centres on ovulation: the days when sperm (which survive a couple of days) can meet a freshly released egg. Outside this window, fertilisation is unlikely.

Fertilisation happens in the oviduct: a sperm's nucleus fuses with the egg's nucleus, restoring the full genetic set in the zygote. The zygote then divides again and again as it travels down the oviduct, becoming a ball of cells. Reaching the uterus, this ball implants in the uterus wall — embedding itself in the thickened lining, where it continues to develop. That is the syllabus)s simple storyline: fusion → dividing ball of cells → implantation in the uterus wall.

Keep the geography straight in answers: fertilisation = oviduct; development and implantation = uterus. Swapping the two sites is the most common single error in this topic.

HIV is a virus spread in body fluids: during sexual contact with an infected person; by sharing needles; through infected blood (transfusions before screening); and from an infected mother to her baby. It is not spread by everyday contact — handshakes, sharing food, mosquitoes or swimming pools. Left untreated, HIV damages the immune system over years, leaving the body unable to fight infections.

Exam answers should pair each route with its reduction: condoms block fluid exchange during sex; sterile, unshared needles block the blood route; screening keeps infected blood out of transfusions. Reduction is about blocking the fluid route, never about avoiding the person — HIV does not spread through ordinary care and contact.