Reproduction (Sexual, Asexual & Human)

Asexual vs sexual reproduction: it's about variation

The defining contrast: asexual reproduction involves one parent and produces genetically identical offspring (clones); sexual reproduction involves two parents, the fusion of gametes, and produces offspring showing genetic variation. Fertilisation is defined precisely as the fusion of the nuclei of a male gamete and a female gamete, 'fusion of gametes' alone sometimes scores, but 'fusion of nuclei' is the safe wording. The advantage questions follow from variation: variation helps a species survive environmental change (sexual), while asexual reproduction is fast and reliable in a stable environment with no need for a mate.

Pollination: insect vs wind comparison

Pollination is the transfer of pollen from the anther to the stigma. The comparison table is a standing question:

Each feature must be linked to its job ('feathery stigma so a large surface area catches windblown pollen'). Distinguish pollination (pollen transfer) from fertilisation (the pollen nucleus fusing with the egg nucleus inside the ovule). Students merge the two and lose marks.

The menstrual cycle: four hormones, four jobs

You need the roles, not just the names:

• FSH (from the pituitary). Stimulates an egg to mature in the ovary and stimulates the ovary to produce oestrogen.
• Oestrogen (from the ovary). Repairs and thickens the uterus lining; stops FSH and stimulates a surge of LH.
• LH (from the pituitary). Causes ovulation (release of the egg) around day 14.
• Progesterone (from the ovary). Maintains the thick uterus lining; when it falls, the lining breaks down (menstruation).

The most-tested idea is that ovulation occurs mid-cycle (about day 14) and that a fall in progesterone triggers menstruation. Questions show a hormone graph and ask you to read off the day of ovulation or explain why the lining breaks down. Link it to falling progesterone.

Adolescence, sex hormones and STIs

Testosterone (males) and oestrogen (females) cause the secondary sexual characteristics at puberty. Be ready to list a few (e.g. Growth of body hair, voice deepening, breast development) and attribute them to the correct hormone. HIV/AIDS is the named sexually transmitted infection: it is caused by a virus (HIV), transmitted through body fluids (sexual contact, shared needles, blood, mother to baby), and methods of control are examinable (using condoms, not sharing needles, screening blood). State HIV is a virus that infects and destroys lymphocytes, weakening the immune system. Linking back to the diseases-and-immunity topic.

Key terms to connect with this topic

Use these definitions inside explanations and questions rather than memorising them as isolated sentences.

• Amniotic fluid: The fluid that surrounds and protects the developing fetus inside the amniotic sac in the uterus. It cushions the fetus against physical shocks, supports its weight, maintains a constant temperature and allows the fetus to move freely.
• Asexual reproduction: A process resulting in the production of genetically identical offspring from one parent.
• Contraception: Methods used to prevent pregnancy. These include barrier methods (condoms, which also prevent STI transmission), hormonal methods (the contraceptive pill, containing oestrogen and/or progesterone, which inhibit ovulation), intrauterine devices and natural methods such as abstinence.
• Fertilisation: The fusion of the nucleus of a male gamete (sperm) with the nucleus of a female gamete (egg cell) to form a zygote. In humans, fertilisation usually occurs in the oviduct (fallopian tube). The zygote is diploid, having received one set of chromosomes from each parent.
• Gamete: A sex cell (reproductive cell) that is haploid, containing half the number of chromosomes of a normal body cell. In humans, the male gamete is the sperm cell and the female gamete is the egg cell (ovum). Gametes are produced by meiosis.
• Germination: The process by which a seed develops into a new plant. Germination requires water (to activate enzymes and soften the seed coat), oxygen (for aerobic respiration to release energy for growth) and a suitable temperature (for enzyme activity). Light is not always required initially.
• Implantation: The process by which a developing embryo embeds itself into the thick, blood-rich lining of the uterus, approximately six to seven days after fertilisation. Once implanted, the embryo receives nutrients and oxygen from the mother's blood supply through the developing placenta.
• Menstrual cycle: A repeating cycle of approximately 28 days in which the uterus lining is prepared for possible pregnancy. It involves menstruation (shedding of the lining), repair and thickening of the lining (controlled by oestrogen), ovulation around day 14, and maintenance of the lining (controlled by progesterone).
• Oestrogen: A hormone produced by the ovaries that controls the development of female secondary sexual characteristics during puberty and plays a key role in the menstrual cycle, where it stimulates the repair and thickening of the uterus lining after menstruation.
• Ovary: The female reproductive organ that produces egg cells (ova) and secretes the hormones oestrogen and progesterone. In plants, the ovary is the part of the flower that contains ovules, which develop into seeds after fertilisation, while the ovary wall becomes the fruit.
• Ovulation: The release of a mature egg cell (ovum) from an ovary, which usually occurs around day 14 of the menstrual cycle. The egg then travels along the oviduct towards the uterus, and can be fertilised by a sperm cell during this journey.
• Placenta: An organ that develops in the uterus during pregnancy, providing the interface between the mother's blood and the fetus's blood. Nutrients and oxygen diffuse from the mother's blood to the fetus, while carbon dioxide and urea diffuse from the fetus to the mother. The two blood supplies do not mix.
• Pollen: A fine powder produced by the anthers of a flower, containing the male gametes. Pollen grains are transferred from the anther to the stigma of a flower (pollination) by wind or insects. Once on the stigma, a pollen tube grows down the style to deliver the male nucleus to the ovule for fertilisation.
• Pollination: The transfer of pollen grains from the anther of a flower to the stigma. Self-pollination occurs when pollen is transferred within the same flower or between flowers on the same plant. Cross-pollination involves transfer between flowers on different plants of the same species.
• Progesterone: A hormone produced by the corpus luteum (in the ovary after ovulation) that maintains the thick, blood-rich lining of the uterus during the second half of the menstrual cycle. If an embryo implants, progesterone levels remain high to support pregnancy. If not, progesterone drops and menstruation begins.
• Sexual reproduction: A process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote and the production of offspring that are genetically different from each other.
• Testis: The male reproductive organ that produces sperm cells and secretes the hormone testosterone. The testes are located outside the body in the scrotum, where the temperature is slightly lower than body temperature, which is necessary for sperm production.
• Testosterone: A hormone produced by the testes that controls the development of male secondary sexual characteristics during puberty, including deepening of the voice, growth of facial and body hair, increased muscle mass and sperm production.
• Umbilical cord: A flexible tube connecting the fetus to the placenta, containing two umbilical arteries (carrying deoxygenated blood from fetus to placenta) and one umbilical vein (carrying oxygenated blood from placenta to fetus).
• Zygote: The cell formed when two haploid gametes fuse at fertilisation. The zygote is diploid, containing the full number of chromosomes (46 in humans, with 23 from each parent). It divides by mitosis to develop into an embryo.

Browse the full IGCSE Biology glossary when a related term needs checking.

Apply the topic in practical work

These practical guides show how the Biology appears in methods, variables, measurements, graphs and evaluation.

• Investigating Conditions Needed for Germination: Seeds need three conditions to germinate: water (to activate enzymes and expand cells), oxygen (for aerobic respiration to provide energy), and a suitable temperature (for enzyme activity). Light is NOT required for germination.