Pearson Edexcel International GCSE Science Single Award 3: Biology: Reproduction and inheritance

Topic 3 follows biological information from parents to offspring and then asks how inherited variation can affect populations over generations. These notes follow the Science Single Award points in Pearson 4SS0 Issue 4. They exclude the additional menstrual-cycle, pregnancy, DNA-structure, cell-division, mutation and genetic-technology points owned by Double Award or separate Biology.

Sexual and asexual reproduction

Sexual reproduction involves the fusion of a male gamete and a female gamete. Their nuclei fuse at fertilisation to produce a zygote. The zygote divides repeatedly and develops into an embryo. Offspring show genetic variation because genetic material from two parents is combined.

Asexual reproduction involves one parent and no fusion of gametes. It can produce many offspring quickly, and the offspring are genetically identical to the parent unless genetic change occurs. This can preserve a successful combination of characteristics in stable conditions, but low inherited variation can make a population similarly vulnerable when conditions change.

The distinction is a mechanism, not simply the number of organisms involved. A process is sexual because gametes fuse, and a process is asexual because they do not.

Reproduction in flowering plants

Insect-pollinated flowers attract animals with conspicuous petals, scent or nectar. Their anthers are usually held inside the flower, and their pollen grains tend to be relatively large and sticky so they attach to an insect. The stigma is also sticky and positioned to contact visiting insects.

Wind-pollinated flowers do not need to attract animals. Their petals are often small, their anthers hang outside the flower and their stigmas are large and feathery. They produce abundant, light pollen because transfer by wind is less directed.

Pollination transfers pollen from an anther to a stigma of a flower of the same species. A pollen tube grows down through the style. The male nucleus travels through the tube to an ovule, where fertilisation occurs. The fertilised ovule develops into a seed. Do not treat pollination and fertilisation as synonyms: transfer happens before nuclear fusion.

Human reproductive systems

The male system is adapted to make and deliver sperm. Testes produce sperm and testosterone. The scrotum holds the testes outside the abdominal cavity, helping maintain a suitable temperature. Sperm travel through sperm ducts, mix with fluid from glands to form semen, and leave through the urethra in the penis.

The female system is adapted to produce eggs and support fertilisation and development. Ovaries produce eggs and oestrogen. Oviducts carry an egg and are the usual site of fertilisation. The uterus has a muscular wall and lining that can support development, the cervix forms its opening, and the vagina receives the penis and sperm.

At puberty, testosterone contributes to male secondary sexual characteristics such as a deeper voice and facial hair. Oestrogen contributes to female secondary sexual characteristics such as breast development and wider hips. These hormones influence development but do not alone define a person's sex or every individual characteristic.