Reproduction, Genetics and Health: notes

Reproduction

• Sexual reproduction means two parents contribute gametes (sex cells). Their fusion forms a zygote, which grows into a new organism with mixed traits.
• Asexual reproduction needs only one parent. Offspring are genetically identical to the parent, for example by binary fission in bacteria or runners in strawberry plants.
• Fertilisation is the joining of a sperm and an ovum (egg) to form a zygote, usually in the fallopian tube.
• Pregnancy lasts about 9 months. The zygote implants in the uterus, and the placenta supplies food and oxygen to the foetus.
• Breast feeding gives the baby antibodies and nutrients from breast milk, which protects it from infections.

Human reproductive system and menstrual cycle

• Ovary produces ova and female hormones such as oestrogen and progesterone.
• Uterus is where the foetus develops. Its lining thickens each month to receive an embryo.
• Menstrual cycle lasts about 28 days. An ovum is released around day 14 (ovulation). If no fertilisation happens, the lining breaks down and is shed as menstruation.

Genetics

• Chromosome is a thread-like structure in the nucleus made of DNA. Humans have 46 chromosomes arranged in 23 pairs.
• Gene is a short section of DNA that controls one trait.
• Mitosis makes two identical body cells with the same chromosome number (diploid, 2n).
• Meiosis makes gametes with half the chromosome number (haploid, n). This keeps the number constant after fertilisation.
• Monohybrid cross follows one gene. Example: if Tt × Tt, the ratio is 3 dominant : 1 recessive.
• Mutation is a sudden change in a gene or chromosome. It is a source of variation.
• Natural selection means organisms best suited to their environment survive and reproduce more.

Diseases, immunity and drugs

• Malaria, cholera, bilharzia and TB are infectious diseases. Prevention includes mosquito nets, clean water, sanitation and treatment.
• Sexually transmitted infections (STIs) include HIV, which weakens the immune system and can lead to AIDS. Use condoms and get tested.
• Immunity is the body's ability to fight a pathogen. Vaccination introduces a harmless form of a pathogen so memory cells can respond quickly later.
• Drugs can be medicines or harmful substances. Misuse damages health and can cause addiction.

Common slips

• Mixing up mitosis (identical cells) and meiosis (gametes).
• Saying a zygote is the same as an embryo or a foetus. The zygote is the first cell.
• Thinking the menstrual cycle starts at fertilisation. Menstruation is the shedding of the lining.
• Writing Tt × Tt as 3:1 for genotypes. The phenotype ratio is 3:1, but the genotype ratio is 1:2:1.