Heredity
What this domain covers
Heredity tests how genetic information is stored, copied, divided and passed on, and how that produces variation. You should be able to follow a monohybrid cross, use genetic terms precisely, and explain sex determination and common genetic disorders.
Cell division
- Mitosis: one division producing two genetically identical diploid cells; used for growth, repair and asexual reproduction.
- Meiosis: two divisions producing four genetically different haploid gametes; crossing over and independent assortment create variation.
- DNA to gene to chromosome: DNA is a double helix; a gene is a length of DNA coding for a protein; chromosomes are packaged DNA, and human body cells have 46 (23 pairs).
Inheritance rules
- Alleles are alternative forms of a gene; genotype is the allele combination, phenotype is the visible characteristic.
- Homozygous means two identical alleles; heterozygous means two different alleles.
- A dominant allele is expressed even when heterozygous; a recessive allele is expressed only when homozygous.
- Sex determination: females are XX and males XY; the father's sperm determines the sex of the child.
- Variation: continuous (height, mass) is controlled by many genes and the environment; discontinuous (blood group) falls into clear categories.
- Mutation is a change in the DNA base sequence; examples include sickle cell anaemia (HbS) and cystic fibrosis (recessive).
Worked example
A heterozygous group A parent (Iᴬi) has a child with a heterozygous group B parent (Iᴮi). What blood groups are possible? Parent 1 gametes: Iᴬ and i. Parent 2 gametes: Iᴮ and i. The Punnett square gives IᴬIᴮ (AB), Iᴬi (A), Iᴮi (B) and ii (O). The ratio is 1 AB : 1 A : 1 B : 1 O, so each child has a one in four chance of each group. Here the phenotype ratio equals the genotype ratio because all four genotypes differ.
Traps that cost marks
- Using gene and allele as if they are the same.
- Assuming a dominant allele must be the most common in a population - it need not be.
- Giving a genotype ratio when the question asks for a phenotype ratio, or vice versa.
- Forgetting that meiosis produces haploid, not diploid, cells.
- Saying the mother determines sex; she contributes only an X.
- Calling a heterozygous carrier affected - carriers of a recessive allele are usually unaffected.
- Forgetting that blood group alleles Iᴬ and Iᴮ are codominant.
Diseases and the Environment
What this domain covers
Diseases and the Environment tests how pathogens and vectors spread disease, how the body defends itself, how immunity and vaccination work, and how environmental conditions such as water supply, sanitation and pollution affect health.
Pathogens and transmission
- Pathogens: bacteria, viruses, fungi and protozoa. Viruses cannot reproduce outside a host cell, so antibiotics do not affect them.
- Vectors: the female Anopheles mosquito transmits malaria (Plasmodium); Aedes aegypti transmits dengue and yellow fever.
- Transmission routes: airborne droplets (tuberculosis, influenza), contaminated water or food (cholera, typhoid), vectors (malaria), direct contact, blood and sexual contact (HIV).
Defence and immunity
- First line: skin, mucus, cilia, stomach acid.
- Second line: phagocytes engulf pathogens; inflammation and fever.
- Third line: lymphocytes produce antibodies specific to an antigen; memory cells remain.
- Active immunity: the body makes its own antibodies after infection or vaccination; slow to develop but long-lasting.
- Passive immunity: antibodies are supplied (breast milk, antiserum); immediate but short-lived.
- Vaccination introduces weakened, inactivated or toxoid antigens so the primary response and memory cells develop without severe disease.
Environmental health
Clean water, safe sewage disposal, vector control (draining stagnant water, bed nets, larvicides) and food hygiene break transmission. Pollution - untreated sewage, fertilizer runoff causing eutrophication, smoke and vehicle emissions - raises disease risk.
Worked example
A child is vaccinated against a disease and later meets the real pathogen but does not become ill. Explain. The vaccine contains antigens from the pathogen. After this first exposure, specific B lymphocytes divide into plasma cells that secrete antibodies and into memory cells; antibody levels then fall. On later exposure, memory cells recognise the antigen and divide rapidly, producing a much faster and larger antibody response that destroys the pathogen before symptoms appear. This is active immunity because the child's own immune system made the antibodies.
Traps that cost marks
- Saying the mosquito causes malaria; it is the vector, while Plasmodium is the pathogen.
- Writing that antibiotics cure viral infections.
- Confusing antigen (the marker) with antibody (the protein made against it).
- Mixing active and passive immunity, or claiming passive immunity lasts for years.
- Saying a vaccine gives you the disease; most vaccines use weakened or inactivated forms.
- Forgetting to name the specific vector or transmission route when asked.