AQA GCSE Biology 8461: Inheritance, variation and evolution – Revision Notes

Condensed recall for the final weeks. For full explanations and worked examples, use the Inheritance, variation and evolution study guide.

These notes cover topic 6, Inheritance, variation and evolution (sections 4.6.1–4.6.4), of the AQA GCSE Biology (8461) specification, for teaching from September 2016 and exams from 2018 onwards. It is examined on Paper 2 at Foundation and Higher tier; lines marked Higher tier only match the specification’s “(HT only)” content. Course hub: AQA GCSE Biology. Track progress with the printable checklist, and test yourself with the practice questions.

4.6.1 Reproduction

	Sexual	Asexual
Parents	Two	One
Gametes fuse?	Yes	No
Cell division	Meiosis makes gametes	Mitosis only
Offspring	Varied	Identical clones

Meiosis in one box

Genetic information copied → cell divides twice → four gametes → each has a single set of chromosomes → all genetically different. Fertilisation restores the full number (23 + 23 = 46 in humans); the zygote then divides by mitosis and cells differentiate.

Advantages to quote. Sexual: variation; survival advantage by natural selection if the environment changes; humans can use it in selective breeding. Asexual: one parent; saves time and energy; faster; many identical offspring when conditions are good.

Both methods (only these examples are required): malarial parasites (asexual in humans, sexual in mosquitoes); fungi (asexual spores, sexual for variation); plants (seeds sexually; runners in strawberries, bulb division in daffodils).

DNA

• DNA: polymer, two strands, double helix, found in chromosomes.
• Gene: small section of DNA coding for a sequence of amino acids → a specific protein.
• Genome: all the genetic material of an organism. Uses of the human genome: genes linked to disease; understanding and treating inherited disorders; tracing past human migration.
• Nucleotide: sugar + phosphate + one base (A, C, G or T). Backbone alternates sugar and phosphate.
• Three bases = one amino acid. Base order → amino acid order → protein.

Higher tier only

• Pairing: A–T, C–G.
• Protein synthesis: made on ribosomes from a template; carrier molecules bring amino acids in order; chain folds into a unique shape (enzyme, hormone, collagen).
• Most mutations do not change the protein, or change it only slightly. A few change its shape: an enzyme’s active site no longer fits its substrate, or a structural protein loses strength.
• Non-coding DNA switches genes on and off; variants there change gene expression.

Genetic crosses – method in steps

1. Write a key: capital = dominant, lower case = recessive.
2. Write the parents’ genotypes.
3. Write each parent’s possible gametes.
4. Fill in a Punnett square.
5. List offspring genotypes and phenotypes.
6. State the answer in the form asked: ratio, fraction, percentage or expected number.

Cross	Genotypes	Phenotype ratio
Aa × Aa	1 AA : 2 Aa : 1 aa	3 dominant : 1 recessive
Aa × aa	1 Aa : 1 aa	1 : 1
AA × aa	all Aa	all dominant
XX × XY	1 XX : 1 XY	1 female : 1 male

Small reminder. A ratio of 3 : 1 means probability 3/4 and 1/4. With 20 offspring, expect 15 and 5.

Disorders. Polydactyly – dominant allele. Cystic fibrosis – recessive allele (cell membranes). Two carriers: 1/4 chance of an affected child.

Family trees. Two unaffected parents with an affected child → the allele is recessive and both parents are heterozygous.

Embryo screening issues. Cost; risk of the procedure; destroying embryos; fear of choosing characteristics. Judge from the information given.

4.6.2 Variation and evolution

• Variation causes: genes, environment, or both.
• All variants come from mutations: most no effect; some influence phenotype; very few determine it.
• Evolution: change in inherited characteristics of a population over time by natural selection; may form a new species.
• New species: populations can no longer interbreed to produce fertile offspring.

Natural selection – method in steps

1. Variation exists in the population (from mutation).
2. Some individuals have a characteristic better suited to the environment.
3. They are more likely to survive and breed.
4. They pass on the alleles for that characteristic.
5. Over many generations the allele becomes more common.

Human intervention

Process	Key steps	Risk or concern
Selective breeding	Choose parents with the trait → breed → select best offspring → repeat for many generations	Inbreeding: disease, inherited defects
Genetic engineering	Gene from one organism put into another’s genome	Effects on wild flowers and insects; health effects not fully explored
Tissue culture	Small groups of plant cells grown into plants	Identical plants, no variation
Cuttings	Pieces of parent plant grown	As above
Embryo transplant	Split embryo cells before specialisation → host mothers	–
Adult cell cloning	Enucleated egg + body-cell nucleus → electric shock → embryo → womb	Ethical objections

Higher tier only – genetic engineering steps: enzymes isolate the gene → insert it into a vector (plasmid or virus) → vector carries gene into cells → done at an early stage of development.

4.6.3 Developing the theory

Scientist	Contribution
Darwin	Natural selection; On the Origin of Species, 1859
Wallace	Independent theory; joint writings 1858; warning colouration; speciation
Lamarck	Changes in a lifetime inherited – now known not to happen in most cases
Mendel	Inherited “units” passed on unchanged; recognised only after his death
Linnaeus	Kingdom → species; binomial names
Woese	Three domains: archaea, bacteria, eukaryota

Why Darwin’s idea was slowly accepted: challenged the idea that God made all living things; too little evidence then; the mechanism of inheritance was unknown for about 50 years.

Genetics timeline: Mendel’s units (mid-19th century) → chromosomes seen in cell division (late 19th) → units linked to chromosomes (early 20th) → DNA structure (mid-20th).

Fossils form: parts that did not decay; parts replaced by minerals; traces (footprints, burrows, rootlet traces). The record is incomplete: soft-bodied early life and geological activity.

Resistant bacteria in steps: mutation → resistant strain → antibiotic kills non-resistant bacteria → resistant ones survive and reproduce → strain spreads. Reduce by: no antibiotics for viral or non-serious infections; finish the course; restrict agricultural use.

Speciation in steps (Wallace’s work): a population is split and isolated → each group has genetic variation → different conditions select different variants → over many generations the groups become so different they cannot interbreed to produce fertile offspring.

Extinction: no individuals of a species remain alive. Factors to describe include a new disease, a new predator or competitor, a change in habitat or climate, and a catastrophic event.

4.6.4 Classification – reading an evolutionary tree

1. Find the two organisms at the tips of the branches.
2. Trace back to where their branches join: that point is their most recent common ancestor.
3. The more recent the join, the more closely related they are.
4. Extinct organisms are placed on the tree using fossil data; living ones using current classification data.

Must-know distinctions

• Gene (section of DNA) vs allele (a version of a gene).
• Genotype (alleles) vs phenotype (characteristic).
• Homozygous (same alleles) vs heterozygous (different).
• Meiosis (four different gametes) vs mitosis (two identical cells).
• Natural selection (environment selects) vs selective breeding (humans select).
• Darwin (natural selection) vs Lamarck (acquired characteristics).

Quick self-test

1. How many chromosomes are in a human egg cell?
2. Name the type of cell division that produces gametes.
3. Give two advantages of asexual reproduction.
4. A cross gives 90 tall plants and 30 short plants. Write this as a simplest whole-number ratio.
5. What is the probability of a Tt × tt cross producing a tt offspring?
6. (Higher tier only) Write the complementary strand for GATTC.
7. How many amino acids are coded for by 300 bases?
8. Is cystic fibrosis caused by a dominant or recessive allele?
9. What is the chance that a baby is female?
10. Name the vector usually used in genetic engineering. (Higher tier only)
11. State the three domains in Woese’s system.
12. Give one way to slow the development of antibiotic resistance.

Answers

2. Meiosis.
3. Any two: only one parent; no need to find a mate, saving time and energy; faster; many identical offspring when conditions are favourable.
4. 90 : 30 = 3 : 1.
5. 1/2 (50%).
6. CTAAG.
7. 300 ÷ 3 = 100.
8. Recessive.
9. 1/2 (50%).
10. A bacterial plasmid (or a virus).
11. Archaea, bacteria, eukaryota.
12. Do not prescribe antibiotics for viral or non-serious infections (or finish the course, or restrict agricultural use).

Where marks are usually lost

• Saying meiosis produces two identical cells.
• Leaving out the gametes stage of a Punnett cross, so no marks for method.
• Giving “3 : 1” when the question asks for a percentage or a probability.
• Writing that a dominant allele is “more common” – dominance is about expression, not frequency.
• Saying organisms “adapt to survive” in a natural selection answer instead of “variation already existed”.
• Forgetting “many generations” in selective breeding descriptions.
• Saying the clone is identical to the egg donor or surrogate rather than the body-cell donor.
• Defining a new species as “cannot breed” without “fertile offspring”.
• Saying antibiotics “make” bacteria mutate.
• Listing only two of the three reasons Darwin’s theory was slowly accepted.

Official syllabus

AQA GCSE Biology (8461) specification, Version 1.0 (21 April 2016), for teaching from September 2016 and exams from 2018 onwards, AQA – section 4.6, Inheritance, variation and evolution.