InheritanceEdexcel GCSE Biology: Revision notes
Section 1
DNA Structure and the Genome
DNA is a polymer, two strands coiled into a double helix, held by complementary base pairs joined by weak hydrogen bonds. A nucleotide = sugar + phosphate + base. The genome is the entire DNA of an organism; a gene is a section of DNA coding for a specific protein.
DNA extraction from fruit: mash, add detergent, add salt, filter, add cold alcohol — DNA precipitates as visible strands.
Section 2
Protein Synthesis: Transcription and Translation
- RNA polymerase binds non-coding DNA in front of the gene
- Produces complementary mRNA
- mRNA attaches to ribosome
- Codons (3 bases) code for amino acids
- tRNA delivers matching amino acids
- Amino acids link into a polypeptide
Variants in non-coding DNA change protein quantity; variants in coding DNA change amino acid sequence and protein activity.
Sequence: RNA polymerase binds non-coding DNA → transcribes mRNA → ribosome reads codons → tRNA delivers amino acids → polypeptide forms.
Section 3
Mendel and Genetic Terms
Mendel proposed inheritance via separate hereditary units (alleles); not understood until genes/DNA discovered.
| Term | Meaning |
|---|---|
| Chromosome | DNA molecule carrying genes |
| Allele | Alternative version of a gene |
| Dominant | Expressed even with one copy |
| Recessive | Only expressed with two copies |
| Homozygous | Two identical alleles |
| Heterozygous | Two different alleles |
| Genotype | Alleles present |
| Phenotype | Observable characteristic |
| Gamete | Sex cell |
| Zygote | Cell formed at fertilisation |
Genotype = alleles; phenotype = the resulting characteristic.
Section 4
Monohybrid Inheritance and Calculations
Monohybrid crosses track one gene: list parent genotypes, gametes, combine to find offspring genotypes/phenotypes/ratios. Pedigrees show inheritance patterns across generations.
Sex determination: eggs all X; sperm X or Y. XX=female, XY=male, 1:1 ratio.
ABO blood groups: I^A, I^B codominant; I^O recessive — multiple alleles.
Sex-linked disorders (e.g. colour blindness) on X chromosome affect males more (only one X).
Outcomes expressed as probabilities, ratios, or percentages.
Both parents Bb (brown dominant): offspring BB, Bb, Bb, bb → 3 brown:1 blue, 25% blue-eyed.
Section 5
Variation, Mutation and the Human Genome Project
Most features result from multiple genes. Variation: genetic (mutation, sexual reproduction) and environmental (acquired characteristics). Mutations are random DNA changes; most have no effect, some small effect, rarely large effect.
Human Genome Project mapped human genes — enables disorder diagnosis, personalised medicine, genetic counselling.
Must Know
- DNA double helix, weak H bonds; nucleotide=sugar+phosphate+base.
- Transcription (mRNA) then translation (polypeptide).
- Non-coding variants change quantity; coding variants change activity.
- Genotype vs phenotype; monohybrid crosses, pedigrees, probabilities/ratios/%.
- ABO codominance/multiple alleles; sex-linked disorders affect males more.
- Mutations mostly neutral; Human Genome Project aids medicine.
That's the notes covered.
Carry on to the next subtopic.