InheritanceEdexcel IGCSE Biology: Revision notes
Section 1
What are genes, chromosomes and the genome, and what is the structure of DNA and RNA?
The genome is the entire set of DNA in an organism. A gene is a section of DNA that codes for a specific protein. Genes are found on chromosomes, thread-like structures made of DNA found in the nucleus of a cell.
DNA is a molecule made of two strands coiled together to form a double helix. The strands are linked by pairs of bases:
- Adenine (A) always pairs with thymine (T)
- Cytosine (C) always pairs with guanine (G)
RNA differs from DNA: it is single-stranded (not double-stranded), and it contains the base uracil (U) instead of thymine.
Always give the specific base pairing (A-T, C-G) when asked to describe DNA structure — 'complementary bases' alone rarely earns full marks.
Section 2
How is a protein made from a gene?
Protein synthesis converts the genetic code in a gene into a protein, in two stages:
- Transcription — occurs in the nucleus; the DNA base sequence of a gene is copied into a single strand of messenger RNA (mRNA)
- Translation — occurs at ribosomes; the mRNA moves out of the nucleus to a ribosome, where its sequence of three-base codons is read; tRNA molecules, each carrying a specific amino acid and a three-base anticodon that pairs with a codon, bring the correct amino acids to the ribosome in order, joining them together to build the protein
Section 3
What are alleles and key genetics terms?
A gene can exist in different forms called alleles, which cause variation in inherited characteristics (e.g. different alleles of a eye-colour gene).
| Term | Meaning |
|---|---|
| Dominant | an allele that is expressed in the phenotype even when only one copy is present |
| Recessive | an allele only expressed in the phenotype when two copies are present |
| Homozygous | having two identical alleles for a gene |
| Heterozygous | having two different alleles for a gene |
| Phenotype | the observable characteristics of an organism |
| Genotype | the genetic make-up of an organism (the alleles it carries) |
| Codominance | when both alleles in a heterozygous individual are expressed in the phenotype, with neither masking the other |
Most visible characteristics (such as height or skin colour) are controlled by many genes acting together, not a single gene — this is called polygenic inheritance, and produces continuous variation rather than distinct categories.
Section 4
How do monohybrid crosses, pedigrees and sex determination work?
A monohybrid cross tracks the inheritance of a single gene between two parents, shown using a genetic diagram (e.g. a Punnett square) with parental genotypes, gametes, and possible offspring genotypes/phenotypes.
A family pedigree is a diagram showing how a characteristic has been inherited across generations of a family; it can be used to work out whether an allele is dominant or recessive, and to predict the probability of future offspring inheriting a characteristic.
Sex determination: sex is controlled by one pair of chromosomes — females are XX, males are XY. All eggs carry an X chromosome; sperm carry either an X or a Y chromosome. Fertilisation by an X-carrying sperm produces a female (XX); fertilisation by a Y-carrying sperm produces a male (XY) — this gives an expected 50:50 ratio, shown using a genetic diagram.
Cross Bb x Bb (B dominant, b recessive) gives offspring genotypes in the ratio 1 BB : 2 Bb : 1 bb, so a 3:1 phenotype ratio of dominant to recessive.
Section 5
How do mitosis and meiosis differ, and how do mutations and natural selection drive variation?
Mitosis produces two daughter cells with identical sets of chromosomes to the parent cell; it occurs during growth, repair, and asexual reproduction/cloning.
Meiosis produces four genetically different haploid gametes, each with half the number of chromosomes; it occurs during the production of sex cells. In humans, the diploid number (in body cells) is 46, and the haploid number (in gametes) is 23. Random fertilisation, combining a haploid gamete from each parent, produces genetic variation in offspring.
Variation between individuals of a species can be genetic (caused by different alleles), environmental (caused by external conditions), or a combination of both.
A mutation is a rare, random change in genetic material that can be inherited. Most mutations have no effect on the phenotype, some have a small effect, and rarely do they have a significant effect. The rate of mutation is increased by exposure to ionising radiation and some chemical mutagens.
Natural selection (Darwin's theory): individuals within a species show variation; those with characteristics best suited to their environment are more likely to survive and reproduce, passing on their advantageous alleles to the next generation — over many generations, this changes the characteristics of the population. A real example is antibiotic resistance: random mutation gives some bacteria resistance to an antibiotic; when the antibiotic is used, resistant bacteria survive and reproduce while non-resistant bacteria die, so the resistant population grows, making infections harder to control.
Do not say bacteria 'develop' resistance in response to antibiotics — the resistance mutation already exists randomly in the population; the antibiotic selects for it by killing non-resistant bacteria.
Must Know
- DNA is a double helix with base pairing A-T and C-G; RNA is single-stranded and uses uracil instead of thymine
- Protein synthesis: transcription (DNA to mRNA, in the nucleus) then translation (mRNA to protein, at ribosomes, using tRNA and codons/anticodons)
- Dominant alleles are expressed with one copy; recessive alleles need two copies; most characteristics are polygenic
- Genetic diagrams and pedigrees are used to predict monohybrid inheritance and probabilities
- Sex is determined by X and Y chromosomes: XX = female, XY = male
- Mitosis makes identical cells (growth/repair); meiosis makes four different haploid gametes (23 chromosomes in humans, diploid = 46)
- Mutations are rare, random changes in DNA; natural selection acts on this variation, as shown by antibiotic resistance in bacteria
That's the notes covered.
Carry on to the next subtopic.