Lesson focus: Inheritance transfers DNAThe molecule that carries the genetic code; it is made of two strands twisted into a double helix and is found in chromosomes. through gametes, while geneA section of DNA on a chromosome that codes for a particular protein, which affects a characteristic such as eye colour. and alleleOne version of a gene, such as the version for blue eyes or for brown eyes. provide a simplified model for predicting some offspring variation.
Relate DNA, genes, alleles and chromosomeA long, coiled thread of DNA in the nucleus that carries many genes. and use a simple genetic cross without assuming all traits follow one-gene dominance.
Build on earlier learning
Continuous and discontinuous variationVariation data show phenotypic patterns. Inheritance explains how some genetic differences pass between generations.
Practical
Extracting DNA
- Mash fruit with salty washing-up liquid to break the membranes.
- Filter the mixture.
- Pour ice-cold ethanol gently down the side of the tube: white strands of DNA appear at the boundary.
Record: Observations and what each step does.
Safety: Keep ethanol away from flames.
You will be successful when you can…
- Relate DNA, genes and chromosomes.
- Explain allele inheritance through gametes.
- Complete a simple genetic cross.
- State the limits of a one-gene model.
Words that carry the science
- DNA
- A polymer whose base sequence stores hereditary information.
- gene
- A DNA sequence that contributes to a functional product or trait.
- chromosome
- A long DNA molecule packaged with proteins.
- allele
- An alternative version of a gene or genetic sequence.
- dominant
- An allele expressed in the phenotype of a heterozygote in a specified simple model.
- recessive
- An allele whose effect in a simple model is masked in a heterozygote.
- heterozygous
- Having two different alleles at a locus.
Link molecular information to probability in offspring
01Chromosomes carry long DNA molecules
A chromosomeA long, coiled thread of DNA in the nucleus that carries many genes. contains one long DNAThe molecule that carries the genetic code; it is made of two strands twisted into a double helix and is found in chromosomes. molecule with many geneA section of DNA on a chromosome that codes for a particular protein, which affects a characteristic such as eye colour. and regulatory regions. A gene occupies a locus, and alleleOne version of a gene, such as the version for blue eyes or for brown eyes. are sequence variants at that locus. Most body cells contain chromosome pairs, one set inherited from each parent.
Gametes contain one chromosome from each pair. Fertilisation combines gametes, restoring paired chromosomes in the zygote.
02Simple crosses calculate probabilities
For a one-gene complete-dominance model, letters can represent alleles. A heterozygote Ff produces gametes carrying F or f, not gametes carrying Ff. A Punnett square combines possible gametes.
Each fertilisation is an independent probability event. A predicted 3:1 ratio does not require every family of four to contain exactly three dominantDescribes a version of a gene that shows its effect even when only one copy is present. phenotypes.
03Most phenotypes need richer models
Codominance, incomplete dominance, sex linkage, multiple alleles, polygenic inheritance and environmental effects do not fit one universal dominant–recessiveDescribes a version of a gene that only shows its effect when two copies are present. pattern.
Use a simple cross only when evidence supports its assumptions. Human characteristics such as intelligence, athletic performance and most appearance traits are complex and should not be reduced to one allele pair.
Inheritance: from body to DNA
Inherited characteristics are passed on in DNA. Zoom in from the body to a cell, its nucleus, a chromosome, a gene and finally the DNA itself. Each sperm and egg cell carries one chromosome from each pair, so a baby gets half its DNA from each parent.
Controls
A to T and C to G: the bases always pair this way. Genes come in different versions called alleles.
Complete a heterozygous cross
Two Ff parents reproduce in a complete-dominance model.
- Each parent produces F and f gametes.
- Combine gametes to give FF, Ff, Ff and ff.
- Genotype probabilities are 1/4 FF, 1/2 Ff and 1/4 ff.
- If F is completely dominant, phenotype probabilities are 3/4 dominant and 1/4 recessive.
How to make the reasoning visible: These are probabilities for each offspring, not a guaranteed sequence or family composition.
Connect levels of inheritance
Order each statement from molecule to offspring.
| Level | Example | Key relationship | Common error to avoid |
|---|---|---|---|
| DNA | Base sequence | Stores hereditary information | DNA is not only inside genes |
| Gene | Pigment-enzyme gene | DNA sequence with a function | One gene may affect several traits |
| Allele | F or f | Sequence variant at a locus | Dominant does not mean common |
| Gamete | F or f | Carries one allele in this model | A gamete is not Ff |
| Offspring genotype | FF, Ff or ff | Combination from two gametes | Probability is not certainty |
Why can two unaffected heterozygous parents have an affected recessive child?
Why does four offspring not guarantee a 3:1 phenotype ratio?
Check your understanding
Answer from memory, then check against the suggested answer.
What is an allele?
What does heterozygous mean?
What does a gamete carry for one locus?
A rare allele is dominant. Is that a contradiction?
Explain how meiosis and fertilisation generate genetic variation even without new mutations.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
In a simple model, F is dominant to f. Two Ff parents are crossed.
Complete the Punnett square.
(3)Parent gametes: F and f from each parent. Offspring: FF, ____, ____, ____
Calculate the probability that one offspring is ff. Give your answer as a percentage.
(1)A family has four children. Explain why they will not necessarily have exactly one ff child.
(1)Total for this question: 5 marks
Command word: For calculateA command word: work out a number answer, showing your working and the unit., show the gametes and combinations before giving the probability and interpreting it.