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Heredity and evolution
Heredity and evolution · Year 10 Science · Science understanding
This lesson explains how traits are inherited and how evolution by natural selection accounts for diversity and adaptation. For the short official checklist wording, see Heredity and evolution (curriculum). Test yourself: Heredity and evolution quiz.
1. What you should be able to do
- Compare mitosis and meiosis and say what each is for.
- Link chromosomes, DNA, genes and alleles in one coherent explanation.
- Use simple Punnett squares or ratios for dominant/recessive monohybrid crosses.
- State natural selection in your own words and name several kinds of evidence for evolution.
2. Cells, division and genetic material
Why two kinds of division?
- Mitosis — one cell becomes two genetically identical diploid body cells. Used for growth, repair and asexual reproduction in some organisms. Chromosome number stays the same (each daughter cell is 2n if the parent was 2n).
- Meiosis — one diploid cell (in ovaries/testes or plant equivalents) produces four haploid gametes (n chromosomes each). Crossing over and independent assortment shuffle alleles into new combinations.
Memory hook
Mitosis = Maintain (same genetics in somatic cells). Meiosis = Make gametes (half the chromosomes, new combos).
Chromosomes, DNA, genes, alleles
- A chromosome is a long DNA molecule packaged with proteins. Humans have pairs of chromosomes in body cells (one set from each parent).
- DNA is the chemical that stores instructions. A gene is a segment of DNA that usually codes for a product (often a protein) that influences a trait.
- Alleles are different versions of the same gene (e.g. allele for purple flowers vs white). An organism is homozygous if both alleles match, heterozygous if they differ.
Worked example — vocabulary in one paragraph
A diploid body cell in a species has 12 chromosomes (6 pairs). It divides by mitosis for skin repair: each daughter cell still has 12 chromosomes and the same alleles as the parent cell. A germ-line cell in the same species divides by meiosis: each functional gamete ends up with 6 chromosomes; which maternal and paternal chromosomes end up together varies, so gametes carry different allele combinations.
3. Mendelian patterns (Year 10 level)
For one gene with complete dominance:
- Phenotype = what you observe; genotype = allele letters (e.g. Aa).
- A monohybrid cross Aa × Aa gives genotypes 1 AA : 2 Aa : 1 aa → phenotypes often 3 dominant : 1 recessive if A is dominant.
Worked example — reading a Punnett square
Both parents are Aa for a trait where A is dominant.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
- Probability aa (recessive phenotype) = 1/4.
- Probability dominant phenotype = 3/4 (AA or Aa).
4. Evolution by natural selection
Natural selection is not “nature wants” or individual effort. It is a population-level outcome:
- Variation in traits (some of it heritable).
- Differential survival and reproduction — some variants leave more offspring in a given environment.
- Over many generations, favourable heritable traits become more common; unfavourable ones may decline.
Evidence you should be able to discuss includes: fossil sequences, homologous structures, molecular similarities (DNA/proteins), biogeography, and direct observation of change (e.g. antibiotic resistance, industrial melanism).
5. Your turn
Try without peeking, then reveal the answers.
Q1. One sentence: why does meiosis increase genetic variation in offspring compared with only mitosis?
Sample answer
Meiosis makes gametes with new combinations of alleles (assortment and crossing over) and half the chromosome number so fertilisation combines DNA from two different individuals.
Q2. In peas, purple (P) is dominant to white (p). Two heterozygous purple plants are crossed. What fraction of offspring are expected to be white?
Sample answer
Pp × Pp → 1/4 pp (white phenotype).
Q3. Explain in two sentences why “the giraffe stretched its neck” is not a scientific explanation of long necks in giraffes.
Sample answer
Acquired stretching is not inherited. Scientific explanations invoke heritable variation in neck length and differential survival/reproduction so longer-necked individuals leave more descendants over time (natural selection).