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  1. 1-biological-molecules

    1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-cell-structure
    2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    7 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-exchange-and-transport
    3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    10 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    8 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-genetics-variation-and-interdependence
    4-1-dna-genes-and-chromosomes
    7 主题
  20. 4-2-dna-and-protein-synthesis
    6 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
  25. 5-energy-transfers-in-and-between-organisms-a-level-only
    5-1-photosynthesis-a-level-only
    5 主题
  26. 5-2-respiration-a-level-only
    7 主题
  27. 5-3-energy-and-ecosystems-a-level-only
    9 主题
  28. 5-4-nutrient-cycles-a-level-only
    4 主题
  29. 6-organisms-respond-to-changes-in-their-environments-a-level-only
    6-1-response-to-stimuli-a-level-only
    12 主题
  30. 6-2-nervous-coordination-a-level-only
    10 主题
  31. 6-3-skeletal-muscles-a-level-only
    6 主题
  32. 6-4-homeostasis-a-level-only
    11 主题
  33. 7-genetics-populations-evolution-and-ecosystems-a-level-only
    7-1-inheritance-a-level-only
    6 主题
  34. 7-2-populations-a-level-only
    3 主题
  35. 7-3-evolution-a-level-only
    5 主题
  36. 7-4-populations-in-ecosystems-a-level-only
    7 主题
  37. 8-the-control-of-gene-expression-a-level-only
    8-1-genetic-mutations-a-level-only
    2 主题
  38. 8-2-regulation-of-gene-expression-a-level-only
    11 主题
  39. 8-3-using-genome-projects-a-level-only
    4 主题
  40. 8-4-gene-technologies-a-level-only
    13 主题
  41. exam-guidance-and-skills
    essay-guidance
    3 主题
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Investigating genetic ratios

  • Genetic diagrams (e.g. Punnett squares) predict offspring genotypes and phenotypes using known parental genotypes and meiosis

  • In sexual reproduction, each allele from a homologous pair has an equal chance of entering a gamete and being inherited

  • Predictions from genetic diagrams are based on probability; actual offspring ratios may differ due to random fertilisation of gametes

  • Model organisms like Fast Plant® (rapid-cycling Brassica rapa) and Drosophila (fruit flies) are used to study inheritance because:

    • They have short life cycles, meaning rapid breeding

    • Traits are controlled by single genes that determine easily identifiable traits

  • For these experiments, the assumption has been made that gamete fertilisation occurs randomly

Monohybrid inheritance in Fast Plant®

  • A particular type of Fast Plant® is bred from the species Brassica rapa for research

  • Usually, these plants produce a purple pigment called anthocyanin in their stems

    • This trait is controlled by a single gene with two alleles:

      • A = dominant, anthocyanin present (purple stem)

      • a = recessive, no anthocyanin (green stem)

  • A monohybrid cross can be carried out to study the inheritance of this single pair of alleles

Example 1: a homozygous cross using Fast Plant®

  • A homozygous dominant Fast Plant® is crossed with a homozygous recessive Fast Plant®

Parental phenotypes

Purple stem × Green stem

Parental genotypes

AA × aa

Gamete genotypes

A (from AA)

a (from aa)

Offspring genotypes

All Aa

Offspring phenotypes

All purple stem
Diagram shows inheritance of stem colour in plants. Parental genotypes AA (purple), aa (green) produce offspring Aa, resulting in purple stems.
A homozygous monohybrid cross

Examiner Tips and Tricks

When both parents are homozygous, no Punnett square is needed — offspring genotypes are predictable directly.

Example 2: a heterozygous cross using Fast Plant®

  • A monohybrid cross can also be used to predict the phenotypes of offspring that would be produced from two heterozygous Fast Plants® interbreeding

Parental Phenotypes

Purple stem × Purple stem

Parental Genotypes

Aa × Aa

Gamete Genotypes

A or a from each parent

Offspring Genotypes

1 AA : 2 Aa : 1 aa

Offspring Phenotypes

3 purple stem : 1 green stem (3:1 ratio)
Fast Plant monohybrid 2 (1), downloadable AS & A Level Biology revision notes
Diagram showing offspring genotypes AA, Aa, Aa, aa with phenotypes: purple stem for AA and Aa, green for aa. Expected phenotype ratio is 3:1.
The crossing of two heterozygous individuals results in a predicted phenotype ratio of 3:1

Examiner Tips and Tricks

Use a Punnett square when both parents are heterozygous, and always include genotype and phenotype labels in diagrams

Monohybrid inheritance in Drosophila

  • Small fruit flies from the genus Drosophila have been used in genetic experiments for many years

  • Their wing length is controlled by a single gene with two alleles:

    • L = dominant, produces long or wild-type wing

    • l = recessive, produces the stunted or vestigial wing

  • A monohybrid cross can be carried out to study the inheritance of this single pair of alleles

Example: a heterozygous cross in Drosophila

  • A monohybrid cross between two heterozygous Drosophila can be completed as follows:

Parental phenotypes

Wild type wing × Wild type wing

Parental genotypes

Ll × Ll

Gamete genotypes

L or l from each parent

Offspring genotypes

1 LL : 2 Ll : 1 ll

Offspring phenotypes

3 wild type wing : 1 vestigial wing

(≈75% : 25% ratio)
Genetics diagram showing parental phenotypes and genotypes. Punnett square with offspring genotypes: LL, Ll, ll. Expected phenotypic ratio of 3:1.
Two parents with the wild type wing length can produce offspring with the vestigial wing length if both of them are heterozygous

Examiner Tips and Tricks

Remember when dealing with genetic diagrams and questions in the exam, you always know the genotype of the individual displaying the recessive phenotype; they have to be homozygous recessive!

Probability of inheritance

  • Genetic diagrams (like Punnett squares) help predict the chance of offspring inheriting certain phenotypes

  • These predictions are shown as ratios, which represent the probability of each phenotype appearing

Example: a dihybrid cross

  • In a dihybrid cross, a 9:3:3:1 ratio is typical when two individuals heterozygous for both genes are crossed

  • This ratio means there are 4 possible phenotypes in the offspring:

Phenotype

Expected number (out of 16)

Percentage probability

W

9

56.25%

X

3

18.75%

Y

3

18.75%

Z

1

6.25%

  • Percentage probabilities are often used when dealing with the probability of inheriting genetic diseases