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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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Dihybrid crosses

  • Monohybrid crosses track the inheritance of one gene

  • Dihybrid crosses track the inheritance of two genes simultaneously

  • Genetic diagrams for both follow a similar structure, but dihybrid crosses involve:

    • More genotype combinations

    • More phenotype combinations

  • Genotype notation in dihybrid crosses:

    • Write both alleles of one gene first, then both alleles of the second gene

    • Example: for genes with alleles Y/y and G/g, a correct genotype is YyGg

    • Do not mix alleles between genes (e.g. avoid YGyg)

Worked Example

  • Horses have a single gene for coat colour that has two alleles:

    • B, a dominant allele produces a black coat

    • b, a recessive allele produces a chestnut coat

  • Horses also have a single gene for eye colour

    • E, a dominant allele produces brown eyes

    • e, a recessive allele produces blue eyes

  • In this example, a horse which is heterozygous for both genes has been crossed with a horse that is homozygous for one gene and heterozygous for the other

Parental phenotypes: black coat, brown eyes x chestnut coat, brown eyes

Parental genotypes: BbEe bbEe

Parental gametes: BE or Be or bE or be bE or be

Punnett square showing genetic traits from two parents. Traits include coat colour (black or chestnut) and eye colour (brown or blue).
Dihybrid cross represented in a Punnett square

  • Predicted ratio of phenotypes in offspring:

    • 3 black coat, brown eyes : 3 chestnut coat, brown eyes: 1 black coat, blue eyes: 1 chestnut coat, blue eyes

Autosomal linkage

  • Dihybrid cross predictions assume independent assortment of genes during meiosis

  • However, when genes are on the same chromosome, they may show autosomal linkage

  • Linked genes tend to be inherited together, maintaining the parental allele combination

  • This affects expected offspring ratios, reducing variation

  • Genotype notation for linked genes:

    • Linked alleles are often written in brackets to show they are inherited together

      • e.g. (FG)(FG) instead of FFGG

Worked Example

  • The genes for tail length and scale colour in a species of newt have displayed autosomal linkage

  • The gene for tail length has two alleles :

    • Dominant allele T produces a normal length tail

    • Recessive allele t produces a shorter length tail

  • The gene for scale colour has two alleles:

    • Dominant allele G produces green scales

    • Recessive allele g produces white scales

  • A newt heterozygous for a normal tail and green scales is crossed with a newt that has a shorter tail and white scales

Parental phenotypes: normal tail, green scales x short tail, white scales

Parental genotypes: (TG)(tg) (tg)(tg)

Parental gametes: (TG) or (tg) (tg)

Punnett square diagram showing gametes from two parents. Parent One: TG, tg. Parent Two: tg. Outcomes: normal tail, green scales or short tail, white scales.
Dihybrid cross with linkage represented in a Punnett square

  • Predicted ratio of phenotypes in offspring:

    • 1 normal tail, green scales: 1 short tail, white scales

  • Predicted ratio of genotypes in offspring :

    • 1 (TG)(tg) : 1 (tg)(tg)

Examiner Tips and Tricks

If phenotypic ratios deviate from standard dihybrid predictions (e.g. not 9:3:3:1), consider autosomal linkage as a possible reason

Epistasis

  • Epistasis occurs when one gene affects the expression of another gene

  • It involves two genes on different chromosomes influencing the same trait

  • The interaction between alleles determines the resulting phenotype

  • Epistasis must be considered when predicting phenotypic ratios in genetic crosses

Worked Example

  • There is a gene that dictates the feather colour of pigeons

  • The gene has two alleles (R / r) :

    • Allele R codes for a pigment that produces grey feathers

    • Allele r doesn’t produce a pigment, resulting in white feathers

  • Another gene has also been found to have an effect on feather colour

  • This gene has two alleles (F / f) :

    • The dominant allele F stops grey feathers from being produced even if the allele R is present

  • These are the possible phenotypes:

    • RRFF white feathers

    • RrFF white feathers

    • rrFF white feathers

    • RRFf white feathers

    • RrFf white feathers

    • rrFf white feathers

    • rrff white feathers

    • RRff grey feathers

    • Rrff grey feathers

Examiner Tips and Tricks

In genetics questions, you may notice that crosses involving autosomal linkage predict offspring that all have the same combination of characteristics as their parents. In reality, recombinant offspring are often produced; this is due to crossing over during meiosis.