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Biology_A-level_Cie

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  1. 1-1-the-microscope-in-cell-studies
    5 主题
  2. 1-2-cells-as-the-basic-units-of-living-organisms
    5 主题
  3. 2-1-testing-for-biological-molecules
    3 主题
  4. 2-2-carbohydrates-and-lipids
    8 主题
  5. 2-3-proteins
    6 主题
  6. 2-4-water
    2 主题
  7. 3-1-mode-of-action-of-enzymes
    5 主题
  8. 3-2-factors-that-affect-enzyme-action
    8 主题
  9. 4-1-fluid-mosaic-membranes
    4 主题
  10. 4-2-movement-into-and-out-of-cells
    12 主题
  11. 5-1-replication-and-division-of-nuclei-and-cells
    6 主题
  12. 5-2-chromosome-behaviour-in-mitosis
    2 主题
  13. 6-1-structure-of-nucleic-acids-and-replication-of-dna
    4 主题
  14. 6-2-protein-synthesis
    5 主题
  15. 7-1-structure-of-transport-tissues
    4 主题
  16. 7-2-transport-mechanisms
    7 主题
  17. 8-1-the-circulatory-system
    7 主题
  18. 8-2-transport-of-oxygen-and-carbon-dioxide
    5 主题
  19. 8-3-the-heart
    4 主题
  20. 9-1-the-gas-exchange-system
    6 主题
  21. 10-1-infectious-diseases
    3 主题
  22. 10-2-antibiotics
    3 主题
  23. 11-1-the-immune-system
    4 主题
  24. 11-2-antibodies-and-vaccination
    6 主题
  25. 12-1-energy
    5 主题
  26. 12-2-respiration
    11 主题
  27. 13-1-photosynthesis-as-an-energy-transfer-process
    8 主题
  28. 13-2-investigation-of-limiting-factors
    2 主题
  29. 14-1-homeostasis-in-mammals
    8 主题
  30. 14-2-homeostasis-in-plants
    3 主题
  31. 15-1-control-and-coordination-in-mammals
    12 主题
  32. 15-2-control-and-coordination-in-plants
    3 主题
  33. 16-1-passage-of-information-from-parents-to-offspring
    5 主题
  34. 16-2-the-roles-of-genes-in-determining-the-phenotype
    7 主题
  35. 16-3-gene-control
    3 主题
  36. 17-1-variation
    4 主题
  37. 17-2-natural-and-artificial-selection
    7 主题
  38. 17-3-evolution
    2 主题
  39. 18-1-classification
    5 主题
  40. 18-2-biodiversity
    7 主题
  41. 18-3-conservation
    6 主题
  42. 19-1-principles-of-genetic-technology
    11 主题
  43. 19-2-genetic-technology-applied-to-medicine
    4 主题
  44. 19-3-genetically-modified-organisms-in-agriculture
    2 主题
  45. 1-1-the-microscope-in-cell-studies
  46. 1-2-cells-as-the-basic-units-of-living-organisms
  47. 2-1-testing-for-biological-molecules
  48. 2-2-carbohydrates-and-lipids
  49. 2-3-proteins
  50. 2-4-water
  51. 3-1-mode-of-action-of-enzymes
  52. 3-2-factors-that-affect-enzyme-action
  53. 4-1-fluid-mosaic-membranes
  54. 4-2-movement-into-and-out-of-cells
  55. 5-1-replication-and-division-of-nuclei-and-cells
  56. 5-2-chromosome-behaviour-in-mitosis
  57. 6-1-structure-of-nucleic-acids-and-replication-of-dna
  58. 6-2-protein-synthesis
  59. 7-1-structure-of-transport-tissues
  60. 7-2-transport-mechanisms
  61. 8-1-the-circulatory-system
  62. 8-2-transport-of-oxygen-and-carbon-dioxide
  63. 8-3-the-heart
  64. 9-1-the-gas-exchange-system
  65. 10-1-infectious-diseases
  66. 10-2-antibiotics
  67. 11-1-the-immune-system
  68. 11-2-antibodies-and-vaccination
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Predicting inheritance: test crosses

  • A test cross can be used to deduce the genotype of an unknown individual that is expressing a dominant phenotype

  • The individual in question is crossed with an individual that is expressing the recessive phenotype

  • This is because an individual with a recessive phenotype has a known genotype

  • The resulting phenotypes of the offspring provides sufficient information to suggest the genotype of the unknown individual

Results

  • For a monohybrid test cross:

    • If no offspring exhibit the recessive phenotype then the unknown genotype is homozygous dominant

    • If at least one of the offspring exhibit the recessive phenotype then the unknown genotype is heterozygous

  • For a dihybrid test cross:

    • If no offspring exhibit the recessive phenotype for either gene then the unknown genotype is homozygous dominant for both genes

    • If at least one of the offspring exhibit the recessive phenotype for one gene but not the other, then the unknown genotype is heterozygous for one gene and homozygous dominant for the other

    • If at least one of the offspring exhibit the recessive phenotype for both genes then the unknown genotype is heterozygous for both genes

Worked example: test crosses

  • Rabbits have a single gene for ear length that has two alleles:

    • D, a dominant allele that produces long ears

    • d, a recessive allele that produces shorter ears

  • A breeder has a rabbit with long ears and they want to know the genotype of the rabbit

    • There are two possibilities: DD or Dd

  • The breeder crosses the long-eared rabbit with a short-eared rabbit

    • A rabbit displaying the recessive short ear phenotype has to have the genotype dd

Punnett square showing genetic cross. Rows and columns labelled with alleles, resulting genotypes Dd producing long ears. Labels: "Known gametes", "Possible gametes Option 1".
Test cross possibility one

Predicted ratio of phenotypes of offspring – 1 long ears

Predicted ratio of genotypes of offspring – 1 Dd

Punnett square diagram showing genetic outcomes for ear length: "Dd" for long ears and "dd" for short ears. Options include "D" and "d" gametes.
Test cross possibility two

Predicted ratio of phenotypes of offspring – 1 long ears : 1 short ears

Predicted ratio of genotypes of offspring – 1 Dd : 1 dd

  • The breeder identifies the different phenotypes present in the offspring

  • There is at least one offspring with the short ear phenotype

  • This tells the breeder that their rabbit has the genotype Dd

  • If the rabbit was genotype DD none of the offspring would have short ears

Examiner Tips and Tricks

Make sure before you start a test cross you think about the following: how many genes are there, how many alleles of each gene are there, which is the dominant allele, what type of dominance is it and is there linkage or epistasis between genes?