DP Biology (first assessment 2025)

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Question SPM.2.HL.TZ0.7

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Date May Specimen-2023 Marks available [Maximum mark: 5] Reference code SPM.2.HL.TZ0.7
Level HL Paper 2 Time zone TZ0
Command term Explain, Identify, Outline Question number 7 Adapted from N/A
7.
[Maximum mark: 5]
SPM.2.HL.TZ0.7

Two alleles of a gene influence body colour in Drosophila. The dominant allele B results in a brown body colour while the recessive allele b codes for black body. In addition, two alleles of another gene determine wing shape; normal wings (V) are dominant over vestigial wings (v).

In a dihybrid cross between a double heterozygote for body colour and wing shape (BbVv) and a homozygous recessive for both traits (bbvv), the following offspring were obtained:

  • brown body, normal wings: 29
  • brown body, vestigial wings: 8
  • black body, normal wings: 10
  • black body, vestigial wings: 33
(a)

Identify the recombinant phenotypes amongst the offspring.

[1]

Markscheme

brown body vestigial wings AND black body, normal wings

(b)

Explain whether these results fit the predicted Mendelian ratios for this cross.

[2]

Markscheme
  1.  independent assortment predicts 1:1:1:1
     OR
     this (mendelian) ratio not seen / recombinants less frequent/parental types more frequent;
  2.  suggests linkage
     OR
     small numbers so could be a result of differential survival;
(c)

Outline the statistical test that could be used to provide more evidence of the type of inheritance.

[2]

Markscheme
  1.  calculate expected numbers;
  2.  assume no selective advantage to any of the genotypes;
  3.  calculate chi square value;
  4.  (if tabulated value is exceeded then) significant difference/reject null hypothesis/alleles do not sort independently/alleles linked;

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