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Date November 2019 Marks available 1 Reference code 19N.2.HL.TZ0.2
Level Higher level Paper Paper 2 Time zone TZ0 / no time zone
Command term State Question number 2 Adapted from N/A

Question

The diagram shows one of Thomas Hunt Morgan’s crosses of Drosophila in the early 20th century.

State the type of inheritance shown.

[1]
a.

Identify the recombinants.

[1]
b.

The chi-squared value was calculated as shown. Deduce, with reasons, whether the observed ratio differed significantly from the expected Mendelian ratio.

[2]
c.

Markscheme

«gene/autosomal» linkage ✔

Reject sex linkage

a.

grey vestigial and black normal ✔

Accept Ggvv and ggVv or alternative acceptable upper/lower case genotypes.

b.

a. yes/observed ratio did differ significantly «from the expected Mendelian ratio»
OR
expected ratio is 1:1:1:1 / 575 of each type / 25 % of each type ✔ Correct ratio not needed in first alternative of mpa

b. 3 degrees of freedom ✔

c. critical value is 7.815 «at the 5 % level / 11.345 «at the 1 % level» ✔ Accept mpc if candidates indicate the critical value of chi squared by circling it.

d. chi-squared value «of 1002.6» exceeds the critical value ✔

Allow other levels of significance as long as the critical value is correctly stated for the chosen level.

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Core » Topic 3: Genetics » 3.4 Inheritance
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