Biology 9700 · AS & A Level · Passage of information from parents to offspring
Passage of information from parents to offspring — practice question
In the fruit fly Drosophila melanogaster, genes determine eye colour and wing shape. E/e are alleles of a gene controlling eye colour: E gives red eyes, while e gives purple eyes. N/n are alleles of a gene controlling wing shape: N gives normal (functional) wings, while n gives vestigial (short, non-functional) wings. Both of these genes are autosomal. A dihybrid cross was performed between one fly with red eyes and normal wings and another with purple eyes and vestigial wings. Each parent was homozygous for both genes. The F1 offspring were crossed to produce the F2 generation. The results are shown in Table 2.1.
(a(i))[2]
Fill in the missing expected number of individuals and the expected F2 ratio in Table 2.1.
(a(ii))[4]
A chi-squared test showed that the F2 generation results in Table 2.1 differed significantly from the values expected. To investigate this, test crosses were done with flies taken from the F1 generation and flies homozygous recessive for both genes. The investigator assumed that the genes were unlinked and predicted a ratio of $1:1:1:1$. Draw a genetic diagram of this test cross. Use the symbols E/e and N/n.
(b(i))[2]
Flies with red eye, vestigial wing and flies with purple eye, normal wing phenotypes are called recombinant. Identify the stage of meiosis in which these recombinants are formed and explain how this happens.
(b(ii))[2]
Explain why the results in Table 2.2 do not fit the expected ratio of $1:1:1:1$.
Worked solution & mark scheme
This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Table of expected numbers and ratios completed correctly” …