Biology Forums - Study Force

The fungus Neurospora crassa is a popular model organism for geneticists. During sexual reproduction, haploid cells from two compatible strains of N. crassa fuse together, forming a diploid zygote. The zygote undergoes meiosis to produce four haploid spores, which are conveniently bound together in a sac.

You have identified two genes, A and B, which control pigmentation in N. crassa. Spores with genotype AB are dark, while spores with genotype a_ or _b are light. Genes A and B are located on different chromosomes. You mate a strain with genotype AB with a strain of genotype ab. What is the expected result?

A. 100% of sacs contain 1 dark spore and 3 light spores.
B. 100% of sacs contain 2 dark spores and 2 light spores.
C. 25% of sacs contain 4 dark spores; the others contain 4 light spores.
D. 50% of sacs contain 4 dark spores; the others contain 4 light spores.
E. 50% of sacs contain 2 dark and 2 light spores; the others contain 4 light spores.

The zygote is AaBb, and it will produce spores AB, Ab, aB, and ab → 1 (AB) will be dark and 3 will be light, so there is a 1:3 ratio for dark:light.

I don't understand why the correct answer would be E, instead of A - both have a 1:3 ratio, what is the difference between the two?

Thanks.

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