In bread mold (Neurospora crassa), two genes A and B are on the same chromosome.

Question

In bread mold (Neurospora crassa), two genes A and B are on the same chromosome. Gene A has two alleles (allele A and a), as well as gene B (allele B and b). One haploid cell with genotype AB mate with another haploid cell with genotype ab. The genotype for the diploid sexual zygote is AB/ab. After meiosis and the following mitosis, the 8 ascospores have following genotypes: aL aB AB A3 Given that only two crossovers happen during meiosis, are gene A and B on the same arm of the chromosome or different arms?

Explanation / Answer

Crossing over is the exchange of chromosome segments between nonsister chromatids in meisois.

Crossing over ususally occurs between distant genes than close genes, therefore these two genes are on different arms of the chromosome.

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