Question

what evidence did alfred sturtevant use to support his conclusion that drosophila have four pairs of chromosomes?

all of the genes he identified had a recombination frequency of 50%.
drosophila genes cluster into four distinct groups of linked genes.
the overall number of genes in drosophila is a multiple of four.
all of the genes he identified had recombination frequencies of less than 50%.

Explanation:

To determine the correct answer, we analyze each option:

• Option 1: Recombination frequency of 50% implies genes are unlinked (on different chromosomes or far apart). Not all genes can have 50% recombination frequency as some are linked, so this is incorrect.
• Option 2: Alfred Sturtevant's work on Drosophila gene linkage showed that genes clustered into four groups of linked genes, corresponding to the four pairs of chromosomes. This matches his conclusion about chromosome number.
• Option 3: The number of genes being a multiple of four is not relevant to chromosome pairing or Sturtevant's evidence, so this is incorrect.
• Option 4: While linked genes have recombination frequencies less than 50%, this alone doesn't support the four - pair chromosome conclusion. The key was the grouping into four linked gene groups.

Answer:

B. Drosophila genes cluster into four distinct groups of linked genes.