Polytene chromosomes

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Polytene chromosomes
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• Irradiate a Drosophila line
• Select different deletion lines
• Cross each deletion line with a homozygotic
  recessive line
• Determine the frequency of phenotypes for each
  deletion line
• Make a map indicating all loci

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• Suppose a polytene chromosome consists of
  segments A1.2.3.4.5.B1.2.3.4.5.6.C1.2.3. 4.
  Suppose you have six deletion lines, A5 -B3,
  B5-C2, A2-A5, B2-C3 B3-C4 and that you obtain
  the following phenotypes
• Deletion line Phenotype
• A5-B3 a c
• B5-C2
• A2-A5 a
• B2-C3 b c e
• B3-C4 b e
• To determine the position and sequence of the
  genes

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Six bands in a salivary gland chromosome of
Drosophila are shown in the accompanying figure,
along with the extent of five deletions

• 1 2 3 4 5 6
• Chr
• D1
• D2
• D3
• D4
• D5

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Recessive alleles a, b, c, d, e and f are known
to be in the region, but their order is unknown.
When the deletions are heterozygous with each
allele, the following results are obtained
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• The following results are all applicable to
  chromosome 3 of Drosophila melanogaster with a
  segmental sequence of A1-5, B1-6, C1-4. Indicate
  the position of the genes
• Deletion line Phenotype
• A5-B3 d e
• B5-C2
• A2-A5 b
• B2-C3 a d e
• B3-C4 a c d

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• The following results are all applicable to
  chromosome 3 of Drosophila melanogaster with a
  segmental sequence of A1-5, B1-6, C1-4. Indicate
  the position of the genes
• Deletion line Phenotype
• B2-C3 b c
• A5-B6 b c
• A2-B5 a b
• A3-B3 a b
• B5-C4 c d
• B3-C2 c
• A4-C1 a b c