Polytene chromosomes from the salivary glands of larval Drosophila. A

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Polytene chromosomes from the salivary glands of
larval Drosophila. A powerful and convenient
tool for studying chromosomal rearrangements.
Note scale
Many (thousands) of chromatids aligned
side-by-side in register. Homologs are
synapsed and chromomeres are visible as bands.
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Can you locate the inverted segment?
Inversion loop
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Deletion loop
Deletions often form deletion loops that are
visible in polytene chromosomes
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Cri-du-chat syndrome
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Same dosage of bar but different phenotypes
Note the position effect
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Can you locate the inverted segment?
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The cross-over products of inversions are
seldom viable---only non-recombinant (parental)
gametes typically survive meiosis. Therefore,
inversions seem to suppress crossing
over. What effect might this have on apparent
map distances in mapping crosses? Remember that
map distance recomb./total
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Variegated position effect
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Effects of inversions

• 1. Suppression of crossing over.
• 2. Reduction of fertility.
• 3. Position effects (variegated)

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There apparently was a pericentric inversion in
the common ancestor of humans, chimps and
gorillas.
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Pericentric inversions can change arm ratios
(chromosome shape)
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This is a classical Robertson- ian translocation
or centric fusion. It occurred in a herd of
Danish dairy cattle. What would you predict
about the fertility of the translocation
heterozygotes like this one?
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Heterozygotes for Robertsonian translocations
have problems at meiosis because of the formation
of multivalents. Consider a heterozygote with
original chromosomes A and B (not homologus)
and the fused metacentric chromosome A---B. A
trivalent forms at prophase of meiosis
A B
A---B There are three
possible types of M1 segregation A, A---B / B
A / A---B, B A, B / A---B Only
the last segregation pattern leads to balanced
gametes or zygotes---the others result in
aneuploidy---zygotes with an extra copy of A or
B.