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#chromosome inversion

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Chromosome variations

In inversion heterozygotes, crossing over results in

In inversion heterozygotes, normal and inverted chromosomes form an inversion loop at pachytene to maximize homology pairing. A single crossover inside the loop exchanges segments but produces unbalanced chromatids: duplicated outside regions with deletions, plus in paracentric cases dicentric chromatid and acentric fragment that cause breakage or loss. Most recombinant products are inviable, so recovered progeny show only parental nonrecombinant chromosomes, giving apparent suppression of recombination. This property preserves coadapted gene complexes and is used as balancer chromosomes in Drosophila genetics to prevent loss of lethal mutations.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 8: Recombination in Inversion Heterozygotes

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Chromosome variations

Inversion that does not include centromere is called

An inversion arises when a chromosome breaks at two points and the intervening segment reinserts reversed, changing gene order 180 degrees. Classification depends on centromere position relative to breakpoints. If both breaks occur in same arm and the inverted segment excludes centromere, it is paracentric, meaning alongside centromere. Because centromere remains outside, crossover inside inversion loop produces dicentric bridge and acentric fragment causing inviability. Pericentric inversions include centromere and alter arm ratio. Breakpoint mapping and cytogenetic banding patterns distinguish types for clinical diagnosis and risk counseling.

Ref: Nussbaum et al., Thompson & Thompson, Chapter 5: Paracentric vs Pericentric Inversions

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