Inversion

An inversion is a chromosome mutation in which a segment of DNA breaks off and reattaches in the opposite orientation. In an inversion, the order of genes in that segment is reversed. Importantly, inversions typically do not involve a gain or loss of genetic material - just a flipped segment.

• Because no DNA is lost or gained, many inversions have no overt effect on the carrier - unless the breakpoints occur within an important gene, disrupting its function.
• Inversions are categorized by whether they include the centromere: pericentric inversions involve the centromere, while paracentric inversions do not. This detail can matter for genetic outcomes during meiosis.
• During meiosis, an individual with one normal chromosome and one inverted chromosome segment can have trouble aligning them properly. This can lead to abnormal gametes (some with duplications or deletions) if crossover happens within the inversion loop - a consideration in fertility or offspring viability.

• If a karyotype or chromosome diagram is shown with a segment reversed in orientation (but all parts present), that indicates an inversion. A question might ask which mutation doesn't change the amount of DNA - the answer would be inversions (or translocations) as opposed to deletions/duplications.
• Exam questions on inversions may test understanding that gene order is changed. For instance, they could describe a chromosomal mutation where gene sequence ABCDE changes to ADCBE - recognizing this as an inversion of C-D segment.
• Also be prepared for questions about why an inversion carrier might have increased risk of miscarriages or infertile offspring - relating to production of unbalanced gametes if crossovers occur inside the inversion.