Question

Contrast the genetic composition of gametes derived from tetrads of inversion heterozygotes where crossing over occurs within a paracentric versus a pericentric inversion.

Short answer

Short Answer: Crossing over within both paracentric and pericentric inversions in inversion heterozygotes adversely affects the genetic composition of the resulting gametes. In paracentric inversions, unbalanced and likely inviable gametes are produced due to the formation of dicentric and acentric chromatids. In pericentric inversions, gametes with unbalanced gene dosage are formed due to the presence of chromosomes with gene duplications and deletions, potentially leading to genetic disorders or inviability.

Step-by-step solution

Understand inversion heterozygotes and inversions

Inversion heterozygotes are individuals carrying one copy of a normal chromosome and one copy with an inversion. Inversions are chromosomal mutations where a segment of the chromosome is reversed. There are two types of inversions: paracentric and pericentric inversions.

Describe paracentric inversion

In a paracentric inversion, the centromere is not included in the inverted segment. This means that both breakpoints of the inversion are on the same arm of the chromosome.

Describe pericentric inversion

In a pericentric inversion, the centromere is included within the inverted segment. The breakpoints for the inversion occur on different arms of the chromosome.

Understand crossing over within paracentric inversion

When crossing over occurs within a paracentric inversion in a heterozygote, a loop is formed by synapsis of inverted and non-inverted chromosomes, and reciprocal exchange may happen. After the exchange during prophase I of meiosis, two types of chromatids are formed: dicentric (two centromeres) and acentric (no centromere) chromatids. These chromatids will not be properly segregated during anaphase I of meiosis, leading to unbalanced gametes that most likely will not survive.

Understand crossing over within pericentric inversion

When crossing over occurs within a pericentric inversion in a heterozygote, a loop is also formed by synapsis of inverted and non-inverted chromosomes. However, after the exchange during prophase I of meiosis, chromosomes with duplications and deletions of certain genes are formed. The resulting gametes will have an unbalanced genetic composition, which may lead to genetic disorders or inviability.

Compare genetic composition of gametes from paracentric and pericentric inversion

In both cases, the genetic composition of gametes is affected by crossing over within inversions. For paracentric inversions, the formation of dicentric and acentric chromatids leads to unbalanced and most likely inviable gametes. For pericentric inversions, the formation of chromosomes with duplications and deletions of certain genes results in gametes with unbalanced gene dosage, which may lead to genetic disorders or inviability.