Final answer:
Unequal recombination during crossover in meiosis can lead to one chromatid having a duplication and another a deletion. It happens due to misaligned sequences on homologous chromosomes.
Step-by-step explanation:
A misalignment of repetitive sequences on two chromosomes can cause an abnormal crossover to happen. This is called unequal recombination, and it results in one chromatid carrying a duplication and another containing a deletion. During prophase I of meiosis, homologous chromosomes align gene-for-gene forming a tetrad structure. At this stage, crossover or recombination can occur where genetic material is exchanged between non-sister chromatids. However, when misalignment of repetitive sequences occurs, it leads to unequal recombination, resulting in one chromatid with an extra segment of DNA (duplication) and another with a missing segment (deletion).
Cytologists have found that structural rearrangements such as chromosomal inversions and translocations can lead to adaptive pairing during meiosis. If proper recombination does not occur, it can result in aneuploid gametes with genetic imbalances. Moreover, errors such as nondisjunction may result in the gain or loss of entire chromosomes during meiosis, contributing to disorders of chromosome number.