Chromosomal Crossover
Chromosomal crossover (also called crossing over or recombination) is the exchange of genetic material between homologous chromosomes at sites called chiasmata during prophase I of meiosis, resulting in recombinant chromosomes with new combinations of alleles. This process is a major source of genetic variation in sexually reproducing organisms and is essential for proper chromosome segregation in most eukaryotes. Crossover frequency between two loci is used to calculate genetic map distances and construct linkage maps.
Stages of Chromosomal Crossover During Meiosis I
| Sub-stage of Prophase I | Event | Significance |
|---|---|---|
| Leptotene | Chromosomes condense and become visible | Chromosomes begin to compact |
| Zygotene | Homologous chromosomes synapse (pair) | Formation of bivalents begins |
| Pachytene | Synaptonemal complex fully formed; crossing over occurs | Genetic recombination takes place at chiasmata |
| Diplotene | Synaptonemal complex dissolves; chiasmata visible | Crossover points become observable |
| Diakinesis | Chromosomes maximally condensed | Bivalents align at cell equator, ready for metaphase I |
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Related Terms
Genetic Linkage
Genetic linkage refers to the tendency of genes located close together on the same chromosome to be inherited together during meiosis, rather than independently assort as predicted by Mendel's Law of Independent Assortment. The degree of linkage between two genes is inversely proportional to the physical distance between them; closely linked genes rarely recombine, while distantly located genes recombine more frequently and approach independent assortment. Linkage mapping uses recombination frequencies to construct genetic maps that estimate distances between genes in centimorgans (cM).
Non-disjunction
Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis I, meiosis II, or mitosis, resulting in daughter cells with an abnormal number of chromosomes (aneuploidy). When non-disjunction occurs during meiosis, the resulting gametes may have one extra chromosome (n+1, called trisomy after fertilisation) or one fewer chromosome (n-1, called monosomy after fertilisation). Non-disjunction is the most common cause of chromosomal abnormalities in humans, with its frequency increasing with maternal age.
Karyotype
A karyotype is the complete set of chromosomes in a cell, organised and displayed by size, shape, and banding pattern, typically as a standardised photographic arrangement used for diagnostic purposes. Karyotyping is performed on metaphase chromosomes stained with Giemsa or other dyes to reveal characteristic banding patterns that allow identification of individual chromosomes. It is used clinically to diagnose chromosomal abnormalities such as trisomies (e.g., Down syndrome, Trisomy 21), deletions, and translocations.
From Greek khiasma (a cross-shaped figure), referring to the X-shaped junction observed between chromatids during crossing over. The term "crossing over" was introduced by Thomas Hunt Morgan in 1911 based on his experiments with Drosophila. "Recombination" derives from Latin re- (again) + combinare (to combine).