⤭ Crossing Over

• The term crossing over was first used by Morgan and Cattell in 1912.
• The exchange of precisely homologous segments between non-sister chromatids of homologous chromosomes is called crossing over.

Mechanism of crossing over

• It is responsible for recombination between linked genes and takes place during pachytene stage of meiosis i.e. after the homologous chromosomes have undergone pairing and before they begin to separate.
• It occurs through the process of breakage and reunion of chromatids. During pachytene, each chromosome of a bivalent (chromosome pair) has two chromatids so that each bivalent has four chromatids or strands (four-strand stage).
• Generally one chromatid from each of the two homologues of a bivalent is involved in crossing over.
• In this process, a segment of one of the chromatids becomes attached in place of the homologous segment of the non-sister chromatid and vice-versa.
• It is assumed that breaks occur at precisely homologous points in the two non-sister chromatids involved in crossing over; this is followed by reunion of the acentric segments.
• This produces a cross (x) like figure at the point of exchange of the chromatid segments. This figure is called chiasma (which is seen in diplotene stage of meiosis) (plural-chiasmata).

• Obviously, each event of crossing over produces two recombinant chromatids (involved in the crossing over) called cross over chromatids and two original chromatids (not involved in crossing over) referred to as non-crossover chromatids.
• The crossover chromatids will have new combinations of the linked genes, i.e. will be recombinant; gametes carrying them will produce the recombinant phenotypes in test-crosses, which are called crossover types.
• Similarly, the non-crossover chromatids will contain the parental gene combinations and the gametes carrying them will give rise to the parental phenotypes or non-crossover types.
• Therefore the frequency of crossing over between two genes can be estimated as the frequency of recombinant progeny from a test-cross for these genes. This frequency is usually expressed as percent.
• Thus, the frequency of crossing over (%) can be calculated using the formula:

Types of crossing over

👉🏻 Depending upon the number of chiasmata involved, crossing over is of three types.

• Single crossing over: It refers to the formation of single chiasma between non-sister chromatids of homologous chromosomes. It involves two linked genes (Two point test cross).
• Double crossing over: It refers to the formation of two chiasmata between non-sister chromatids of homologous chromosomes. It involves three linked genes (Three point test cross).
• Multiple crossing over: Occurrence of more than two crossing overs between non-sister chromatids of homologous chromosomes is known as multiple crossing over. However, the frequency of such type of crossing over is extremely low.

Factors affecting crossing over

👉🏻 The frequency of crossing over between the linked genes is affected by several factors.

• Distance between the genes: The frequency of crossing over between the two genes is positively associated with the distance between their locations in the chromosome. Crossing over between the two genes would increase with an increase in distance between them.
• Sex: The frequency of recombination is markedly influenced by the sex of heterozygotes for linked genes. In general, the heterogametic sex shows relatively lower recombination frequencies than the homogametic sex of the same species. Eg: No crossing over occurs between linked genes in Drosophila males and females of silkworm.
• Age of female: The frequency of crossing over shows a progressive decline with the advancing age of Drosophila females.
• Temperature: In Drosophila, the lowest frequency of crossing over is observed when females are cultured at 22°C. The frequency of recombination tends to increase both at the lower and higher temperatures than 22°C.
• Nutrition: The frequency of crossing over in Drosophila is affected by the presence of metallic ions Eg: Ca⁺² and Mg⁺² in its food. Higher the amount, lower will be the crossing over frequency and vice-versa.
• Chemicals: Treatment of Drosophila females with certain antibiotics like mitomycin D and actinomycin D and certain alkylating agents such as ethylmethane sulphonate promotes crossing over.
• Radiations: An increase in frequency of crossing over is observed when Drosophila females are irradiated with X-rays and γ-rays.
• Plasmagenes: In some species, plasma genes reduce the frequency of crossing over. Eg: The Tifton male sterile cytoplasm reduces the frequency of crossing over in bajra.
• Genotype: Many genes are known to affect the occurrence as well as the rate of crossing over. For example C3G gene of Drosophila located in chromosome 3 prevents crossing over when present in homozygous state while it promotes crossing over in the heterozygous state.
• Chromosomal aberrations: In Drosophila, some chromosomal aberrations Eg: paracentric inversions, reduce recombination between the genes located within the inverted segment.
• Distance from centromere: Centromere tends to suppress recombination. Therefore genes located in the vicinity of centromeres show a relatively lower frequency of crossing over than those located away from them.

Significance of crossing over in Plant Breeding

• It increases variability
• It helps to break linkages
• It makes possible to construct chromosome maps

Coincidence

• It refers to the occurrence of two or more distinct crossing overs at the same time in the same region of a pair of homologous chromosomes and as a result, a double cross over product is obtained.
• Coefficient of coincidence is estimated by using the formula:

• The ratio between the observed and the expected frequencies of double crossovers is called coefficient of coincidence.

Interference

• The occurrence of crossing over in one region of a chromosome interferes with its occurrence in the neighbouring segment. This is known as interference.
• The term interference was coined by Muller.
• It may also be defined as the tendency of one crossing over to prevent another crossing over from occurring in its vicinity. This is called positive interference.
• Sometimes, one crossing over enhances the chance of another crossing over in the adjacent region. This is termed as negative interference. Eg: Aspergillus, bacteriophages.
• The effect of interference reduces as the distance from the first crossing over increases.
• The intensity of interference may be estimated as coefficient of interference.
  • Coefficient of interference = 1 - Coefficient of coincidence

Differences between crossing over and linkage