Crossing over may be defined as the recombination of linked genes brought as a result of interchange of corresponding parts between the chromatid of a homologous pair of chromosomes, so as to produce new combinations of old genes.
Types of crossing over
There could be following types of crossing over. The types depend upon the number of chiasmata in the chromosome.
- Single crossing over : There is only one chiasma where chromatids of homologous chromosomes form contact. The crossing over, thus, occurs at one point. This results in two crossovers or recombinant chromatids and two non-crossovers or parental chromatids are produced.
- Double crossing over: The crossing over occurs at two points in the same chromosome pair. The two chiasmata thus formed, produce a ring shaped structure.
- Multiple crossing over: The crossing over occurs at more than two points on the same chromosome. As a result, chiasma produce loops lying at right angles to each other.
Mechanism of crossing over
It involves following four stages. At the end of these stages, chromosome would show recombination of genes.
- Synapsis: This process occurs during prophase 1 of meiosis. A pair of homologous chromosomes, one from the male parent and the other from the female parent, come close together during zygotene. The process is termed as synapsis. This pair of chromosomes is known as bivalent.
- Chromosome duplication: In pachytene, each chromosome of the homologous pair differentiates into two sister chromatids. Thus there are four chromatids at this stage. This is termed as tetrad.
- Crossing over: Later, during pachytene stage, the non sister chromatids of homologous chromosomes exchange equal segments. This process is termed as crossing over. These points of contact where exchange of segments had been completed are termed as chiasmata. The two of the four chromatids now include some exchanged parts from non sister chromatid of homologous partner.
- Terminalization : On completion of crossing over, non sister chromatids move away from one another. In this process separation begins at the centromere and gradually proceeds towards the chromosomal ends. This is termed as terminalization. The process also involves the movement of chiasmata towards the ends of chromosomes.
Crossing over at four stranded stage
Neurospora, the pink mold, an ascomycetous fungus is helpful in demonstrating crossing over at 4-stranded stage. The ascomycetous fungus, Neurospora, has the following advantages as experimental organism.
- It is haploid and there is only one allele at each locus. Hence, dominant-recessive relationship does not interfere with observations and analysis.
- The products of single meiosis can be easily analysed.
- The products of meiosis occur in the form of ‘ordered tetrads’ i.e., the eight ascospores formed are linearly arranged in a sac like structure termed as ascus.
In Neurospora, as a result of meiosis, there is a formation of four haploid nuclei as usual, each of which further undergoes mitosis. Thus, it forms 8 haploid ascospores . In fact these should be termed as octads, but they are termed as tetrads since they actually represent four pairs or four tetrads.
The technique of using each of the individual spores of a tetrad to understand linkage and crossing over is termed as tetrad analysis.
First and Second Division Segregation
If crossing over does not take place, the ascospores shows 4 + 4 arrangement i.e., AAAA, aaaa as in this case. This is termed as first division segregation.
However, Neurospora shows any one of the following paired arrangements-AAaa AAaa or AAaa aaAA or aaAA AAaa. This is termed as second division segregation.
Similarly if A and B are located on different chromosomes and undergo independent assortment, the genotype of linearly arranged ascospores can be studied. If crossing over takes place at 2-stranded stage, the ascospores would show Ab, Ab, Ab, Ab, aB, aB, aB, aB (4 Ab+4 aB) arrangement. But if it takes place at 4-stranded stage, the ascospores would show AB, AB, Ab, Ab, aB, aB, ab, ab, (2AB+2Ab+2Ab+2ab) arrangement or 2:2:2:2 arrangement. Tetrad analysis demonstrates the presence of such an arrangement. Thus it confirms the occurrence of crossing over at 4-stranded stage.
Drosophila as experimental models
According to Morgan, Drosophila melanogaster i.e., the fruit fly suits better for studies in genetics. He therefore, used it as an experimental organism. The following are the advantages of fruit fly.
- It is easy to grow on a simple synthetic medium in the laboratory.
- It takes about two weeks to complete its life cycle.
- Single mating results in the production of large number of offsprings.
- Male and female flies with desired genotypes can be easily interbred.
Crossing over in Drosophila melanogaster
In a cross between a grey body vestigial wing (BBvv) and black body long wing (bbVV), Drosophila produces F1 hybrid all of them having grey body and long wings (BbVv).
Crossing of female flies of F1 generation with double recessive male having black body and vestigial wings (bbvv), produces following four types of offspring .
Grey vestigial- 41.5%
Black long -41.5 %
Black vestigial -8.5%
Grey long- 8.5%
The formation of these new combinations has occurred due to the exchange of chromosome parts between analogous chromosomes.
Significance of Crossing Over
- Genetic variability : It produces new combinations of characters. It produces inexhaustible genetic variability which passes through generations.
- Microevolution : The new combination produced, play an important role in microevolution.
- Mapping of chromosomes: Crossover values are useful in determining the position of genes on a chromosome.
- Arrangement of genes: It also offers proof of linear arrangement of genes on a chromosome.
- Development of new characters: The new combination is helpful in plant breeding.