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Home >> Linkage and Crossing Over Between Genes >>Crossing Over Between Genes
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Crossing Over Between Genes -In linkage analysis, two crosses are described, regardless of whether the cross was done during coupling or repulsion.

The F I produces gametes, 18% of which carried recombination progeny.

The reason for the production of the recombinant progeny with a fixed frequency is due to the occurrence of a phenomenon called recombination or crossing over.

Crossing over is the exchange of strictly homologous segments between non sister chromatids of homologous chromosomes.

Crossing over occurs during pachytene stage of meiosis and is responsible for recombination between linked genes.

During pachytene stage, each chromosome of a bivalent (chromosome pair) has two chromatids. Thus each bivalent contains four chromatids or strands (four strand stage). Generally, one chromatid from each homologue is involved in crossing over. In the crossing over a segment of one chromatid becomes attached in place of the homologous segment of the non sister chromatid and vice versa.

Crossing over involves the breakage each of two homologous chromosomes at precisely points in the two non sister chromatids and then exchange of parts. This produces an X-like structure at the point of exchange of the chromatid segments. This structure is called chiasma (plural chaismata).

Each homologous pair can show one to several chaismata. Chiasmata can be anywhere along the length of a pair but normally it is seen in different positions in different meiocytes.

Chaismata occur more or less randomly. The mechanism of crossing over.

Crossing over occurs at the pachytene stage after the synapsis of the homologous chromosomes has occurred in prophase of meiosis.

Since chromosome replication occurs during interphase, meiotic crossing over occurs in the post replication tetrad stage, that is, after each chromosome has doubled such that four chromatids are present for each pair of homologous chromosomes.

Each event of crossing over produces two recombinant chromatids called as crossover chromatids and two original chromosomes (Le., which have not undergone any crossing over) are called as non crossover chromatids.

The crossover chromatids will have new combinations of the linked genes, Le., will be recombinant. Gametes carrying them will produce the recombinant phenotype in test.

Hence, these phenotypes are called as crossover types, whereas the other chromatids will give rise to the parental type character and are therefore called as parental phenotypes or non crossover types.

Genes on one chromosome are said to be linked for the obvious reason that they are physically linked or joined together by the segment of chromosome between them. The position of alleles of one gene on a chromosome is always the same and fixed.

The location of a gene oil a chromosome is called a locus (plural loci). Thus, we can say that two genes whose loci are on the same chromosome are said to be linked.

If both the genes are dominant or both are recessive, then such an arrangement is called as cis hybrid.

But if one gene is dominant and other is recessive and if both the genes are linked, then it is called as trans hybrid.

Chromosomes with recombinant combinations of linked genes are formed by the occurrence of crossing over in the region between the two loci.

The probability that crossing over will occur between two loci increases with increasing distance between the two genes.

The frequency of crossing over between two genes can be estimated as the frequency of recombinant progeny obtained in a test cross for these genes.
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