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Home >> Linkage and Crossing Over Between Genes >> Coupling Phase of Genes
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Coupling Phase of Genes - Linkage is further classified into two groups, complete or incomplete linkage depending upon the absence (complete) or presence of recombinant types in the test cross progeny.

For example, if genes Land G (long wings and grey bodies) are present in one chromosome, while their recessive alleles I and g (short wing and black bodies) are on the homologous chromosome.

Each chromosome behaves as a unit during cell division.

Therefore, gene L and G would move to one pole, while I and g would move to the opposite pole.

In drosophila, a dominant L produces long wings, while its recessive allele, I produces short wings.Another dominant gene G governs grey bodies, whereas its recessive allele governs black bodies. When long-winged grey coloured Drosophila (LLGG) were crossed with short winged black coloured (llgg) ones, Fl progeny was long winged and grey coloured. The F1 (llgg) was test crossed with the double recessive drosophila (llgg).

The result showed that 82% of progeny show parental combination. While the 18% showed recombinant progeny properties (Le., long winged black coloured or short winged grey coloured). Clearly, the four phenotypic classes are not present in the expected ratio of 1:1:1:1. The phenotypic classes long winged grey coloured and short winged black coloured have a much higher frequency than the expected 25%.

These two character combinations are referred to as parental combination or parental phenotypes or parental types because they have the same character combinations that were present in the two parents of Fl'. The remaining two phenotypic classes, coloured shrunken and colourless full are far less frequent than the expected 25%.

These two character combinations are called recombinant phenotypes or recombinant types, since they are obtained by reshuffling of the characters present in the two parents of the F1 From the results, it is seen that dominant genes Land G have strong attractions for each other.

This phase is called as coupling phase or cis configuration. In other words, coupling phase is one in which two dominant genes are linked and they do not get separated or transferred on to other chromosomes by recombination.
 

 
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