Repulsion Phase of Genes,Repulsion Phase,Recombinant Types,Trans Conguration,Coupling Phases,Repulsion Phases,Linkage Map

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Linkage and Crossing Over Between Genes
Linkage Between Genes
Repulsion Phase of Genes

	Repulsion Phase of Genes - When long winged black bodies (LLDD) are crossed with short winged grey bodies (llBB), the FI were long-winged and grey bodied (LlGg). But when the FI were test crossed with the double recessive strain (llgg), the parental type long winged black bodies and short winged, grey bodies were more frequent than the recombinant types. It appears as if in this cross, the dominant genes did not like one another, hence they got separated. This situation is referred to as repulsion phase or trans configuration.
Coupling and repulsion phases are obviously two phenomena of linkage. The only point to consider is whether two dominant genes or characters are existing simultaneously on one chromosome or not. In other words, if the parental combination continues to exist in FI, F2 and in test cross, i.e., in every generation, then such combination of linkage is called complete linkage. On the other hand, if the parental combination continues to exist only in every alternate generation, i.e., FI' F3, Fs F n-I then such combination of linkage is called as incomplete linkage. Linkage between two dominant genes produces significant deviation from the typical dihybrid ratio 1: 1: 1: 1 (test cross) and 9:3: 3: 1 (in F2 generation). This deviation is the most easily detected in test cross. In the case of linkage, the two parental types !ire most frequent.
In the linkage group, the percentage of parental types will be more than 50% and the recombinant percentage type will be less than 50%. It may be seen that the two parental types have comparable frequencies. Similarly the frequencies of the two recombinant types are also comparable. This type of test cross data is a sure indication of linkage. In fact, this relationship can be used as a safe guide to identify the two types in the test cross data whenever the identity of the parental type is not known. All the genes present on single chromosomes are grouped as one linkage group. Thus, the number of linkage groups present in a species is equal to the number of chromosomes present in that species.
For example, the number of linkage groups in humans is 23 and that of maize is 10.Genes present in one linkage group can be represented on a single straight line in: the same order in which they are present in the chromosome, and the distance between two linked genes is proportional to the frequency of recombination between them which can be depicted in a diagram. Such a diagram with the linear order and recombination frequencies depicted is called as linkage map or genetic map or chromosome map. Before preparing a chromosome map, the sequence of genes in the chromosome and frequencies of recombination between linked genes must be known. An appropriate test cross will determine the recombination frequencies between linked genes. Each recombination frequency is used as a map unit for preparing the linkage map.
	A map unit is defined as that distance between genes for which one product of meiosis out of 100 is recombinant or the recombination frequency between two genes is one. Then the two genes are separated or present on the chromosome at a distance of a map unit. Map unit is an imaginary distance and does not represent the actual physical distance between two linked genes in the chromosomes. Sometimes a map unit is referred to as centimorgan in the honour of Thomas Hunt Morgan. The sequence of linked genes IS determined by studying the test cross for three linked genes at a time. The data from such a test cross provides information on the order of the three genes in the chromosome as well as the frequency of recombination among them.

To begin with, three linked genes are mapped subsequently, a three point test cross involving only two of the three already mapped linked genes and a new gene expected to be linked with them is studied to map the new gene.

It is desirable to include only those genes that show less than 20% recombination with each other.