Friday, September 25, 2009

Linkage and Crossing over Linkage Groups and Linkage Maps

In four pairs of chromosome of Drosophila there are thousands of genes. Each chromosome contains large number of genes; same is the case with all organisms. The chromosomes behave as single units. All the genes in a given chromosome tend to remain together during inheritance. This tendency of genes in a chromosome to remain together is called ‘Linkage’. This Linkage is not absolute and the genes do not remain locked up in the same chromosome for ever. Otherwise the interitence of traits would also have remained constant. During Meiosis, the homologous chromosomes come together and form pairs, a process is called ‘Synapsis’. Soon after sometimes they exchange segments mutually and the process is called ‘Crossing over’. This exchange occurs at random along the length of chromosomes. After separation, the chromosomes carry some genes that were earlier located in different members of their pair of homologous chromosome. Such exchange of chromosomal segments or crossing over may occur at more than one point in homologous chromosomes in single meiotic division. In Drosophila the dominant gene V for normal wings and its recessive allele V for ventigeal wings and the dominant gene B for grey body colour and b for black body colour are located in the same pair of chromosomes. As they are linked, they tend to be inherited together i.e. V with B and v with b.

It implies that when a homozygous VVBB fly is crossed with a homozygous vvbb, all of offsping would be VvBb i.e. normal winged, grey body colour flies. When one of these heterozygous flies is crossed with a homozygous recessive vvbb and the genes remain completely linked and no crossing over takes place, then only two types of individual, would appear equally in the offspring that would resemble the two parents i.e. grey bodied normal winged (VvBb) and black bodied, vestigial winged (vvbb).

The tendency of the parental Combinations to remain together is called linkage, the genes show linkage because they are located on the same chromosome. The magnitude of linkage depends upon the strength of linkage.

Coupling and repulsion: If two dominant genes are located on the same chromosome, the linkage relationship is called coupling. If one dominant gene and one recessive gene are both located on the same chromosome, the linkage relationship is called repulsion.

Linkage cross: The members of linkage groups corresponds to the number of chromosome pairs because such pair represents on linkage group. In Drosophila, there are four pairs of linkage groups because there are 4 pairs of chromosomes. Similarly, in corn there are 10 pairs of Linkage groups because it has 10 pairs of chromosomes.

Crossing over: The genes show linkage because they are located on the same chromosome. The combination of the linked genes can arise by a process during which the chromatids of homologous chromosomes exchange parts, the process is known as Crossing over.

Some basic principles of genes are:

(1) The genes on the chromosome are arranged in a linear colour in a specific manner like the beads on a string.

(2) When a gene A and its allele ‘a’ both are present in different members of a pair of homologous chromosomes the genes and its alleles occupy corresponding locations on homologous chromosomes.

(3) In order to produce recombinations between two different allelic pairs which are situated on the same chromosome, the crossing over must occur.

(4) The crossing over characteristically occur during first meiotic division.

(5) Meiotic crossing over occur when four chromatids are present in each pair of chromosome.

Sometimes no crossing over occurs in male Drosophila. In Sinkworm no crossing over occurs in female. Reason of absence of crossing over is not known.

Linkage groups: If a certain gene A is linked to two other genes B and C and C as well are linked, if many genes are present in an organism, Crosses can be arranged to determine independently assorting genes or genes linked to one another in pairs or groups. The group of genes linked together and not showing independent assortment is called linkage groups.

Linkage Maps: Linkage maps have been prepared in Large number of animals and plants with the help of frequencies of recombinations. These maps are condensed graphic representations of the relative distances expressed in percentages of recombination among the genes in one linkage group, Consequently located in a single chromosome.

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