Concept of Crossing Over

Crossing Over:

Crossing Over may be defined as “the recombination of linked genes brought about as a result of interchange of corresponding parts between the nonsister chromatids of a homologous pair of chromosomes, so as to produce new combination of old genes”.

Actually, crossing over is a physical exchange of chromatid parts. Recombination is the result of crossing over. It represents new association of linked genes in the same chromosome. The chromosomes with new recombinations are called cross-over chromosomes or recombinant chromosomes. Crossing over and independent assortment are mechanisms that produce new combinations of genes. The new combinations are useful for evolution.

Concept of Crossing Over:

The important features of crossing over are as follows:

1. A gene is located on a chromosome at a specific site, this is called gene locus. The gene locus on a chromosome are arranged in a linear sequence.

2. In a hetero-zygote, the two alleles of a gene occupy corresponding position in the homologous chromosomes. The position of a gene in a given chromosome of a species is fixed or constant.

3. Crossing over involves the breakage of the nonsister chromatids of homologous chromosomes and their rejoining. This results in reciprocal exchange of corresponding segments between them.

4. As a result of crossing over new combination of genes are formed. This is called recombination.

5. Crossing over occurs more or less at random along the length of a chromosome pair. The chances of occurrence of crossing over between two autosomal genes increase with the distance between them along the chromosome length.