Linkage: T.H. Morgan performed a series of experiments with Drosophila (1910-1914). He showed that two genes were found in coupling phase (cis) or repulsion phase (trans), because these were present on the same chromosome pair. Such genes are called linked genes and the phenomenon was called linkage.
The genes on a given chromosome cannot assort independently and are called linked genes. The state of many genes being present on the same chromosome is called Linkage.
The number of linkage groups for any organism is equal to the haploid chromosome number. In man the number of linkage groups is 23, corresponding to the chromosome number n=23. Drosophila has 4 linkage groups.
Crossing Over: Recombination results from crossing over. The process of crossing over occurs by a mechanism called breakage and reunion theory (Darlington; 1937) and comprises four steps: synapsis, tetrad formation, crossing over and disjunction.
During prophase of meiosis I chromosomes physically exchange their segments. At zygotene homologous chromosomes are brought together and they are synapsed (side to side pairing). Each homologue has two chromatids at this stage. At pachytene chromatids exchange segments, resulting crossover types.
Linkage vs Crossing Over
1. It is tendency of genes on a chromosome to remain together and passed as such in next generation.
2. It brings more parental types.
3. Strength of linkage between two genes increases if they are closely placed on a chromosome.
4. With increase in age, link age increases
5. It helps to maintain a newly improved variety.
1. It is exchange of genes or chromosomal parts to break established linkages and formation of new linkages.
2. It produces recombinations.
3. Frequency of crossing over between two genes decreases if they are closely placed.
4. Crossing over decreases.
5. It is the source of variations for producing new varieties.