21. upon studying a considerable number of different crosses in Drosophila, Morgan reached the conclusion that all genes of this fly were clustered into four linked groups corresponding to the four pairs of chromosomes. Further studies revealed that linkage is not absolute and it is broken frequently. It is broken in prophase by a process called
(1) Recombination.    (2) Jumping of genes.
(3) Integration.        (4) Mutation.

Explanation of options

1. Recombination – correct

• During meiotic prophase I (especially pachytene), homologous chromosomes pair and exchange segments in a process called crossing over.

• This exchange reshuffles linked alleles, producing new combinations and effectively breaking linkage between genes on the same chromosome. This is genetic recombination.

2. Jumping of genes

• Refers to transposition of mobile genetic elements (transposons).

• Although transposons can move, this is not the mechanism Morgan referred to for routinely breaking linkage in meiosis.

3. Integration

• Usually describes insertion of viral or foreign DNA into a genome (e.g., prophage integration), not normal meiotic exchange between homologs.

• It is not the standard term for the process that disrupts linkage groups in meiosis.

4. Mutation

• Mutation is a change in DNA sequence at a locus.

• While mutations create new alleles, they do not systematically break linkage groups between existing genes during meiosis.

Therefore, the process that breaks linkage frequently during prophase I of meiosis is meiotic crossing over, i.e., genetic recombination (option 1).