What is Crossing Over? – Definition
The term crossing over was coined by Morgan.
Crossing over is a process that produces new combinations of genes by exchanging segments between non-sister chromatids of homologous chromosomes normally reciprocally but sometimes unequally.
Crossovers are chromatins resulting from crossing over i.e. interchange of chromosomal parts.
Characteristics of Crossing Over
- Occurs at two levels, at gross chromosomal level (chromosomal recombination) and at DNA level (genetic recombination).
- Occurs between non-sister chromatids of homologous chromosomes.
- Exchange is normally reciprocal but sometimes unequal.
- Frequency of crossing over is closely related to physical distance between genes located on chromosomes.
Types of Crossing Over
- Somatic or Mitotic Crossing Over
- Germinal or Meiotic Crossing Over
Somatic or Mitotic Crossing Over
When crossing over occurs in chromosomes of somatic cells of an organism during mitotic cell division, it is called as mitotic crossing over. Occurrence is rarely. It has no genetical significance.
Germinal or Meiotic Crossing Over
When crossing over occurs during meiosis, it is called as meiotic crossing over.
Pre-requirements of Crossing Over
- Replication of about 99.7% DNA and 75% of histone synthesis should be completed before prophase I.
- Attachment of all chromosomes to nuclear envelop by their both ends, during leptotene.
Kinds of Crossing Over
- Single crossing over
- Double crossing over (reciprocal and complimentary)
- Multiple crossing over
Stages of Crossing Over or Mechanism of Crossing Over
The page Mechanism or Stages of Crossing-over provides the sufficient information on the topic.
Theories about the mechanism of Crossing Over
- Duplication theory (John Belling, 1928)
- Copy choice hypothesis / Switch model (J. Laderbeg)
- Break and exchange theory
- Hybrid DNA model
Factors affecting Crossing Over
- Physical distance between genes
Significance of Crossing Over
- Crossing over is universal in occurrence, occurs in plants, animals, bacteria, viruses and moulds.
- Meiotic crossing over allows a more independent selection between the two alleles that occupy the positions of single genes, as recombination shuffles the allele content between sister chromatids
- Helps in proving linear arrangement of genes.
- Recombination does not have any influence on the statistical probability that another offspring will have the same combination. This theory of “independent assortment” of alleles is fundamental to genetic inheritance.
- Origin of new character
- Necessary for natural selection, as it increases chances of variation.