What Is Crossing Over In Meiosis? Mechanism, Errors, And Significance

Meiosis is a process of chromosomal reduction through which eukaryotic cells reproduce sexually. An essential part of this process is crossing over which occurs in the prophase stage of meiosis I. Crossing over is the most crucial step in meiosis.

The essentiality of this process is evident in its potential to give rise to genetic variation. Therefore, crossing over has become the basis upon which biodiversity is studied among a population of living organisms and from one generation to another.

There is a lot about crossing over in meiosis that you need to learn about. Keep reading.

What is crossing over?

Crossing over is a crucial process in meiosis that involves the exchange of genetic material between two non-sister chromatids of homologous chromosomes (one from each parent) to produce new combinations of alleles in the gametes.

This process usually results in genetic variation in gametes. Crossing over in meiosis occurs in the prophase-1 stage during sexual reproduction. It exerts genetic recombination between chromosomes.

The exchange of genetic material occurs at the chiasma (plural: chiasmata), which is the primary point of contact. Depending on the number of chiasmata formed, there can be three types of crossing over.

A single crossover involves only two out of four chromatids that form one chiasma. It can be a double crossover if two chiasmata are formed with two, three, or four chromatids. A multiple crossover involves the formation of more than two chiasmata with a low frequency of crossing over.

How does crossing over occur in meiosis?

Crossing over is triggered by synapsis, the fusion of homologous pairs of chromosomes at the beginning of meiosis 1.  It is a cellular process regulated by the enzyme recombinase which is present in the recombination nodule.

Crossing over occurs at the chiasma, where the exchange of DNA between chromatids occurs. Then it progresses into the formation of new combinations of alleles in offspring of identical genetic combinations.

The homologous chromosomes (one from the father and one from the mother) are lined up in a way that they can be switched. The two chromosomes have the same genes but are usually in different forms.

The genes from one chromosome can move to the other chromosome to create new and unique but identical combinations in offspring. This process is crucial to genetic variation and helps to increase the diversity of a species.

At the chiasma, sections of the chromosomes break off and reconnect with the other chromosome, thereby rearranging the genetic material. Crossing over is responsible for the creation of genetically unique gametes.

Diversity in a species is necessary for the adaptation of species to changing environments and evolution which usually occurs later.

Does crossing over contribute to genetic diversity?

Crossing over increases genetic diversity among species. As segments of chromosomes exchange during crossing over, hybrid chromosomes with unique genetic material form. This results in the formation of genetically different gametes, which allows for genetic diversity.

Genetic diversity helps cells to engage in survival of the fittest through harsh environmental conditions. It also helps them undergo evolution. Furthermore, as crossing over allows for the rearrangement of genetic code within the chromosome, it creates diversity in sperm and egg cells during meiosis.

Can there be errors in crossing over?

Errors in crossing over can occur when homologous chromosomes misalign or non-homologous chromosomes pair. These errors could lead to:

Chromosome deletion or duplication

Chromosome deletion or duplication occurs when a segment of the chromosome is deleted or duplicated. This can occur anywhere on the chromosome. It can happen at the middle or terminal ends of the chromosome.

Some deletions have more effect than others. The size of the deleted genes determines the severity of the effect. The larger the deletion, the more severe its clinical effect. Microdeletions are more common than large deletions.

An example of a microdeletion is the deletion of 22q11, a region of chromosome 22. This causes the Di George syndrome, which manifests as health conditions like heart abnormalities, a characteristic facial appearance in children, defects of the palate, and immunity problems.


Translocation is another type of error that can occur during crossing-over. It occurs when one segment of a chromosome is relocated to another chromosome. This sounds normal but it could lead to a net gain or loss of genetic material.

This error is usually classified as Reciprocal or Robertsonian translocations. Reciprocal translocation occurs when any two chromosomes exchange non-homologous segments. This results in offspring with unbalanced translocation.

Robertsonian translocation occurs when two acrocentric (chromosomes that have their centromeres very close to their ends and include chromosomes 13, 14, 15, 21, 22 &Y) chromosomes are stuck together.

Chromosome inversion

A chromosome inversion occurs when a segment of the chromosome breaks away, twists around 180°, and is re-inserted back into the chromosome. Chromosome inversion can be pericentric if it spans the centromere or paracentric if it doesn’t get to the centromere.

This form of error does not usually have severe consequences except when it involves the disruption of a critical gene at the breakpoints. People with chromosome inversion are asymptomatic. One may or may not be aware of this condition.

People only find out when they have a child with an unbalanced chromosome arrangement. Other ways of finding out are during infertility investigation or extensive examination after recurrent miscarriages.

Significance of crossing over

  • Crossing over validates the claim that genes are arranged linearly
  • Crossing over increases the frequency of variation. During crossing over, numerous combinations are formed which provide the basis for natural selection. Natural selection, in turn, supports evolutionary fitness
  • It also supports the construction of chromosomal maps and tracing linkage groups
  • Linkage groups and the established linear arrangement of genes validate the structure and operation of genes
  • Crossing over cuts across all living organisms, including bacteria, fungi, yeast, higher plants, and mammals
  • The interchange of segments of chromosomes is important for evolution
  • During crossing over, very unique combinations can be formed, and they can be adopted for plant breeding


Does crossing over occur in mitosis?

Crossing over does not occur in mitosis. This is because synapsis, the trigger of crossing over in meiosis, does not occur in mitosis.

So, the homologous chromosomes do not pair with their counterparts. Unlike in meiosis where genetically unique cells are created, mitosis produces genetically identical cells.

Can crossing over be manipulated or controlled?

Yes. The frequency of crossing over depends on how close the genes are to each other on the chromosome. The closer the genes, the higher the frequency of crossover events. Conversely, the farther apart they are, the equal the chances of crossover and non-crossover events.

Other factors that affect crossing over are age, sex, mutation, nutrition, X-radiation, temperature, and inversion. The chances of crossing over are higher in males than in females and older people than young people.

Is crossing over the same as recombination?

Crossing over is not the same as recombination. Although the process looks quite similar and occurs at the prophase-1 stage of meiosis-1, they have their unique differences.

Crossing over is the pairing of homologous chromosomes and the crossing over between non-sister chromatids that results in recombination. Therefore, crossing over leads to recombination. However, recombination can be defined as the production of different combinations of alleles in the offspring.

Is crossing over the same as DNA repair?

Crossing over and DNA repair are two different processes. However, crossing over can be used to replace damaged DNA sections. Both processes use several similar protein complexes and occur in similar chromosome regions.


In simple words, crossing over is the exchange of DNA segments between non-sister chromatids to produce a new combination of alleles. Crossing over, among other processes like independent assortment and random fertilization, produces the potential for genetic variation.

Crossing over is not the same as recombination. It gives rise to genetic recombination between chromosomes. In other words, recombination is the end of crossing over. Also, keep in mind that whenever you hear crossing over, it must be meiosis.

Finally, learn about the mechanism of meiosis and its significance to living organisms.

Thanks for reading.