When the non-sister chromatids of homologous chromosomes exchange segments between them during meiosis is called crossing over.
Linked genes can be separated by crossing over. When genes are on closer loci, they are strongly linked. If the genes lie farther apart, the chance of their separation through crossing over is greater.
Process of Crossing Over
Let say, there is a pair of the single homologous chromosome which pair up lengthwise. This is point to point and locus to locus pairing. One homologue carries genes A and B. Another homologue has a and b.
- Chiasmata formation was first observed by Janssens in 1909. He observed that during meiosis chromosomes are wrapped around each other. They form chiasmata. The genetic exchange takes place at chiasmata. The two chromatids exchange exactly equivalent segments that neither of chromatid gains or losses any genes.
Chiasmata open up during meiosis I and the homologous chromosomes are separated. The sister chromatids also separated from each other during meiosis II. Each chromatid becomes an independent chromosome.
- Chiasmata are formed at many places between non-sister chromatids. Crossing over occurs at 4 strand-stage between non-sister chromatids. Crossing over can take place in more than one place.
- Each chromatid moves individually in every four haploid gametes. Four types of gametes are formed;
- Two gametes have parental combinations of linked genes, i.e., AB and ab,
- Two with recombination of genes i.e., Ab and aB.
Only two parental types of gametes are formed without crossing over. Parental types of gametes produce the parental types of offspring. But recombinant gametes produce recombinant offspring.
Single and Multiple Crossovers
A crossing over that occurs between non-sister chromatids is called single crossover. In some cases, the distance between different gene loci located on non-sister chromatids is smaller. There is less chance of crossover if the two loci are closer to each other.
Thus, chances of occurring crossing over are reduced. In such cases, only one crossing over can occur and this is what we called a single cross over.
The occurrence of many crossovers between non-sister chromatids is called multiple crossovers. In some, chromosomes, the distance between gene loci is greater; here chances of occurrence of crossing over are increased. In such cases, multiple crossovers can occur.
In the double crossover, two exchanges take place between chromatids of the tetrad. To study a double crossover, three genes must be considered, each heterozygous for two alleles.
Mitotic Crossing Over
It is a rare type of genetic recombination. It may occur in some types of somatic cells during mitosis. Mitotic crossover may occur in organisms that do not contain the cycle of sexual reproduction.
But it can only occur in diploid cells and pair of chromosomes is required for the crossover. The mitotic crossover produces homozygous allele combinations in all heterozygous genes located on that chromosome arm.
So, when a mitotic crossover takes place, the genes that were previously recessive are expressed creating a new phenotype. Mitotic crossovers occur in some asexually reproducing fungi and in human cancer cells.
Crossing Over in Drosophila
Let’s take the example of crossing over in drosophila. Linked genes are located on the same chromosome so, they do not assort independently. They move together during meiosis and fertilization.
Therefore, linked genes should not recombine by the assortments of alleles. But, in fact, recombination between linked genes occurs.
- The offspring of testcross did not give a 1:1:1:1 phenotypic ratio. This ratio comes if the genes were two traits were present on different chromosomes and assort independently.
- But if the two genes were completely linked then we should observe a 1:1 ratio. The parental phenotypes should be present among the offspring.
The actual results are different from both these expectations. Most of the offspring had parental phenotypes. It suggests a linkage between the two genes. But about 17 % of the flies were recombinant. Thus, linkage appeared incomplete.