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Pairing of the homologous chromosomes in the prophase of the first meiotic division.




Crossing over and exchange of pieces between two non-homologous chromatids.

Separation of the chromosomes in the first anaphase of the meiotic division.

Separation of the chromatids into chromosomes in the second anaphase and its distribution in the gametes.

An example that illustrates the genetic ratios in the case of incomplete linkage (where crossing over occurs):

In Drosophila, when mating occurred between dihybrid grey long-winged female (heterozygous in the two characteristics) and black vestigial-winged male (homozygous in the two characteristics). This mating represents a test cross for the dihybrid individual. Small percent of the offspring carried recombinant characteristics are resulted, whereas, most of the offspring looks like parents as follows:

 

The offspring resulted:

1. Grey long-winged 41.5% parental individuals.

2. Black vestigial-winged 41.5% parental individuals.

3. Grey vestigial-winged 8.5% recombinant individuals.

4. Black long-winged 8.5% recombinant individuals.

Analysis of these results:

If independent assortment occurred, these individuals would have appeared in equal ratios: 1 : 1 : 1 : 1 , but the parental traits appeared in 83% of the offspring, and 17% had new traits due to crossing over, and thus the linkage is not completed. We can make use of this result in deducing a relative distance between the two linked genes, which is 17 units, and the rate of occurrence of crossing over equals double this number, that is 34%

The importance of crossing over:

Crossing over increases the chances of diversity in the traits among members of the same species, which helps in their survival, and evolution.

2. Crossing over is used in drawing maps that illustrate the distribution of the genes in what is known as chromosomal maps.

The chromosomal maps:

The American geneticist Morgan introduced a method to explain the results of linkage and crossing over and to draw maps that determine the location of genes on the chromosomes of some plants, and animals, and for some chromosomes in Man.

Ex.:

In the previous example of Drosophila, the percentage of crossing over between the black colour gene, and the vestigial wing gene was 17%. If another ratio of crossing over of 5% appeared between the black colour gene and the gene for purple colour of the eye, and both are located on the chromosome no. 2. Then the percentage of crossing over between the gene for the purple colour of eye, and the gene for the vestigial wing is 12%, and then it is possible to draw a map for its chromosome as follows:

 

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