How does recombination of linked genes occur

The allelic composition of parental and recombinant gametes depends upon whether the original cross involved genes in coupling or repulsion phase. The figure below depicts the gamete composition for linked genes from coupling and repulsion crosses.

It is usually a simple matter to determine which of the gametes are recombinant. These are the gametes that are found in the lowest frequency. This is the direct result of the reduced recombination that occurs between two genes that are located close to each other on the same chromosome.

Also by looking at the gametes that are most abundant you will be able to determine if the original cross was a coupling or repulsion phase cross. For a coupling phase cross, the most prevalent gametes will be those with two dominant alleles or those with two recessive alleles.

For repulsion phase crosses, gametes containing one dominant and one recessive allele will be most abundant. Understanding this fact will be important when you actually calculate a linkage distance estimate from your data. The important question is how many recombinant chromosomes will be produced. If the genes are far apart on the chromosome a cross over will occur every time that pairing occurs and an equal number of parental and recombinant chromosomes will be produced.

Test cross data will then generate a ratio. But as two genes are closer and closer on the chromosome, fewer cross over events will occur between them and thus fewer recombinant chromosomes will be derived.

We then see a deviation from the expected ratio. How can we decide how close two genes are on a chromosome? Because fewer crossover events are seen between two genes physically close togehter on a chromosome, the lower the percentage of recombinant phenotypes will be seen in the testcross data.

It is now known that the pairing and interaction between homologous chromosomes, known as synapsis, does more than simply organize the homologs for migration to separate daughter cells. To construct a chromosome map, Sturtevant assumed that genes were ordered serially on threadlike chromosomes. He also assumed that the incidence of recombination between two homologous chromosomes could occur with equal likelihood anywhere along the length of the chromosome.

Operating under these assumptions, Sturtevant hypothesized alleles that were far apart on a chromosome were more likely to dissociate during meiosis simply because there was a larger region over which recombination could occur. Conversely, alleles that were close to each other on the chromosome were likely to be inherited together.

The average number of crossovers between two alleles, or their recombination frequency, correlated with their genetic distance from each other, relative to the locations of other genes on that chromosome.

Sturtevant divided his genetic map into map units, or centimorgans cM , in which a recombination frequency of 0. Perfectly unlinked genes correspond to the frequencies predicted by Mendel to assort independently in a dihybrid cross. A recombination frequency of 0. That is, every type of allele combination is represented with equal frequency. Gregor Mendel and the Principles of Inheritance.

Mitosis, Meiosis, and Inheritance. Multifactorial Inheritance and Genetic Disease. Non-nuclear Genes and Their Inheritance. Polygenic Inheritance and Gene Mapping. Sex Chromosomes and Sex Determination. Sex Determination in Honeybees. Test Crosses. Biological Complexity and Integrative Levels of Organization.

Genetics of Dog Breeding. Human Evolutionary Tree. Mendelian Ratios and Lethal Genes. Environmental Influences on Gene Expression. Epistasis: Gene Interaction and Phenotype Effects. Genetic Dominance: Genotype-Phenotype Relationships. Phenotype Variability: Penetrance and Expressivity. Citation: Lobo, I. Nature Education 1 1 How would you feel if you had to be the one to challenge Gregor Mendel's paradigm-shifting laws of inheritance?

Yet Thomas Hunt Morgan did exactly this and in the process made gene mapping possible. Aa Aa Aa. Figure 4: Phenotypes used in Sturtevant's cross. Sturtevant crossed flies with long wings M and vermillion eyes p with flies with rudimentary wings m and red eyes P. These traits are X-linked. Mapping Genes Using Recombination Frequency. Figure 5: An illustration of Sturtevant's cross, showing the chromosomes, illustrates his logic. Sturtevant illustrated the crosses and offspring resulting from a parental strain of gray-eosin female flies and yellow-red male flies.

In order to calculate the recombination frequency we use the following formula:. Substituting the values from our data set, we arrive at the following:. Sturtevant also described the fact that, for genes that were distant from one another, there was a discrepancy in the predicted number of crossovers. For example, the distance between B and M on his map was His recombination data using those two genes, however, did not suggest this distance.

Instead, Sturtevant found recombinants in male progeny, which, when plugged into the equation, produced a result of How, then, did Sturtevant explain the deviation? Figure 6: Data collected by Sturtevant. Number of possible combinations in forms having from 2 to 36 chromosomes in the pre-synaptic cells. Complete and Incomplete Linkage. References and Recommended Reading Blixt, S.

Journal of Heredity 26 , 60—64 ———. Journal of Heredity 29 , Hillers, K. Science 34 , Passarge, E. Genetics: A Conceptual Approach. New York, W. Journal of Genetics 13 , — ———. Journal of Experimental Zoology 14 , 43—59 Weiner, J.

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