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Chromosome Behavior and Gene Linkage

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Only two chromatids are involved in any single crossover, and crossing over occurs at different points along the chromosome. Every meiotic process does not involve crossing over in the same set of genes each time, but there is at least one crossover per homologous pair. Still, when crossing over is observed in a meiotic division, up to half of the products of that division result in a recombination of DNA between the homologues. This recombination means the homologues are now different than the parents' chromosomes, therefore, each may carry a different set of genetic information.

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View this animation to observe how recombinants arise from crossing over between genes on nonsister chromatids. 

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Crossing Over is Random

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Many factors affect crossing over, so the position on the chromosome where crossing over will occur is unpredictable. Crossing over is a random event based on chance. The location of the break points on the DNA sequence of the chromosomes are somewhat random, but the recombination frequency is relatively constant between homologous chromosomes. (For a given chromosome, N number of breaks will occur, but where they will occur is random.)

The probability of crossing over between genes on a chromosome is dependent on the distances between the genes. This shouldn't surprise you because the greater the distance between two genes, the greater the chance a break will occur.

Genes that are located on the same chromosome and that tend to be inherited together are termed linked genes because the DNA sequence containing the genes is passed along as a unit during meiosis. The closer that genes reside on a particular chromosome, the higher the probability that they will be inherited as a unit, since crossing over between two linked genes is not as common.

Linked genes do not follow the expected inheritance patterns predicted by Mendel's Theory of Independent Assortment when observed across several generations of crosses. Usually, crossing over between nonsister chromatids will occur between genes when they are relatively far apart on the homologous chromosomes when pairing occurs. This results in the production of an equal number of nonrecombinant and recombinant chromosomes. Thus, the ratio of offspring produced from test crosses will be 1:1:1:1 (fully paternal, paternal-maternal recombinant, maternal-paternal recombinant, and fully maternal). When half of all offspring have recombinant chromosomes, a 50% frequency of recombination is observed. Recall that in a test cross, a 1:1:1:1 ratio indicates that the genes are unlinked. Therefore, unlinked genes may either reside on different chromosomes or reside far apart on the same chromosome.

When two genes are very close together on each homologue, break points for crossing over between the two genes will be rare and fewer recombinant chromosomes will be produced. Under this circumstance, a ratio that deviates from the usual 1:1:1:1 will be observed, indicating that the genes are linked. Thus, crossing over between two particular genes on the same chromosome can be used as an indirect indicator of the distance between the two genes.

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 Use this interactive animation to practice genetic mapping:

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Genetic Recombination and Meiosis

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