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Chromosome Behavior and Gene Linkage

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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,  and the position on the chromosome where crossing over will occur is unpredictable. Crossing over is a random event. While the location of the break points on the DNA sequence of the chromosomes are  fairly random, the recombination frequency is relatively constant between homologous chromosomes. (For a given chromosome, N number of  cross overs 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  called linked genes because the DNA sequence containing the genes is passed along as a unit during meiosis unless they are separated by crossing over. The closer together that genes  are located on a particular chromosome, the higher the probability that they will be inherited as a unit, since crossing over between two linked genes is less frequent the closer together the two genes are (genes with complete linkage are close enough together on a chromosome that they never recombine and are always inherited as a unit).

Because of this, linked genes do not follow the expected inheritance patterns predicted by Mendel's Theory of Independent Assortment when observed across several generations of crosses. For two heterozygous genes that are unlinked and undergoing independent assortment, you expect to see parental and recombinant gametes in a ratio of 1:1:1:1 (if you don’t remember why, please review Tutorial 29). 

When two genes are linked on a chromosome,crossing over between the two genes will be less common than having no crossing over, so 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.  The number of parental genotypes in the gametes will be higher and the number of recombinant genotypes will be lower.

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