Access Keys:
Skip to content (Access Key - 0)
Biology 110 - Basic Concepts and Biodiversity

Contents

Toggle Sidebar
Chromosome Behavior and Gene Linkage

Versions Compared

Key

  • This line was added.
  • This line was removed.
  • Formatting was changed.

...

Section
Column
width70%

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.

Transcript for Gene Linkage - Part I

Column
width30%
 

Test Your Understanding: Crossing Over and Recombination of Alleles

...

Section
bordertrue
Column
width60%

Thus, using crossover data, Sturtevant and his coworkers mapped other Drosophila genes in linear arrays at particular genetic locations. Figure 8  shows an abbreviated genetic map of chromosome II in Drosophila.

As with many rules, there are exceptions. The maximum recombination frequency that can be calculated between two genes is 49%.  Once two genes are 50 map units apart, or further, the number of recombinant offspring produced would be equal to the number of offspring with the parental pehnotypes.  These genes would appear to assort independently and may mistakenly be thought to be located on different chromosomes because they are far enough apart on their chromosome that linkage is not observed in genetic crosses. These genes are mapped by adding the recombination frequencies from crosses involving intermediate genes, and by determining the approximate distance of each gene from the intermediates. For example, in Figure 8, the genes for black body and brown eyes are move than 50 map units apart, so if you performed a test cross you would see a 1:1:1:1 ratio of phenotypes in the offspring.  However, the genes for vestigial wings and cinnabar eyes are between these two genes at a distance of less than 50 map units, and you can to crosses to determine the recombination frequency between those genes and either black body or brown eyes, allowing you to construct a linkage map.

Column
width40%

Figure 8. Genetic Map of Chromosome II in Drosophila. (Click image to enlarge)

Section
Column
width70%

 

Transcript for Gene Linkage - Part II

 

Transcript for Gene Linkage - Part III

 

Transcript for Gene Linkage - Part IV

Column
width30%

 

 

Summary

Section
Column
width70%

This tutorial examined the consequences of gene linkage and how crossing over in meiosis can alter the segregation pattern of genes that are located on the same chromosome. During Prophase I, homologous chromosomes pair up, gene-for-gene. Occasionally this pairing leads to breakage and rejoining of regions of homologous chromosomes. In other words, portions of a maternally derived chromatid can exchange and become physically connected with regions of a paternally derived chromatid (and vice-versa). Any gene located on either side of this  break point will become unlinked. The absolute point where crossover occurs cannot be predicted with certainty (although some regions of the chromosome are more prone to this than others); however, the greater the distance that two genes are from one another on the same chromosome, the greater the likelihood that crossing over will occur between them.

If the probability for crossing over increases proportionately with the distance between two linked genes, then the frequency at which they become unlinked (due to crossovers) is an indirect measure of how far apart these genes are from one another. If you don't understand why this is so, you might find it helpful to draw a couple of cartoons of chromosomes with two hypothetical genes at different distances from one another. Draw one set with the two genes 1/2 inch apart, and the other set with them one inch apart. If, on average, a crossover occurs once every four inches, then 25% of the time the two genes located one inch apart will become unlinked. However, the two genes located 1/2 inch apart will only become unlinked 12.5% of the time.

The relationship between gene distance and recombination frequency can be used to map the relative location of a gene on a chromosome. Be sure that you understand the logic behind this process that allows geneticists to map the relative order of genes on a chromosome.

Column
width30%
 

...

Section
bordertrue
Column
width80%

Questions?  Either send Send your instructor a message through ANGEL or attend an office hour (the times are posted on ANGEL).Canvas!

Column
width20%