LINKAGE, CROSSING-OVER AND CHROMOSOME MAPPING (Part I)

 

LINKAGE AND CROSSING OVER

žGenes that are on the same chromosome travel through meiosis together. This is called linkage.

žAlleles of chromosomally linked genes can be recombined by crossing over.

žDuring meiosis, homologous chromosomes pair, and physical exchange of chromosome segments recombine genes. This is supported by the cytological observation that homologous chromosomes pair during meiosis.

žAt the switch points, the two homologues are crossed over, as if each has been broken and then reattached to its partner.

žA crossover point is called a chiasma (plural, chiasmata). Crossing over describes the process that creates the chiasmata—that is, the actual process of exchange between paired chromosomes.

žRecombination—the separation of linked genes and the formation of new gene combinations—is a result of the physical event of crossing over.

Linkage

žWhen two or more characters of parents are transmitted together to the offsprings of few generations such as F1 , F2 , F3 etc. without any recombination, they are called linked characters and the phenomenon is called linkage.

žThis is a deviation from the Mendelian principle of independent assortment.

žMendel’s law of independent assortment is applicable to the genes that are situated in separate chromosomes. When genes for different characters are located in the same chromosome, they are tied to one another and are said to be linked.

žThey are inherited together by the offspring and will not be assorted independently.

žThus, the tendency of two or more genes of the same chromosome to remain together in the process of inheritance is called linkage.

Coupling vs. Repulsion

žThe condition of having the dominant alleles for both genes on the same parental chromosome and both recessive alleles on the other parental chromosome is called “coupling”.

žThe opposite condition, having one dominant and one recessive on each parental chromosome is called “repulsion”.

EARLY EVIDENCE FOR LINKAGE AND RECOMBINATION: W. Bateson and R. C. Punnett Experiment

žPlants with red flowers and long pollen grains were crossed to plants with white flowers and short pollen grains.

žAll the F1 plants had red flowers and long pollen grains, indicating that the alleles for these two phenotypes were dominant.

žWhen the F1 plants were self-fertilized, instead of the 9:3:3:1 ratio expected for two independently assorting genes, they obtained a peculiar ratio of 24.3:1.1:1:7.1.

žAmong the F2 plants, the classes that resembled the original parents (called the parental classes) are significantly overrepresented and the two other (non-parental) classes are significantly underrepresented.

žBateson and Punnet could not provide the correct explanation for this observation.

žThe correct explanation for the lack of independent assortment in the data is that the genes for flower color and pollen length are located on the same chromosome—that is, they are linked.

EARLY EVIDENCE FOR LINKAGE AND RECOMBINATION: 

MORGAN’s Experiment

žLater, Thomas Hunt Morgan found a similar deviation from Mendel’s second law while studying two autosomal genes in Drosophila.

žOne of the genes affected eye color:

pr: purple (recessive)         

pr+: red (dominant)

žThe other gene affected wing length:

vg: vestigial (recessive)         

Vg+: normal (dominant)

žMorgan performed a cross to obtain dihybrids, then followed with a testcross:

žMorgan’s testcross results were as follows (listed as the gametic classes from the dihybrid):

žThese numbers deviate drastically from the Mendelian prediction of a 1:1:1:1 ratio. Parental classes are overrepresented while the non-parental classes are underrepresented.

žThe ratio of two parental classes is 1:1 while the ratio of two non-parental classes is also 1:1.

žMorgan performed another cross to obtain dihybrids, then followed with a testcross, where the alleles were in different combinations:

žMorgan’s testcross results were as follows (listed as the gametic classes from the dihybrid):  

žAgain, these numbers deviate drastically from the Mendelian prediction of a 1:1:1:1 ratio. Parental classes are overrepresented while the non-parental classes are underrepresented.

žThe ratio of two parental classes is 1:1 while the ratio of two non-parental classes is also 1:1.

žMorgan suggested that the two genes in his analyses are located on the same pair of homologous chromosomes, i.e. they are linked.

New combinations of alleles arise from crossovers

žThe linkage hypothesis explains why allele combinations from the parental generations remain together—because they are physically attached by the segment of chromosome between them.

žBut how do we explain the appearance of the minority class of non-parental combinations?

žWhen homologous chromosomes pair in meiosis, the chromosomes occasionally break and exchange parts in a process called crossing over. The two new combinations are called crossover products.

Cis- and trans conformation

žWhen the two dominant or wild-type alleles are present on the same homolog, the arrangement is called a cis conformation.

                                              Cis:   RL/rl

žWhen the two dominant or wild-type alleles are present on the different homologs, the arrangement is called a trans conformation.

                                           Trans:   Rl/rL

CROSSING OVER AS THE PHYSICAL BASIS OF RECOMBINATION

žRecombinant gametes are produced as a result of crossing over between homologous chromosomes.

žThis process involves a physical exchange between the chromosomes.

žThe exchange event occurs during the prophase of the first meiotic division.

žAlthough four homologous chromatids are present, forming a tetrad, only two chromatids cross over at any one point.

žEach of these chromatids breaks at the site of the crossover, and the resulting pieces reattach to produce the recombinants.

žThe other two chromatids are not recombinant at this site.

žThere is a possibility for multiple exchanges in a tetrad of chromatids.

žThere may, for example, be two, three, or even four separate exchanges—customarily called double, triple, or quadruple crossovers.

žHowever, that exchange between sister chromatids does not produce genetic recombinants because the sister chromatids are identical.

EVIDENCE THAT CROSSING OVER

CAUSES RECOMBINATION

žIn 1931 Harriet Creighton and Barbara McClintock obtained evidence that genetic recombination was associated with a material exchange between chromosomes.

žCreighton and McClintock studied homologous chromosomes in maize that were morphologically distinguishable.

žTwo forms of chromosome 9 were available for analysis: one was normal and the other had cytological aberrations at each end—a heterochromatic knob at one end and a piece of a different chromosome at the other.

žThese two forms of chromosome 9 were also genetically marked to detect recombination.

žOne marker gene controlled kernel color (C, colored; c, colorless), and the other controlled kernel texture (Wx, starchy; wx, waxy).

žCreighton and McClintock performed the following testcross:

žTheir results showed that the C Wx and c wx recombinants carried a chromosome with only one of the abnormal cytological markers.

žThe other abnormal marker had evidently been lost through an exchange with the normal chromosome 9 in the previous generation. 

žThese findings suggest that recombination was caused by a physical exchange between paired chromosomes.

CHIASMATA AND CROSSING OVER

žThe cytological evidence for crossing over can be seen during late prophase of the first meiotic division when the chiasmata become clearly visible.

žAt this time paired chromosomes repel each other slightly, maintaining close contact only at the centromere and at each chiasma.

žAs we might expect, large chromosomes typically have more chiasmata than small chromosomes.

žThus, the number of chiasmata is roughly proportional to chromosome length.

Recombination frequency

žLet’s assume that genes A and B are on different chromosomes and that an AABB individual is crossed to an aabb individual.

žFrom this cross the AaBb offspring are then testcrossed to the double recessive parent (aabb).

žBecause the A and B genes assort independently, the F2 will consist of two classes (AaBb and aabb) that are phenotypically like the parents in the original cross and two classes (Aabb and aaBb) that are phenotypically recombinant.

žFurthermore, each F2 class will occur with a frequency of 25 percent. Thus, the total frequency of recombinant progeny from a testcross involving two genes on different chromosomes will be 50 percent.

žA frequency of recombination less than 50 percent implies that the genes are linked on the same chromosome.