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Agriculture LibreTexts

Section C

Cytological basis of crosssing over Creighton and McClintock first obtained a strain of corn with an abnormal chromosome belonging to homologous pair number nine.

This chromosome carried a distinct knob at one end and a detectable translocation at the other end. The knob has no effect on the appearance of the plants, but the translocation, when heterozygous with normal chromosomes 8 and 9, results in 50% of pollen grains being sterile and empty and 50% of eggs, embryo sacs, and ovules aborting—consequently, the ear becomes only half-filled, and the kernels are irregularly distributed.

The interchange between chromosome 9 and chromosome 8 was first recognized by its effects on fertility, with 50% pollen and kernel abortion This semisterility results from the formation of deficient gametes following synapsis of four chromosomes in a cross-shaped configuration. This configuration opens out into a ring , from which the centromeres distribute either two alternate chromosomes, with balanced (viable) genomic constitutions, or two adjacent chromosomes, with deficient-duplicate (inviable) constitutions.

By crossing corn carrying only the abnormal chromosomes with corn carrying only normal chromosomes they were able to obtain plants with heteromorphic pairs of chromosomes –– that is, with one normal and one abnormal chromosome number nine. Then, they carried out crosses involving plants with heteromorphic chromosome pairs so that they could look for the occurrence of cytological recombination.

The essential components to the demonstration of cytological and genetic crossing over are:-

  • Differential features along the chromosomes that are morphologically (i.e., physically) recognizable and
  • genes in the region of the cytological markers (groups of traits= a distinguishing characteristic).

Genetic markers (the seed color alleles and their associated inheritance patterns) and cytological markers (the presence of abnormal sets of sex chromosomes).

Two cytological features were sufficient for the experiment::-

  • A dark-staining,heterochromatic ‘‘knob’’ at the end of chromosome 9, and
  • A reciprocal interchange (translocation) of a part of chromosome 9 with a part of chromosome 8.
  • The knob feature is present in some strains and absent in others.

    Crossing over and Linkage Maps Recombination due to crossing over can be used to order and determine distances between loci (chromosome positions) by genetic mapping techniques. Loci that are on the same chromosome are all physically linked to one another, but they can be separated by crossing over. Examining the frequency with which two loci are separated allows a calculation of their distance: The closer they are, the more likely they are to remain together.The percentage of recombinants formed by F1 individuals can range from a fraction of 1% up to the 50% always seen with gene loci on separate chromosomes (independent assortment).

    Example :- Normally, when a heterozygous colored, full kernel(CcSHsh ) corn plant crossed with homozygous( ccshsh )recessive, expected result is:- CcSHsh X ccshsh

    Phenotype Genotype Observed Expected
    CcSHsh color, full 4032 1/4
    Ccshsh color, shrunken 149 1/4
    ccSHsh colorless, full 152 1/4
    ccshsh colorless, shrunken 4035 1/4

    The two types with the higher numbers represent the two parent-type gametes, since not all cells will crossover. The other two represent the single crossovers.Since it is observed that crossing over has occurred, therefore it is possible to map the distance between these two traits on the chromosome. The closer the traits are on the chromosome, the less likely crossing over will occur.

    To figure map distance, the numbers are observed to determine the parent crosses, and whether or not the genes are in trans or cis formation. Then this equation is possible:-

# (Number) of crossover gametes

Map distance = -------------------------------------------------------------------- X 100

Total gametes

This percent can be used as the number of map units apart the two genes are. If the two genes are further than 50 units, crossing over will not be a factor.Therefore, the percentage of recombinant gametes (reflected in the percentage of recombinant offspring) correlates with the distance between two genes on a chromosome. By comparing the recombination rates of multiple different pairs of genes on the same chromosome, the relative position of each gene along the chromosome can be determined. This method of ordering genes on a chromosome is called a linkage map.

The higher the percentage of recombinants for a pair of traits, the greater the distance separating the two loci. By definition, one map unit (m.u.) is equal to one percent recombinant phenotypes. In honor of the work performed by Morgan, one m.u. is also called one centimorgan (cM).

Interference and Coincidence Crossing over does not occur uniformly along a chromosome. For example, fewer crossovers occur in the area around the centromere than in other areas of the chromosome (making the loci appear closer together than they actually are). Recombination is detected in heterozygotes only. It also occurs in homozygotes with the same frequency but not detected. Because Maximum frequency of recombination between two linked genes is 50% and cannot exceed 50%. Due to Even if every meiocyte had a crossing-over between two linked genes, only 2 of 4 chromatids in a bivalent (tetrad) was involved in crossing-over. (2/4 = 50%) Double or multiple crossovers decrease the number of detectable recombinants (cancellation effect).

Also, the formation of one chiasma typically makes it less likely that a second chiasma will form in the immediate vicinity of the first. This seems to may be due to the inability of the chromatids to bend back upon themselves within a certain minimum distance. This lack of independence is called interference and results in the observation of fewer double crossover types than would be expected according to true map distance. Interference varies in different sections of the chromosome and is measured by the Coefficient of Coincidence (C.C.) which is the ratio of observed to expected double crossover types.

C.C. = (observed DCO) / (expected DCO)

Interference = 1 - C.C.