Gene mapping
Genes are
arranged linearly in a chromosome. The point in a chromosome where the gene is
located is called locus. The diagrammatic representation of location and
arrangement of genes and relative distance between linked genes of a chromosome
is called linkage map or genetic map.
Constructing Genetic Maps
When two genes are close together on the same chromosome, they do not assort independently and are said to be linked. Genes located on different chromosomes are not linked and they assort independently and have a recombination frequency of 50%. Linked genes have a recombination frequency less than 50%.
The number
of genetic recombinants produced is characteristic of the two linked genes
involved. Genetic experiments can be used in genetic mapping.
Unlinked
genes assort independently. The best cross to use to test for linkage is the
testcross, a cross of an individual with another individual homozygous
recessive for all genes involved. In a testcross between a+ /a b+ /b and a/a
b/b, where genes a and b are unlinked, progeny phenotypic ratio will be
1:1:1:1. A significant deviation from this ratio (more parental types and few
recombinant types) indicates that the two genes do not assort independently and
that they are linked. The chi-square
test can be used to find the significant deviation.
In an
individual that is doubly heterozygous for the w and m alleles, for example,
the alleles can be arranged in two ways:
In the
arrangement on the left, the two wild-type alleles are on one homolog and the
two recessive mutant alleles are on the other homolog, an arrangement called coupling (or the cis configuration).
Crossing-over between the two loci produces w+m and wm+ recombinants.
In the
arrangement on the right, each homolog carries the wild-type allele of one gene
and the mutant allele of the other gene, an arrangement called repulsion (or the trans configuration). Crossing-over between the two genes produces w+m+
and wm recombinants.
The
recombination frequency for two linked genes is the same, regardless of whether
the alleles of the two genes involved are in coupling or in repulsion. Although
the actual phenotypes of the recombinant classes are different for the two
arrangements, the percentage of recombinants among the total progeny will be
the same in each case.
In 1913, a
student of Morgan’s, Alfred Sturtevant, determined that recombination
frequencies could be used as a quantitative measure of the genetic distance
between two genes on a genetic map. The genetic distance between genes is
measured in map units (mu),where 1 map unit is defined as the interval in which
1 percent crossing-over takes place. The map unit is also called a centimorgan
(cM), a term named by Sturtevant in honor of Morgan.
The unit of genetic map is Morgan or centimorgan. When
the percentage of crossing over between two linked genes is 1 per cent, then
the map distance between the linked genes is one morgan.
There is a greater probability of occurrence of crossing
over, when the two genes are farther apart in a chromatid. The probability of
crossing over between two genes is directly proportional to the distance
between them.
When two genes are nearer, the probability of occurrence of
crossing over between them is limited.
Gene Mapping with
Two-Point Testcrosses
Testcrosses are used for mapping because the homozygous
recessive parent produces only one type of gamete, with alleles that are
recessive to the alleles in gametes produced by the heterozygous parent. So in
a testcross we use one parent that is heterozygous for the genes being mapped
and another parent that has the recessive alleles for those genes.
A two-point
testcross should yield a pair of parental types that occur with about equal
frequency and a pair of recombinant types that also occur with about equal
frequency.
The following formula is used to calculate the recombination frequency:
Number of recombinants X 100 =
Recombination Frequency = Map Units
Total
number of testcross progeny
The recombination frequency is
used directly as an estimate of map units.
The two-point method of mapping is
most accurate when the two genes examined are close together; when genes are
far apart, there are inaccuracies.
Gene Mapping with
Three-Point Testcrosses
Genetic maps can be built by using
a series of two-point testcrosses. Still more complex type of mapping analysis
for three linked genes can be done using a three-point testcross. In diploid
organisms, the three-point testcross is a cross of a triple heterozygote with a
triply homozygous recessive.
Uses of gene mapping
1. It
is useful to determine the location, arrangement and linkage of genes in a
chromosome.
2. It
is useful to predict the results of dihybrid and trihybrid crosses.
Interference and Coincidence:
Besides single crossing over, having
only one chiasma, there may be double or multiple crossing over. It has been
discovered by H.J.Muller (1911) that when there are two double cross-overs
(suppose a and b) then one cross-over (a) tries to prevent the formation of
other cross over (b). This tendency of one cross-over to interfere with the
other cross over is termed as interference. Suppose frequency of ‘a’ crossover
is 10 and frequency of ‘b’ cross-over is 12, then their total frequency will
not be 10+12 = 22 as required but will be less than 22 due to interference.
When the two things happen the same
time and at the same place, they then coincide or intermix and this occurrence
may be considered coincidence. This coincidence refers to the occurrence of two
or more distinct cross-over (double or multiple) at about the same time in the
same chromosomal region. Double cross-overs are the result of coming together
(coincidence) of two single cross-overs.
When doubles occur in regular
expected ratio, coincidence is said to be 100%, whereas interference will be
nil. i.e.,
coincidence is inversely proportional to the interference.
According to Muller (1916) the
coefficient of coincidence is the ratio between the observed and expected
frequencies of double cross overs.
Coefficient of coincidence = Actual
number of double cross-overs/Expected number of double cross-overs.
Interference = 1- Coefficient of
Coincidence
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