Genetic linkage occurs when alleles located near each other on a chromosome tend to be inherited together during meiosis. Genes located closer together are less likely to undergo chromosomal crossover and become separated, making them more likely to be inherited together. The closer two genes are on a chromosome, the lower the chance that a crossover will occur between them. Crossing over during meiosis can lead to new combinations of genes on homologous chromosomes. The frequency of crossing over depends on the distance between genes, with genes farther apart having a greater chance of crossover.

2.  Genetic linkage is the tendency of alleles that
are located close together on a chromosome to
be inherited together during meiosis.
 Genes whose loci are nearer to each other are
less likely to be separated onto different
chromatids during chromosomal crossover, and
are therefore said to be genetically linked.
 In other words, the nearer two genes are on a
chromosome, the lower is the chance of a swap
occurring between them, and the more likely
they are to be inherited together.

3.  For Example, the research of the human genome
discovered that the factor III of clotting gene and the
factor V of clotting gene are located in the same
chromosome (the human chromosome 1). The factor
VII gene however is not linked to those genes since it
is located in the chromosome 13.
 Physical crossing over during meiosis I is a normal
event. The effect of this event is to rearrange
heterozygous homologous chromosomes into new
combinations. The term used for crossing over
is recombination.

4.  Recombination can occur between any two genes on
a chromosome, the amount of crossing over is a function
of how close the genes are to each other on the
chromosome.
 If two genes are far apart, for example at opposite
ends of the chromosome, crossover and non-crossover
events will occur in equal frequency. Genes that are
closer together undergo fewer crossing over events and
non-crossover gametes will exceed than the number of
crossover gametes.

6.  Finally, for two genes are right next to each other on
the chromosome crossing over will be a very rare
event.
 Two types of gametes are possible when following
genes on the same chromosomes.
 1) If crossing over does not occur, the products
are parental gametes.
 2) If crossing over occurs, the products
are recombinant gametes.
 The allelic composition of parental and recombinant
gametes depends upon whether the original cross
involved genes in coupling or repulsion phase.

8. It is usually a simple matter to determine which
of the gametes are recombinants. These are the
gametes that are found in the lowest frequency. This is
the direct result of the reduced recombination that
occurs between two genes that are located close to
each other on the same chromosome.
Also by looking at the gametes that are most
abundant you will be able to determine if the original
cross was a coupling or repulsion phase cross. For a
coupling phase cross, the most prevalent gametes will
be those with two dominant alleles or those with two
recessive alleles. For repulsion phase crosses,
gametes containing one dominant and one recessive
allele will be most abundant.

10.  How can we decide how close two genes are
on a chromosome?
 Because fewer crossover events are seen between two
genes physically close together on a chromosome, the
lower the percentage of recombinant phenotypes will
be seen in the testcross data.
 Now let's determine the linkage distance between the
genes pr and vg. We can actually make two estimates
because we have the results from coupling and
repulsion phases crosses.

12.  Once we have settled on a value, these genes can then be
graphically displayed. Let's say that the true distance
between the pr and vg genes is 11.8 cM that is the average
of our two estimates. We can next display them along a
chromosome in the manner shown below

13. The final point that we need to make regards the maximum
distance that we can measure. Because of the way in which the
calculations are performed, we can never have more that 50%
recombinant gametes.
 Therefore the maximum distance that two genes can be apart
and still measure that distance is just less that 50 cM. If two genes
are greater than 50 cM apart, then we can not determine if they
reside on the same chromosome or are on different
chromosomes.
 In practice though, when experimental error is considered, as
distances approach 50 cM it is difficult to determine if two genes
are linked on the same chromosome. Therefore, other mapping
techniques must be used to determine the linkage relationship
among distantly associated genes.
 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).

14.  “The exchange of chromosomal segments
between two non- sister chromatids”
 Crossing over may happen when the arms of the
chromatids of each homologous are paired during meiosis.
Matching portions of the extremities of two non sister
chromatids (one from one homologous of the pair) break
and the pieces are exchanged, each of them becoming
part of the arm of the other chromatid.

16. Genetic swapping occurs between paired
homologous chromosomes in our sex cells
—The Egg and Sperm. Chromosomes may
swap genetic material
 Genes that have a tendency to remain together
during crossing over are said to be linked. We will
talk about this concept during Crossing Over Basics
New genetic material has been acquired from the
other homologue.

17.  To provide genetic variation during
meiosis.
 Crossing Over ensures a combination
of the maternal and paternal genes we
inherited.
 More than two chromatids exchanged
parts.

18.  Tracking crossing over helps determine where
genes are located on the chromosome. Genes
that are far apart have a GREATER chance of
crossing over. Genes that are closer have a LESS
LIKELY chance of crossing over.
 Maker genes:
One gene can be identified as a maker genes
that can infer the presence of the other gene. This
can be used in identifying disease predisposition.

19.  The idea that intra chromosomal recombinants were
produced by some kind of exchange of material
between homologous chromosomes. But
experimentation was necessary to test this idea.
 In 1931, Harriet Creighton and Barbara McClintock
were studying two loci of chromosome 9 of corn:
 One affecting seed color (C, colored; c, colorless)
and the other affecting endosperm composition (Wx,
waxy; wx, starchy). Furthermore, the chromosome
carrying C and Wx was unusual in that it carried a
large, densely staining element (called a knob) on
the C end and a longer piece of chromosome on
the Wx end.

20. .
Thus, they correlated the genetic and cytological events
of intra chromosomal recombination. The chiasmata
appeared to be the sites of the exchange, but the final
proof of this did not come until 1978.
.

21. Crossovers taking place at the four-chromatid stage of
meiosis. However, just from studying random recombinant
products of meiosis, as in a testcross, it is not possible to
distinguish this possibility from crossing-over at the two
chromosome stage.
This matter was settled through the genetic analysis of
organisms whose four products of meiosis remain together
in groups of four called tetrads. These organisms are mainly
fungi and unicellular algae.
The meiotic products in a single tetrad can be isolated,
which is equivalent to isolating all four chromatids arising
from a single meiosis.

22.  Tetrad analyses of crosses in which genes are
linked clearly show that in many cases tetrads
contain four different genotypes with regard to
these loci; for example, from the cross:
AB x ab
 Some tetrads contain four genotypes:
AB
Ab
aB
ab

23.  This result can be
explained only by the
occurrence of a
crossover at the four
chromatid stage
because, if crossing over
occurred at the two
chromosome stage, then
there could be only two
different genotypes in an
individual meiosis.

24.  Tetrad analysis provides evidence that
enabled geneticists to decide whether
crossing-over occurs at the two-strand (two-
chromosome) or at the four-strand (four-
chromatid) stage of meiosis.
 Because more than two different products of a
single meiosis .Tetrad analysis allow the
exploration of many other aspects of intra
chromosomal recombination.

26. .