Linkage and Crossing
over
Lecture 5
Dr. Attya Bhatti
Linkage
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Linkage is defined genetically as the failure of two genes to
assort independently.
Linkage occurs when two genes are close to each other on
the same chromosome.
Genes far apart on the same
independently: they are not linked.
chromosome
assort
Linkage is based on the frequency of crossing over between
the two genes.
Discovery of linkage
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In the early 1900s, William Bateson and R. C. Punnett studied
two genes: one affecting flower color (P, purple, and p, red)
and the other affecting the shape of pollen grains (L, long,
and l, round).
They crossed pure lines P/P · L/L (purple, long) × p/p · l/l (red,
round), and selfed the F1 P/p · L/l heterozygotes to obtain an
F2.
The F2 phenotypes deviated strikingly from the expected
9:3:3:1 ratio.
Table: Sweet Pea Phenotypes Observed in the F2 by Bateson and
Punnett.
NUMBER OF PROGENY
Phenotype (and
genotype)
Observed
Expected from
9:3:3:1 ratio
purple, long (P/–
· L/–)
4831
3911
purple, round (/–
P· l/l)
390
1303
red, long (p/p · L/–)
393
1303
red, round
(p/p · l/l)
1338
435
6952
6952
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Researchers thought that physical coupling
between the dominant alleles P and L and between
the recessive alleles p and l might have prevented
their independent assortment in the F1.
Later, Thomas Hunt Morgan found a similar
deviation from Mendel’s second law while studying
two autosomal genes in Drosophila.
Morgan suggested that the genes governing both
phenotypes are located on the same pair of
homologous chromosomes.
Crossing over
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Morgan suggested that, when homologous
chromosomes pair in meiosis, the
chromosomes occasionally exchange parts
in a process called crossing-over.
Linkage groups
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A group of genes known to be linked; on a
chromosome are called linkage groups.
Linkage Symbolism
•
Each line represents a chromosome;
the alleles above are on one
chromosome, and those below are on
the other chromosome.
•
A crossover is represented by
placing an X between the two
chromosomes.
•
A wild-type allele is written with a plus sign.
•
Linked genes are designate as a b/a b, never
a b/b a.
Linkage Symbolism
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Genes known to be on different chromosome
pairs are shown separated by a semicolon, for
example, A/a ; B/b.
Genes of unknown linkage are shown separated
by a dot, A/a · B/b.
Linkage of genes on the X
chromosome
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The genotype of the gamete that a mother
contributes to her son is the sole determinant of
the son’s phenotype.
For Example;
We use here the following symbols: y and y+ for
the alleles governing yellow body and brown body,
respectively; w and w+ for alleles for white eye and
red eye; and Y for the Y chromosome.
Linkage of genes on the X
chromosome
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In the F1 the numbers of males in the phenotypic
classes are:
The total frequency of the recombinants will be;
(43 + 22) ÷ 14513 = 1.4 percent.
Linkage maps
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The amount of crossing-over between various
linked genes is different.
Morgan thought that these variations in crossover
frequency might somehow indicate the actual
distances separating genes on the chromosomes.
percentage of recombinants can be used as a
quantitative index of the linear distance between
two genes on a genetic map, or called linkage map.
Linkage maps
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If two specific genes positioned a certain fixed
distance apart.
In some meiotic divisions, nonsister chromatids
cross over by chance in the chromosomal region
between these genes; from these meioses,
recombinants are produced.
In other meiotic divisions, there are no crossovers
between these genes; no recombinants result from
these meioses.
Linkage maps
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Sturtevant postulated a rough proportionality:
The greater the distance between the linked
genes, the greater the chance that nonsister
chromatids would cross over in the region between
the genes and, hence, the greater the proportion
of recombinants that would be produced.
Thus, by
determining
the frequency
of
recombinants, we can obtain a measure of the map
distance between the genes.
Linkage maps
The two genes T and U are farther apart on a chromosome
than V and W. Chromatids cross over between T and U in a
larger proportion of meioses than between V and W, so the
recombinant frequency for T and U is higher than that for V
and W.
Linkage maps
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One genetic map unit (m.u.) is the distance
between genes for which one percent of the
products of meiosis are recombinant.
Or
Recombinant frequency (RF) of 0.01 (1 percent) is
defined as 1 m.u.
A map unit is also referred to as a centimorgan
(cM) in honor of Thomas Hunt Morgan.]
Linkage maps
The place on the map and on the chromosome where a
gene is located is called the gene locus (plural, loci).
Genetic map of the Drosophila genome