1. Genetic mapping involves determining the linear order and distance between linked genes on chromosomes through test crosses. 2. Key processes include determining linkage groups, calculating map distances in morgan or centimorgan units, and using two-point and three-point test crosses to order genes. 3. Maps are constructed by combining map segments and are useful for understanding inheritance, disease diagnosis, and evolution.

1. Chromosomal Mapping in
Eukaryotes
Chromosomal
Mapping in
Eukaryotes

2. Genetic map
Graphical representation of relative distances between
linked genes of a chromosome is called genetic
map, also known as gene map or chromosome map
or cross over map.
Genetic map
Graphical representation of relative distances between
linked genes of a chromosome is called genetic map, also known as
gene map or chromosome map or cross over map.

3. Purpose and uses of Genetic
mapping
 The purpose of genetic mapping is to determine
the linear order and distance of separation among
genes that are linked to each other along the
same chromosome.
 The chromosome maps display the exact
location, arrangement and combination of genes
in a linkage group of chromosomes.
 They are useful in predicting results of dihybrid
and trihybrid crosses.
 It allows geneticists to understand the overall
complexity and genetic organization of a
particular species.
Purpose and uses of Genetic
mapping
The purpose of genetic mapping is to determine the
linear order and distance of separation among genes that
are linked to each other along the same chromosome.
The chromosome maps display the exact location,
arrangement and combination of genes in a linkage
group of chromosomes.
They are useful in predicting results of dihybrid and
trihybrid crosses.
It allows geneticists to understand the overall
complexity and genetic organization of a particular
species.

4. Purpose and uses of Genetic
mapping
 The genetic map of a species portrays the
underlying basis for the inherited traits that an
organism displays.
 In some cases, the known locus of a gene within
a genetic map can help molecular geneticists to
clone that gene and thereby obtain greater
information about its molecular features.
 In addition, genetic maps are useful from an
evolutionary point of view. A comparison of the
genetic maps for different species can improve
our understanding of the evolutionary
relationships among those species.
Purpose and uses of Genetic
mappingThe genetic map of a species portrays the underlying
basis for the inherited traits that an organism displays.
In some cases, the known locus of a gene within a
genetic map can help molecular geneticists to clone that
gene and thereby obtain greater information about its
molecular features.
In addition, genetic maps are useful from an
evolutionary point of view. A comparison of the genetic
maps for different species can improve our
understanding of the evolutionary relationships among
those species.

5. Purpose and uses of Genetic
mapping
 Along with these scientific uses, genetic maps
have many practical benefits. For example, many
human genes that play a role in human disease
have been genetically mapped. This information
can be used to diagnose and perhaps someday
treat inherited human diseases. It can also help
genetic counselors predict the likelihood that a
couple will produce children with certain inherited
diseases.
 In addition, genetic maps are gaining increasing
importance in agriculture.
 A genetic map can provide plant and animal
breeders with helpful information for improving
agriculturally important strains through selective
Purpose and uses of Genetic mapping
Along with these scientific uses, genetic maps
have many practical benefits. For example, many
human genes that play a role in human disease
have been genetically mapped. This information
can be used to diagnose and perhaps someday
treat inherited human diseases. It can also help
genetic counselors predict the likelihood that a
couple will produce children with certain inherited
diseases.
In addition, genetic maps are gaining increasing
importance in agriculture.
A genetic map can provide plant and animal
breeders with helpful information for improving
agriculturally important strains through selective
breeding programs.

6. The method of construction maps of
different chromosomes is called genetic
mapping.
Construction of a Linkage Map
or Genetic Mapping
Construction of a Linkage Map
or Genetic Mapping
The method of construction maps of
different chromosomes is called
genetic mapping.

7. The genetic mapping includes
following processes:
1. Determination of Linkage Groups:
Before starting the genetic mapping of
chromosomes of a species, one has to know
the exact number of chromosomes of that
species and then, he has to determine the total
number of genes of that species by undergoing
hybridization experiments in between wild and
mutant strains.
The genetic mapping includes
following processes:
1. Determination of Linkage
Groups(No. of Chromosomes)
Before starting the genetic mapping
of chromosomes of a species,
 To know the exact number of
chromosomes of that species.
To determine the total number of
genes of that species by undergoing
hybridization experiments in between
wild and mutant strains.

8. 2. Determination of Map
Distance
1. Map unit
 Genetics use an arbitrary unit to measure the
intergene distance on the chromosomes that is map
unit which describe distances between linked genes.
 A map unit is equal to 1 per cent of crossovers
(recombinants); that is, it represents the linear
distance along the chromosome for which a
recombination frequency of 1 per cent is observed.
2. Morgan units
 These distances can also be expressed in morgan
units; one morgan unit represents 100 per cent
crossing over. Thus 1 per cent crossing over can also
be expressed as 1 centimorgan (1cM).
2. Determination of Map
Distance
Map unit
Genetics use an arbitrary unit to measure the
intergene distance on the chromosomes that is
map unit which describe distances between linked
genes.
A map unit is equal to 1 per cent of crossovers
(recombinants); that is, it represents the linear
distance along the chromosome for which a
recombination frequency of 1 per cent is observed.
Morgan units
These distances can also be expressed in
morgan units; one morgan unit represents 100
per cent crossing over. Thus 1 per cent crossing
over can also be expressed as 1 centimorgan
(1cM).

9. 1. Two point test cross
The percentage of crossing over between two
linked genes is calculated by test crosses in
which a F1 dihybrid is crossed with a double
recessive parent. Such crosses because involved
crossing over at two points, so called two point
test crosses.
1. Two point test
cross
The percentage of crossing
over between two linked
genes is calculated by test
crosses in which a F1 dihybrid
is crossed with a double
recessive parent. Such
crosses because involved
crossing over at two points,
so called two point test

10. 2. Three point test cross
Double cross over usually don’t occur between
genes less than 5 centimorgans apart, so for
genes further apart, the three point test crosses
are used. A three point test cross or trihybrid test
cross (involving three genes) gives us information
regarding relative distances between these
genes, and also shows us the linear order in
which these genes should be present on
chromosome. Such a three point test cross may
be carried out if three points or gene loci on a
chromosome pair can be identified by marker
genes.
2. Three point test
cross(Trihybrid cross)
Double cross over usually don’t occur
between genes less than 5 centimorgans
apart, so for genes further apart, the three
point test crosses are used. A three point
test cross or trihybrid test cross (involving
three genes) gives us information
regarding relative distances between
these genes, and also shows us the linear
order in which these genes should be
present on chromosome. Such a three
point test cross may be carried out if three
points or gene loci on a chromosome pair

11. 3. Determination of Gene Order
After determining the relative distances between the
genes of a linkage group, it becomes easy to place
genes in their proper linear order.
For example
If the linear order of three genes ABC is to be
determined, then these three genes may be in any
one of three different orders depending upon that
which gene is in the middle. For the time being we
may ignore left and right end alternatives. If double
crossovers do not occur, map distances may be
treated as completely additive units. Now, if we
suppose that the distance between the genes A-B =
12, B-C = 7, A-C = 5, we can determine the order of
genes correctly in the following manner:
3. Determination of Gene Order
After determining the relative distances between the
genes of a linkage group, it becomes easy to place
genes in their proper linear order.

12. 4. Combining Map Segments
Finally, the different segments of maps of a complete chromosome are
combined to form a complete genetic map of 100 centimorgans long for a
chromosome.
Examples. Suppose we have to combine
following three map segments.
a 8 b 10
c
c 10 b 22
d
c 30
e 2 d

13. 4. Combining Map Segments(cont..)
We can superimpose each of these segments by aligning the genes shared in
common.
a 8 b 10
c
d 22 b 10
c
d 2 e 30
c
Then finally we may combine the three segments into one map:
The a to d distance = (d to b) – (a to b) = 22 – 8 = 14
The a to e distance = (a to d) – (d to e) = 14 – 2 = 12
d 2 e 12 a 8 b
10 c

14. Following terms need to be understoo
Linkage
Linkage Groups
Linkage and Recombination

15. 1. Linkage
The term linkage has two
related meanings
 Two or more genes can
be located on the same
chromosome.
 Genes that are close
together tend to be
transmitted as a unite.

16. 2. Linkage Groups
Chromosomes are called
linkage groups
They contain a group of genes that
are linked together
The number of linkage groups is the
number of types of chromosomes of the
species
For example, in humans
22 autosomal linkage groups
An X chromosome linkage group
A Y chromosome linkage group
Genes that are far apart on the same
chromosome can independently assort
from each other
– This is due to crossing-over or
recombination

17. 3. Linkage and
Recombination
Genes nearby on the same
chromosome tend to stay
together during the formation
of gametes; this is linkage.
The breakage of the
chromosome, the separation of
the genes, and the exchange
of genes between chromatids
is known as recombination.
(We call it crossing over)

18. Mapping Genes in
Maize
Autosomal loci
Gene order unknown
Allele arrangements in
heterozygous female F1 not known
Symbol “+” used for wt alleles
Cross and possibilities shown Fig
Results shown Fig

21. Three-Point Mapping in
Drosophila
Criteria for a 3-point mapping cross
Organism producing crossover games must be
heterozygous at all loci to be studied
All genotypes of gametes must be able to be
determined from offspring phenotypes
Sufficient numbers of offspring must be
produced to have a “representative sample” of all
crossover classes
Three-point mapping experiment
 Offspring are a combination of parental (most
common), single-crossover and double-crossover
(least common) phenotypes

22. Three-Point
Mapping in
Drosophila

23. Genetic Map of Human X Chromosome

24. Gene Mapping in Haploid
Organisms
Common organisms
- Chlamydomonas and Neurospora
species
Neurospora
- Ordered asci (singular ascus)
- Sacks containing 8 spores in a string
- Each spore has genetic information
from one single strand of a DNA
duplex from each tetrad in the original
zygotes