The document discusses the concept of genetic linkage, first introduced by Bateson and Punnett, which describes the tendency of genes to remain together during inheritance due to their proximity on a chromosome. It outlines types of linkage—complete, incomplete, coupling, and repulsion—and their implications in genetic studies, including methods for detecting linkage such as test crosses. The significance of linkage in plant breeding and genetic mapping is highlighted, illustrating its impact on variability and the simultaneous improvement of desirable traits.

1. LINKAGE
Presented By:
Dr. RAJENDRA CHAVHAN
Assistant Professor in Zoology,
Mahatma Gandhi Arts, Science and
late N. P. Commerce College Armori, District Gadchiroli

2. DISCOVERY OF LINKAGE
In 1900, Mendel’s work was re-discovered, and scientists were testing his
theories with as many different genes and organisms as possible.
William Bateson and R. C. Punnett were working with several traits in sweet
peas.
Notably a gene for Purple (P) vs. Red (p) flowers, and a gene for long
pollen grains (L) vs. round pollen grains (l). (PPLL X ppll).
Definition of Linkage: “Tendency of genes to remain together in their
original combination during inheritance is called linkage”
The phenomenon of linkage was firstly
reported by Bateson and Punnet in 1906.
T. H. Morgan put forth the theory of linkage and concluded the
coupling & repulsion were two phases of single phenomenon of
linkage.

3. WHAT IS LINKAGE ?
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 CHROMOSOME ASSORT INDEPENDENTLY
They are NOT LINKED
Linkage is based on the FREQUENCY OF CROSSING OVER
between the two genes
Crossing over occurs in
PROPHASE OF MEIOSIS-I,
where homologous chromosomes break at identical to cation and re
join with each other.

4. Types of LINKAGE
Linkage is generally classified on the basis of three criteria
1. Based on CROSSING OVER
2. Based on GENES INVOLVED
3. Based on CHROMOSOMES INVOLVED.
a) COMPLETE LINKAGE
b) INCOMPLETE/PARTIAL LINKAGE
a) COUPLING PHASE
b) REPULSION PHASE
a) AUTOSOMAL LINKAGE
b) ALLOSOMAL LINKAGE/SEX LINKAGE

5. Types of LINKAGE - Based on Crossing Over
a) COMPLETE LINKAGE
b) INCOMPLETE/PARTIAL LINKAGE
COMPLETE LINKAGE:
There is complete absence of recombinant types due to absence of crossing over
INCOMPLETE/PARTIAL LINKAGE: If some frequency of crossing over also occurs between the
linked genes. It is known as incomplete/partial linkage.
It is known in case of
1. Males of drosophila and
2. Females of Silkworms
Incomplete linkage has been observed in
1. Maize,
2. Pea,
3. Drosophila female and
4. Several other organisms.

6. Types of LINKAGE - Based on Crossing Over
DIFFERENTIATE BETWEEN

7. Types of LINKAGE - Based on Genes involved
Depending on whether all dominant or some dominant and some recessive alleles are linked
together, Linkage can be categorized into:- a) COUPLING PHASE, b) REPULSION PHASE
a) COUPLING PHASE
b) REPULSION PHASE
The key difference between coupling and repulsion is
that coupling refers to the linkage of two dominant or
two recessive alleles while repulsion refers to the linkage of
dominant alleles with recessive alleles.

8. What is Coupling?
Coupling is the linkage of two dominant alleles of two genes in one chromosome and two recessive alleles of two
genes in the other homologous chromosome. Here, dominant alleles of the genes are present in one chromosome,
while their recessive alleles are present in the other chromosome. These linked genes show cis arrangement. It can be
illustrated as AB/ab.
Figure 01: Coupling Conformation
One chromosome carries AB and the other carries ab. This physical coupling between
dominant alleles and between recessive alleles prevents their independent assortment
during the gamete formation. Dominant alleles tend to remain together. Similarly,
recessive alleles also tend to remain together during the gamete formation.
Types of LINKAGE - Based on Genes involved

9. What is Repulsion?
Repulsion is another aspect of linkage which is different from coupling. In repulsion, dominant alleles or recessive
alleles come from different parents, and they tend to remain separate. Here, one parental chromosome carries one
dominant and one recessive allele while the other chromosome carries the other two alleles (dominant and recessive
alleles). It can be illustrated as Ab/aB.
Types of LINKAGE- Based on Genes involved
Figure 02: Repulsion Conformation
One dominant allele is linked with the recessive allele of the
second gene. This type of gene arrangement is
called trans arrangement

10. What are the Similarities Between Coupling and
Repulsion?
•Coupling and Repulsion are two aspects of linkage.
•They behave against Mendel’s law of independent
assortment.
What is the Difference Between Coupling and
Repulsion?
In coupling, there is a tendency in dominant alleles to
remain together; there is a tendency in recessive
alleles to remain together. On the other hand, in
repulsion, two such dominant alleles or two recessive
alleles come from different parents, and they tend to
remain separate. So, this is the key difference
between coupling and repulsion. The observed ratio
in coupling is 7:1:1:7 while the ratio of repulsion is
1:7:7:1. Moreover, coupling is a type of cis
arrangement, while repulsion is a type of trans
arrangement.
Difference Between Coupling and Repulsion

11. Summary – Coupling vs Repulsion
 Coupling and repulsion are two aspects of gene linkages.
 Coupling is the presence of two dominant alleles of two genes on the
same chromosome (AB).
 The remaining recessive genes of the two genes are present on the
other chromosome (ab).
 Hence, the dominant alleles of genes tend to remain together.
 Repulsion is the presence of dominant genes on the two
homologous chromosomes (Ab/aB).
 Therefore, dominant alleles or recessive alleles that came from
different parents tend to remain separate.
 Thus, this is the summary of the difference
between coupling and repulsion.

12. Types of LINKAGE - Based on Chromosomes involved
Based on the location of genes on the chromosomes
Linkage can be categorized into:-
a) AUTOSOMAL LINKAGE
b) ALLOSOMAL LINKAGE/SEX LINKAGE

15. LINKAGE groups
 Linkage group refers to a GROUP OF GENES which are present in one chromosome.
 In other words, ALL THOSE GENES WHICH OF LINKAGE GROUPS is EQUAL to the
HAPLOID CHROMOSOME NUMBER of an organism.
 Chromosome constitute one linkage group.
 The number of linkage groups is limited in each individual.
 The maximum number of linkage groups is equal to
haploid chromosome number of an organism.
For example there are
 Ten linkage groups in Corn (2n = 20)
 Seven in Garden Pea (2n = 14)
 Seven in Barley (2n = 14)
 Four in Drosophila melanogaster (2n=8) and
 Twenty three in human (2n=46)

16. detection of LINKAGE
 Test cross is the most common method of detecting the linkage
 In this method, the F1 heterozygous at two loci (AB/ab) is
crossed to a double recessive parent (ab/ab) and
 the phenotypic ratio of test cross progeny is examined.
 If the phenotypic ratio of test cross progeny shows 1:1:1:1
ratio of parental and recombinant genotypes,
 it indicates absence of linkage.
 If the frequency of parental types and
recombinant types deviate significantly from
the normal dihybrid test cross ratio of
1:1:1:1,
 it reveals presence of linkage between two
genes under study.

17. Detecting LINKAGE throughtest crosses
 Linked genes are used for mapping. They are found by looking for deviation from the
frequencies expected from independent assortment.
 A testcross (one parent is homozygous recessive) works well for analysing linkage.
 If the alleles are not linked, and the second parent is heterozygous, all four possible
combinations of traits will be present in equal numbers in the progeny.
 A significant deviation in this ratio (more parental and fewer recombination types) indicate
linkage.

18. significance of LINKAGE in plant breeding
1. Linkage limit the variability among the individuals
2. Linkage between two or more loci controlling different desirable characters is
advantageous for a plant breeder.
 A linkage between genes controlling two different desirable characters will help in
simultaneous improvement of both the characters.
3. Linkage is undesirable when desirable and undesirable genes are linked together.
4. THE ESTIMATES OF GENETIC VARIANCES FOR QUANTITATIVE CHARACTERS ARE GREATLY
INFLUENCED BY THE PRESENCE OF LINKAGE.

19. LINKAGE mapping
Each gene is found at a fixed position on a particular chromosome.
Making a map of their locations allow us to identify and study them better.
In modern times, we can use the locations to clone the genes so we can better understand
what they do and why they cause genetic diseases when mutated.
The basis of linkage mapping is that since crossing over occurs at random locations, the
closer two genes are to each other, the less likely it is that a crossover between two genes
is a measure of how far apart those two genes are.
 As pointed out by T. H. Morgan and Alfred Sturtvant, who produced the first
Drosophila gene map in 1913.
 Morgan was the founder of Drosophila genetics, and in his honour a recombination
map unit is called a centiMorgan (cM).

20. LINKAGE: Non-Mendelian Inheritance OR
Exception to Mendel’s law of independence Assortment
Application of Mendel’s Rules assumes:
1. One allele completely dominates the other.
2. All genes have two allelic forms.
3. All traits are monogenetic (affected by only one locus).
4. All chromosomes occurs in homologous pairs.
5. ALL GENES ASSORT INDEPENDENTLY.
 Allele pairs separate independently during the
formation of gametes.
 This means that traits are transmitted to offspring
independently of one another.
MENDEL’S LAW OF INDEPENDENT ASSORTMENT

21. LINKAGE: exceptionto mendel’s thirdlaw
Law of independent assortment
William Bateson and R. C. Punnett were working with several traits in sweet
peas.
Notably a gene for Purple (P) vs. Red (p) flowers, and a gene for long
pollen grains (L) vs. round pollen grains (l). (PPLL X ppll).
The phenomenon of linkage was firstly reported by Bateson and Punnet in
1906.
T. H. Morgan put forth the theory of linkage and concluded the
coupling & repulsion were two phases of single phenomenon of
linkage.

24. Thank
you