B.Sc. II Sem IV Linkage, Theories, and types

1. Unit – V Genetics
5.1 Linkage – Concept, Types and theories
5.2 Crossing over: Concept, Types and theories
5.3 Gene mutations- Spontaneous and Induced
5.4 Extra-nuclear Genome- Mitchondrial DNA and
Chloroplast DNA

3. All the genes present on a chromosome are said to be linked to
one another. This type co existance of 2 or more genes in the
same chromosome and their inheritance in group is called as
linkage.
OR
The tendency of two or more genes to stay together during
inheritance is known as linkage.
OR
When two genes are located on the same chromosome, they
tend to stay together in inheritance. This process is known as
linkage.

4. Genetic linkage is the tendency of DNA sequences that are close together on
a chromosome to be inherited together during the meiosis phase of sexual
reproduction

11. Example 1-
P1 : AABB x aabb
P1 gamets: AB ab
F1: AaBb
Testcross: Aa Bb x aa bb
Gamets: AB Ab aB ab ab
F2 AaBb: Aabb: aaBb: aabb or 1:1:1:1
Example 2-
P1 : AA/BB x aa/bb
P1 gamets: AB ab
F1: Aa/Bb
Testcross: Aa/Bb x aa/bb
Gamets: AB ab ab
F2 AB/ab : ab/ab or 1:1

12. Theories of linkage
1)Sutton hypothesis of linkage
2)Sutton – Boveri chromosome theory of heredity
3)Batson and Punnet’s coupling and repulsion hypothesis(1905-08)
4)Morgan’s theory
5)Chromosomal theory of linkage

13. a) Walter Sutton and (b) Theodor Boveri are credited with developing the
Chromosomal Theory of Inheritance, which states that chromosomes carry
the unit of heredity (genes). Thomas Hunt Morgan, who studied fruit flies,
provided the first strong confirmation of the chromosome theory.

14. Sutton hypothesis of linkage(1903)
Walter Sutton, in full Walter Stanborough Sutton, also
called Walter S. Sutton, (born 1877, Utica, New York, U.S.—died
November 10, 1916, Kansas City, Kansas), U.S. geneticist who
provided the first conclusive evidence that chromosomes carry the
units of inheritance and occur in distinct pairs.
The paper ended with the hypothesis that chromosomes carry the
units of inheritance and that their behaviour during division of the
chromosomes of sex cells (meiosis) is the physical basis of the
Mendelian law of heredity. Sutton developed this hypothesis in
“The Chromosomes in Heredity” (1903) and concluded that
chromosomes contain hereditary units and that their behaviour
during meiosis is random. His work formed the basis for the
chromosomal theory of heredity.

17. Yellow Round Green Wrinkle

18. 2)Sutton – Boveri chromosome theory of heredity(1902)
•Boveri and
Sutton's chromosome theory
of inheritance states that
genes are found at specific
locations on chromosomes,
and that the behavior of
chromosomes during meiosis
can explain Mendel’s laws of
inheritance.

19. 3) Batson and Punnet’s coupling and repulsion hypothesis(1905-08)
Bateson and Punnett in 1906,
described a cross in sweat pea,
where failure of gene pairs to
assort independently was
exhibited. Plants of a sweat pea
variety having blue flower (BB)
and long pollen (LL) were
crossed with those of another
variety having red flower (bb)
and round pollen (II).
F1 individuals (BbLl) had blue
flower and long pollen.
These were test crossed with
plants having red flower and
round pollen (bbll).

21. 4) Morgan’s theory
Morgan discovered a mutation that affected fly eye color. He observed that the mutation was inherited
differently by male and female flies. Based on the inheritance pattern, Morgan concluded that the eye color
gene must be located on the X chromosome.

23. TYPES OF LINKAGE
Complete Linkage (Morgan, 1919):
The genes located on the same chromosome do not separate and are inherited together over the
generations due to the absence of crossing over. Complete linkage allows the combination of parental
traits to be inherited as such. It is rare but has been reported in male Drosophila and some other
heterogametic organisms.
Incomplete Linkage:
Genes present in the same chromosome have a tendency to separate due to crossing over and hence
produce recombinant progeny besides the parental type. The number of recombinant individuals is
usually less than the number expected in independent assortment. In independent assortment all the
four types (two parental types and two recombinant types) are each 25%. In case of linkage, each of
the two parental types is more than 25% while each of the recombinant types is less than 25%.

24. Example of Complete linkage

25. Example of Incomplete Linkage