chromosomal theory of inheritance, linkage and crossing over

This slideshare contains all the necessary details to learn the above topics. it mainly talks about how linkage and crossing over occurs ad there effect with example and types. illustrations and crosses are also included for better understanding of the topic.

1. CHROMOSOMAL
THEORYOF
INHERITANCE,LINKAGE,
CROSSOVER
by DEENA.N
by DEENA.N

2. Linkage
Genes on different chromosomes assort independently (Mendel’s
Law)
But some genes on the same chromosome are inherited together
This phenomenon is called linkage and such genes are called
syntenic genes
LINKAGE – it is the tendency of genes located on the same
chromosome to be inherited together during meiosis
Linkage helps preserve parental combinations in offspring
Common in both plants and animals

3. Suttons theory (1903)
Proposed by Walter Sutton
He proposed the Chromosomal Theory of Inheritance.
According to this theory chromosomes are the physical
carriers of heredity.
He stated that each chromosome carries many genes, so
genes on the same chromosomes are usually inherited
together – a phenomenon known as linkage.
He also explained that genes on one chromosome tend to
be transmitted as a group rather than assorting
independently.

4. Features of Linkage
Involves two or more genes on the same chromosome
May include dominant, recessive, or both types of genes
Usually involves closely located genes
Produces more parental types than recombinants
Linkage can be broken by crossing over
Linkage causes a higher frequency of parental type than
recombinants in crosses

5. Linkage Groups
a linkage group is the set of genes located on one chromosome
Each homologous chromosome pair forms one linkage group
Genes close together are strongly linked; distant ones show weak linkage
Linkage can be broken by crossing over during meiosis

6. Significance of linkage
Linkage ensures genes are inherited together, maintaining specific traits
Reduces recombination frequency, preserving parental genotypes
Maintain genetic consistency across generations
Linkage provides strong evidence in support of the chromosome theory of heredity and shows that
genes are located on chromosome in linear order

7. Coupling and Repulsion Theory
By Bateson and Punnet (1905).
GENETICCOUPLING– it is the situation in which one parent introduces the dominant alleles
(AB) of both the genes, and another parent introduces their recessive allele (ab).
GENETIC REPULSION – in this linkage relationship the dominant allele of one gene and the
recessive allele of the other occupy the same chromosome (Ab and aB). Each parent brings in the
dominant allele of one gene and the recessive allele of another

8. MORGAN’S CHROMOSOME THEORY OF LINKAGE
By T.H Morgan
According to this theory, he explained that coupling and repulsion not as two different events but
as the two aspects of one and same phenomenon called linkage
Main postulates include :-
a)genes on the same chromosome, remains together during inheritance and transmitted as single
units, this is called linkage
b) the genes which exhibit linkage are always located on the same chromosome
c) linked genes always have linear arrangements
d) the strength of linkage is inversely related to the distance between the genes on the
chromosomes
e) linked genes will remain in their parental combinations

9. Types of linkage
(1) based on the chromosome involved :- can be of two autosomal linkage and
chromosomal linkage. The former occurs between the genes located on
autosomes and the latter occur between the genes in sex chromosome.
(2) based on crossing over :- it can be of 2 types, complete and incomplete or
partial linkage. This classification is based on the presence or absence of
crossing over and the new non-parental combination of linked genes.

10. there are two
types of linkage
•complete linkage
•incomplete linkage

11. Complete Linkage
 this is the linkage in which the linked genes are always transmitted orderly and regularly as a
single unit over several generations, without affecting their linear sequence and parental
combination.
The genes will not, in any way, enter into new non-parental recombination, since crossing over and
independent assortment are totally absent

12. EXAMPLE OF COMPLETE LINKAGE
In Drosophila, the genes for bent wings (bt) and shaven abdominal bristles (svn)
are located on chromosome 4. The wild type alleles (bt+,svn+) produce long wings
and long bristles, while the mutant alleles (bt,svn) produce bent wings and shaven
bristles. crossing a true breeding double mutant fly (bt,svn) with a true-breeding
double dominant fly (bt+,svn+) yields F1 heterozygotes with normal phenotype. a
test cross of these F1 flies with bent shaven, double recessive mutants produces
progeny showing only the parental phenotypes- normal and bent shaven flies-
without any recombinant types and doesn’t exhibit the 1∶1∶1∶1. The absence of
non-parental phenotypes indicates absence of crossing over and independent
assortment, confirming that the genes are completely linked.

14. Incomplete or Partial Linkage
This is the linkage in which the linked genes will not always stay together.
They will separate out and enter into new non-parental combinations.
Crossing over breaks linkage and brings gene shuffling which alters the linear sequence of genes
and produces new non-parental phenotypes in addition to the parental ones.
This non parental phenotypes produced are called recombinant phenotypes
The genes involved are called incompletely linked genes and they are wide apart from each other
which maximises the chance of crossing over

15. EXAMPLE OF INCOMPLETE OR
PARTIAL LINKAGE
 In Drosophila, complete linkage is seen when the male parent is heterozygous and
when the male parent is homozygous, incomplete linkage is observed. Morgan
crossed two strains- one with red eyes and long wings (wild type) and another with
purple eyes and vestigial wings (double recessive). The F1 hybrids were red-eyed,
long winged heterozygotes. When an F1 male was crossed with a purple-vestigial
female, only parental types (red-long and purple- vestigial) appeared, showing
complete linkage due to the absence of crossing over.
however, when an F1 female was crossed with a purple-vestigial male, four
phenotypic classes appeared – red long, red vestigial, purple long and purple
vestigial in the ratio 8∶1∶1∶6. here [parental types were more frequent, and
recombinants were fewer indicating partial linkage. Thus, Morgan concluded that the
genes responsible for eye color and wing type lie on the same chromosome and
sometimes undergo c.o producing non-parental combinations

17. Crossing Over (Morgan and Cattell 1912)
Crossing over is the mutual exchange and reciprocal recombination of chromatid segments, DNA
fragments between the non-sister chromatids of homologous chromosome
It enables genes to enter into non-parental recombination
Crossing over involves the symmetrical breakage and reciprocal exchange and crosswise reunion
of segments between non-sister chromatids often breaking linkage
This results in the recombination of genes

18. Mitotic and Meiotic Crossing Over
Mitotic crossing over – occurs in somatic cells hence, it is also called somatic crossing over. It is a
rare phenomenon. It has been observed in Dipteran flies (eg:- Drosophila) and in the fungus
Aspergillus. It has no genetic significance, since it is not transmitted from parent to offspring
Meiotic crossing over - it is a common phenomenon in the reproductive cells of sexually
reproducing organisms. Exceptions are seen in male Drosophila and female silkmoth, where
crossing over has not been detected. It usually occurs during the prophase 1 of gametogenesis. It
is genetically significant because it produces heritable non-parental gene combinations.

19. PHYSICAL MECHANISM OF
CROSSING OVER
Meiotic crossing over is essentially a physical exchange of chromatid segments
between the non-sister chromatids of homologous chromosomes. It mainly
involves six major steps:-
 (a) Synapsis and bivalent formation-it is the close pairing of homologous
chromosomes during the zygotene stage of first meiotic division. The zygotene
pairs are generally called bivalents. The paired homologues are very closely
aligned by a proteinous cementing substance between them, called
synaptonemal complex
(b) Duplication of bivalents and tetrad formations - once synapsis is completed,
the bivalents undergo duplications. During this, the sister chromatids of each
homologue dissociate from each other, except at the centromere region. Now,
each bivalent appears to consist of four chromatids and hence it is called tetrad.

20. PHYSICAL MECHANISM OF CROSSING
OVER
(c) Crossing over and chiasma formation – crossing over is the overlapping of
non-sister chromatids. It occurs in the pachytene stage. During this, one
chromatid of each homologue overlaps around a non-sister chromatid of the
other homologue at one or more points. At some of these crossing points, they
establish intimate contact with each other. Such points of close contact are
called chiasmata
(d) breakage and reunion of chromatids – endonuclease causes breakage, and
ligase joins them crosswise. This results in two chromatid sets: one with
unaltered parental gene combinations (non-cross or non-recombinant
chromatids) and another wit altered gene combinations (crossover or
recombinant chromatids).

21. PHYSICAL MECHANISM OF CROSSING OVER
(e) disruption of linkage – symmetrical breakage of non-sister chromatids
disrupts linkage and the linear gene sequence. The reciprocal exchange and
crosswise reunion results in the formation of new non-parental gene
combinations, this process is called genetic recombination. Such gametes are
called recombinant or crossover gametes
(f) dissociation of homologues by chiasma terminalization – after chromatid
exchange, homologous chromosome begin to separate during diplonema due
to reduced attraction and dissolution of the synaptonemal complex. Detachment
proceeds from centromere to chiasma in a zipper like manner and the chiasma
moves toward the telomere until it disappears. This process is called chiasma
terminalization.

22. Significance of chiasmata in
recombination
Chiasmata plays a major role in recombination during meiosis.
Two major hypotheses were put forward to explain the relationship
Janssen’s hypothesis: chiasma formation leads to physical exchange of chromosomal material
Morgan’s hypothesis: physical exchange results in gene recombination.
Both indicate that chiasma formation and recombination are correlated, contributing to genetic
variation

23. Chiasma-type Theory
Proposed by Janssens; supported by Darlington
This is also called One-Plane Theory
According to this theory, chiasma is the visible manifestation of crossing ver
Crossing over occurs during the pachytene stage between non-sister chromatids
Frequency ratio of crossing over to chiasma formation is 1∶1
Symmetrical breakage and reunion of chromatids cause genetic recombination
However, chiasma may occur without recombination (eg.,in male Drosophila)

24. Kinds of Crossing Over
Meiotic crossing over usually involves two non-sister chromatids and one or
more chiasmata, but may also involve three or four chromatids and two or more
chiasmata
Number of chiasmata is proportional to chromosomal length and distance
between linked genes
Based on chiasmata number:
1)Single crossing over
2)Double crossing over
3)Multiple or compound crossing over

25. Single crossing over
1) SINGLE CROSSING OVER – involves two chromatids and one chiasma,
producing two recombinant and two parental chromatids. The resulting gametes
are single crossover gametes
It produces two recombinant chromatids and two parental chromatids.

26. 2) Double Crossing Over
It involves two chiasmata and usually two. Three, or four chromatids
Occurs when the distance between the linked genes are more
Based on the number of participating chromatids, double cross over can be of:
a)Two stranded double crossover – it is also called regressive crossover, involves the same pair of
chromatids in both crossing overs, producing two recombinant and two parental chromatids, and
the two chiasmata formed between the same chromatids are known as reciprocal chiasmata

27. ➢b) Three stranded crossing over – it is a type of double cross over, involving three chromatids with
one common to both chiasmata, producing t5here recombinants and one parental chromatid. It is
also called progressive crossing over.

28. (c) Four stranded double crossing over - double crossing over involving all four chromatids,
producing four recombinant chromatids and no parental chromatids; the two chiasmata formed by
different chromatid pairs are called complementary chiasmata. It is also known as degressive
crossing over
3) Multiple or Compound crossing over – in this case, more than two chiasmata are formed
between the same pair of homologous chromosomes. That is a very rare phenomenon

29. FACTORS INFLUENCING CROSSING
OVER
a.Distance between linked genes: crossover frequency increases with distance
b.Synaptonemal and telomere regions: crossover is minimal near these regions
c.Age: frequency decreases with advancing age
d.Sex: depends on the organism’s sex
e.Temperature: high temperature can increase crossover frequency
f.Irradiation: X-rays and gamma rays enhance crossover frequency

30. SIGNIFICANCE OF CROSSING OVER
➢Gene exchange – enables mutual gene exchange and reciprocal recombination
between homologous chromosomes.
➢New gene combination – reassorts linked parental genes, forming new genes
combinations, which results in the altering of gene frequency and increases
genetic variation
➢Source of variation – major contributor to genetic variation, providing raw
material for evolution
➢Gene mapping evidence: supports the linear arrangement of genes
➢Linkage maps: crossover frequency helps construct genetic/linkage maps

31. CROSSING OVER FREQUENCY
Recombination frequency is the probability of crossing over between linked
genes, indicating their order and relative distance on a chromosome. Crossing
over frequency is directly proportional to the distance between loci, so closely
linked genes show strong linkage and lower crossover frequency

32. CROSSOVER VALUE (COV)
Crossover value (COV) represents the percentage of recombinants in the progeny
of a given cross.

➢COV indicates both the number of crossover events on a chromosome and the
degree of linkage between genes
➢Tighlty linked genes have low COV, while loosely linked genes have high COV
➢COV may vary due to interference and environmental factors affecting chiasma
formation

33. INTERFERENCES AND COINCIDENCE
INTERFERENCE – it is the phenomenon in where the occurrence of one
crossover affects the formation of another crossover nearby. If the occurrence
of one crossover reduces the probability of a second one it is called positive
crossover, if it increases the chance it is called negative crossover. It explains
why crossover are not randomly distributed along the chromosome
TYPES OF INTERFERENCE
a)Chromosomal interference: in this one crossover ether reduces (positive) or
increases (negative) the chance of another crossover nearby. It is the most
common type. It is also known as chiasma position interference
b)Chromatid interference: involves unequal participation of chromatids. Positive
chromatids interference promotes four strand double crossover, while
negative interference promotes two-strand double crossover.
c)Interchromosomal interference: crossover on one chromosome can affect
crossover frequency on another chromosome, either enhancing or reducing it.

34. 2.COINCIDENCE- it refers to the simultaneous formation of two single crossovers between the
same homologous chromosome to form a double crossing over.
COEFFICIENT OF COINCIDENCE – it measures the ratio of observed double crossovers to
expected double crossovers. it is expressed as:
➢c.o.c = 1: no interference; crossover occur independently
➢c.o.c<1: positive interference; fewer double crossover occur than expected
➢c.o.c>1: negative interference; more double crossovers occurs than expected
➢if interference is completed, the value becomes zero

35. RECOMBINATION
Produces new combinations of genes or allele in offspring
Results in combinations different from either parent
Increases genetic variation in populations.
Plays a key role in evolution and heredity
Recombination can be of two types:
1.Homologous recombination - occurs between homologous chromosome
during meiosis (prophase 1). it is the one explained by crossing over and
chiasmata formation
2.Non-homologous recombination – occurs between non- homologous
chromosome or sequence with little similarity; it’s rarer and can cause
chromosomal rearrangements.
Recombination is mainly brought about by crossing over, the physical exchange of
chromosome segments at chiasmata

36. SIGNIFICANCE OF RECOMBINATION
➢ produces new genetic combinations, increasing population variability
➢helps in mapping genes on chromosomes
➢plays a key role in evolution and adaptation
➢maintains chromosomal integrity by repairing damaged DNA through
homologous recombination
Recombination is the outcome of crossing over, ensuring continuous reshuffling of
genes that drives heredity anf variation in sexually reproducing organsims

37. Reference
1.Klug - W.S and Cummings M.R (2003), Conceptes of Genetics 7
th
edn.
Pearson Education Inc.
2.Daniel L. Hartl (1997) Our Uncertain Heritage Genetics and Human Diversity.
Ken Burke and Associates
3.Gupta, P.K. (2017). Genetics: Classical to Modern. Rastogi Publications
4.Gardner, E.J. Simmons, M.J., and Snustad, D.P. (2008). Principles of
Genetics (8
th
ed.). Wiley India Pvt.Ltd
5.Karp, G. (2014). Cell and Molecular Biology: Concepts and Experiments (7
th

ed.). Wiley