This document discusses the concept of heterosis, also known as hybrid vigor. It defines heterosis as the superiority of F1 hybrids over their parents in traits like yield, vigor and adaptation. The document then discusses the history of heterosis research and different hypotheses for the genetic basis of heterosis, including dominance, overdominance and epistasis. It also covers types of heterosis estimates and how heterosis is manifested. Factors affecting heterosis and various methods for heterosis breeding in crops are outlined.

1. HETEROSIS
Dr. NAVEENKUMAR K.L
Assistant Professor
Dept. of GPB
UAHS Shivamogga

2. HETEROSIS
Heterosis- refers to superiority of F1 hybrids in one or
more characters over its parents.
The term hybrid vigour is used as synonym for heterosis.
Heterosis differs from luxuriance.
Heterosis- as increase of F1 over parents in general
vigour, yield and adaptation.
Luxuriance -as increase of F1 over parents in vegetative
growth but not in yield and adaptation.

3. • Hybrid vigour:
According to Shull
The developed superiority of the hybrids is the ‘hybrid
vigour.
• Heterosis:
The mechanism by which this superiority is developed

4. HISTORICAL ASPECTS OF HETEROSIS
Heterosis concepts (Pre – Mendelian)
 1766 Kolreuter – Hybrid vigour in Nicotiana
 1799 A. Knight – Principle of anti-inbreeding
 1865 Mendel – Hybrid vigour in peas
 1877 Darwin – Cross fertilization is beneficial
 1880 Beal – First published report of 51% increase in
yield over parents.
 1891 Johnson – Crossing gave better off-springs.
 1892 Mc Cleur – Inbreeding imparted sterility crossing
imparted vigour.

5. History of Heterosis Concept
 1914 Shull – Coined the term heterosis
 1918 Jones – Practical utilization of heterosis.
 Jones 1918: Double cross.
 France Lamarck 1800s Correns, De vries 1900 Holland
Von Tshermack Mendel Newyork.
 Heterozygosis (East & Hayes, 1912): Crossing produces
heterozygosis,
 Selfing leads to Homozygosis.
 Hybrid Vigour (Jones, 1918): Synonymous to Shull’s
Heterosis

6. Heterobeltiosis (Blitzer, 1968): Increased
performance of hybrid over BP
Euheterosis (Dobzhansky, 1950): Hybrids possessed
higher fitness than their parents
 Luxuriance (Dobzhansky, 1950): Extreme heterosis
for morpho- characters, but no fitness
Positive and Negative heterosis (Powers, 1944): To
include inferiority of hybrid over parents Adaptive
Heterosis (MacKay, 1976): Heterosis for adaptability

7. Types of heterosis
A. On the basis of origin and nature have 2 types:
1. Euheterosis or true heterosis : superiority of F1 hybrids
over parents.
2. Pseudoheterosis/ luxuriance is an overexpression or
quicker growth of characters in a variety or species hybrid
which may not result from either masking of deleterious
genes or from balanced gene combinations brought together
in the hybrid.
 Therefore, luxuriance is a temporary and excessive
vegetative overgrowth which has no adaptive significance.
:

8. B. On the basis of types of estimation :
1. Average or Relative heterosis
2. Heterobeltiosis
3. Useful or standard or Economic heterosis

9. Estimation of Heterosis
1. Average heterosis: It is the heterosis where F1 is superior to mid
parent value. In other words superior to average of two parents.
Average Heterosis= [(F1 – MP)/MP] x 100
Where, F1 = Mean of hybrid
MP = Mid parental value.
(P1 + P2) where P1 = Parent 1 and P2 = Parent 2
This type of heterosis is of no use in agriculture since the
superiority is below the better parent value

10. 2. Heterobeltiosis:
When the heterosis is over the superior or better parent or Superiority of F1 over
the better parent.
Heterobeltiosis= [(F1 – BP)/BP] x 100
Where, BP = Mean value (over replications) of the Better Parent of the particular
cross
3. Economic Heterosis/ Useful Heterosis:
Superiority of the F1 compared to the high yielding commercial check variety in
a particular crop.
Useful Heterosis= [(F1-CC) / CC] x 100
Where, CC = Mean of Commercial check variety.
It is estimated in those crops where hybrids are already available for
comparison. This type of heterosis is of practical value in plant breeding.
4. Negative heterosis:
Performance of F1 inferior to better parent / mid parent value. - e.g. Duration.

11. Genetical Basis of Heterosis
• The genetical basis of heterosis is still following
THREE hypotheses:
A. DOMINANCE HYPOTHESIS.
B. OVER DOMINANCE HYPOTHESIS.
C. EPISTASIS HYPOTHESIS

12. A: Dominance hypothesis of heterosis.
 Heterosis is not due to heterozygosity per se Dominance
Hypothesis
 Proposed by Davenport, (1907), Bruce (1910) & Keeble
and Pellew (1910).
 Superiority of dominant alleles over recessive alleles.
 Heterosis is due to masking of deleterious recessive
alleles by dominant alleles.
 Heterosis is directly proportional to the number of
dominant genes contributed by each parent.

13. A: Dominance hypothesis of heterosis
Example;

14. Objections Dominance hypothesis of heterosis
1. Breeding homozygous individuals for all dominant genes in F2.
2. F2 Curve should be skewed towards dominant genes.
Explanations
1. Dominance of linked gene hypothesis (Jones, 1917):
There may be linkage between some favourable dominant genes and
some unfavourable recessive genes and as a result it is not possible to
obtain true breeding homozygous individual for all dominant genes in
F2 generation.
2. Smooth and symmetrical as yield is governed by polygenes (Collins
1921):
yield governed by large number of genes or polygenes which exhibit
continuous variation resulting in symmetrical distribution of genes.

15. B: Over dominance hypothesis of heterosis.
 Proposed by Shull and East in 1908.
 Supported by East (1936) & Hull (1945).
 This theory called by various names such as stimulation of
heterozygosis, cumulative action of divergent alleles, single
gene heterosis, super dominance and over dominance.
 According to this hypothesis “ heterosis is the result of
superiority of heterozygote over its both homozygous
parents.
Superiority of heterozygote may arise due
1. Production of superior hybrid substance in heterozygote
2. Cumulative action of divergent alleles

16. B: Over dominance hypothesis of heterosis.
Example:

17. C. EPISTASIS HYPOTHESIS
Epistasis refers to interaction between alleles of two or
more different loci.
Its also known as non-allelic interaction.
Non allelic interaction is of three types viz., additive X
additive, additive X dominance, dominance X
dominance.
Epistasis hypothesis Heterosis is mostly attributable to
favorable epistatic interaction between non-allelic
genes (Powell 1944, Williams 1959)

18. • Out of above three genetic hypothesis of heterosis the
dominance hypothesis is most widely accepted.
• Over-dominance and epistasis also operate in the
manifestation of heterosis.

19. Fixation of Heterosis
1. Asexual reproduction: Sugarcane, potato, sweet
potato, banana
2. Apomixis: citrus, black berries, roses, blue grasses,
3. Balanced Lethal System: evening primose (oenothera
spp)
4. Polyploidy: wheat -rye

20. Manifestation of heterosis May be in the
following form.
1. Increased yield.
2. Increased reproductive ability.
3. Increase in size and vigour.
4. Better quality
5. Greater adaptability

21. Factors affecting heterosis
1. Mode of pollination- High in cross pollinated species than self
pollinated species
2. Genetic diversity of parents- ex. In wheat heterosis is associated
with crosses of more distantly related parents.
Heterosis increased with genetic divergence in morphological
characters and flowering time, also with respect to geographical
origin of parents. Thus maximum heterosis occurs at an optimum or
intermediate level of parental diversity.
3. Genetic base of parents- High heterosis is associated with broad
genetic basis of the parents.
4. Adaptability of parents- heterosis is associated with wide
adaptability of the parents. Because there is close association
between adaptability and genetic base.

22. Heterosis Breeding
Development of hybrid varieties for genetic improvement of yield is
referred to as Heterosis Breeding.
In practical plant breeding, heterosis can be fully exploited in the
form of hybrids, and partially in the form of synthetics and
composite varieties.
Important factors in the commercial exploitation of heterosis
1. Enough magnitude of Heterosis
2. High percentage of outcrossing
3. Floral biology
4. Availability of MS and SI

23. Instead of using CGMS lines, Detasseling the female inbred line is
followed in India. Since use of CGMS line is costlier compared to
Detasseling it is not followed.
Crossing the inbreds of indigenous x exotic origin resulted in release of
best hybrids.
Indian x Indian - 24 to 43% yield increase.
Indian x U.S. dent 58 %
Indian dent x Caribbean Flint 47 to 54 %
1. Single cross hybrid – DMH1 (UAS, D), COH1 & 2 (TNAU)
2. Three way cross hybrids - Ganga -5
3. Double cross hybrids - COH 3
4. Double top cross hybrid - White kernel hybrids - Ganga safed 2,
Histarch, Ganga 4.

24. Single crosses
A single cross is a hybrid progeny from a cross between two
unrelated inbreds
A x B
F1 SCH
Double cross hybrids
A double cross hybrid is the hybrid progeny from a cross between
two single crosses.
(A x B) x (C x D)
Eg. Ganga 1, Ranjit, Deccan, Ganga 3, Ganga safed,

25.  The most striking advantage of single crosses over double and three way
crosses is that single cross breeding is simpler and faster (SCH - homogenous).
 The probability of identifying two inbred lines that combine well is more than
selecting for three or four inbreds.
 The expectation is that the best single cross has the highest yield, followed by the
best three way cross and the best double cross.
 Double and three way crosses selected on the basis of their predicted performance
using single cross data are developed and evaluated .
 DCH/TCH crosses require two stage testing whereas the evaluation of single
crosses is a one stage process.
 Prediction has its limitations and there are chances of missing some good
potential hybrids.
 The seed production cost is less in case of single hybrids. But the single cross
hybrids are less stable than other two types of hybrids.

26. Modified single crosses
A modified single cross is the hybrid progeny from a three way cross which utilizes
the progeny from two related inbreds as the seed parent and an unrelated inbred as
the pollen parent.
(A x A' ) x B
Three way crosses
A three way cross is the hybrid progeny from a cross between a single cross and an
inbred
(A x B) x C
Eg. Deccan 107, Deccan 109, Ganga 11, Ganga 5, Trishula
Double modified single crosses
A double modified single cross is the hybrid progeny from two single crosses, each
developed by crossing two related inbreds
(A X A') x (B x B')
Modified three way hybrids
A modified three way hybrid is the progeny of a single cross as female parent and
another single cross between two related inbreds
(A x B) x (C x C')

27. Non conventional hybrids:
Advocated by CIMMYT for countries lacking effective seed
industry
1. INTERVARIETAL HYBRIDS
Generated by crossing two varieties. Equivalent to synthetics
and composites
2. TOP CROSS HYBRIDS
Inbred line x variety hybrids. They can be – IL x Variety, IL x
Exp. Hybrid, IL x Synthetic variety and IL x Family
3. INTER FAMILY HYBRID
Progeny of crosses originating from same/ different population
4. DOUBLE TOP CROSS HYBRIDS
Progeny of SC and a variety. Commercially produced in India
and China