Heterosis, also known as hybrid vigor, refers to the superiority of an F1 hybrid over its parents in terms of traits like yield, vigor, and resistance to diseases. There are two main hypotheses for the genetic basis of heterosis: the dominance hypothesis, which attributes heterosis to the masking of deleterious recessive alleles in hybrids, and the overdominance hypothesis, which posits that heterozygosity at certain gene loci leads to greater vigor. Heterosis can be classified by type, such as individual, maternal, or paternal heterosis, and by origin as either true (eu) heterosis resulting from mutational or balanced genetic combinations, or pseudoheterosis involving only vegetative superiority. Factors like

2. By
Ghulam Shabir
1st Semester MSc(Hons.) Agri.
Department of Plant Breeding and Genetics
Ghazi University Dera Ghazi Khan

3. Heterosis
 Heterosis refers to the superiority of F1 hybrid
in one or more characters over it's parents.

 The term hybrid vigour is used as synonym for
heterosis.

 The term heterosis was first used by shull in
1914.



4. Hybrid vigour:
According to shull the developed
superiority of the hybrid is the hybrid vigour.
Heterosis :
The mechanism by which superiority is
developed.

5. Superiority over parents
Heterosis lead to superiority in
adaptation, yeild, quality, disease resistance, maturity, and general
vigour over its parents .
Generally positive heterosis is considered as desirable but in some
cases negative heterosis is also desirable.

6.  Superiority of over parents
 Confined to F1
 Genetic control
 Reproductive
 Association with specific combining ability
 Effect of heterozygosity
 Conceals recessive gene
 Low frequency

7. 1. Increase yield
ncrease in yield which may be measured in terms of
grain, fruit, seed, leaf, tuber or the whole plant is one of the
most important manifestations of heterosis
2. Increase in Size and General Vigor:
Heterosis results in more vigorous growth which
ultimately leads to healthier and faster growing plants with
increase in size than the parents.
3. Better Quality:
In many cases heterosis yields better quality which
may be accompanied with higher yield.

8. Manifestations of heterosis
4. Greater Adaptability:
Hybrids are generally more adapted to environmental
changes than the inbred lines due to heterozygosity.
5. More Disease Resistant:
Heterosis sometimes results into development of
more disease resistant character in the hybrids.
6. Increased Reproductive Ability:
Hybrids exhibit heterosis by expressing high fertility
rate or reproductive ability, which is ultimately expressed in yield
character.

9. Manifestations of heterosis
7. Increase in Growth Rate:
In many cases the hybrids show faster growth rate
than the parents, but that does not always produce larger plant size
than the parents.
8. Early Flowering and Maturity:
In many cases the hybrids may show early-ness in
flowering and maturity than the parents, for some crops these are
the desirable characters for crop improvement. All these
manifestations of heterosis can be traced at all levels of hybrid plant
organization.

10. Manifestations of heterosis
Molecular Level:
Heterosis is manifested in increased rate of DNA
reduplication, transcription and translation influencing the
formation of genetic information, enzymatic activity, other
regulatory mechanisms and also hybrid protein molecule formation.
Functional Level:
Heterosis is expressed as an effective regulation in
metabolic processes and morphogenesis in hybrid organism.
Cellular Level:
Due to change in electro-kinetic properties of
hybrid cell nuclei, the heterosis is manifested by increased mitosis.

11. • The genetical basis of heterosis is still following two hypotheses:
1) Dominance hypothesis of heterosis
2) Over dominance hypothesis of heterosis.

12. A: Dominance hypothesis of heterosis
 Holds that increased vigour and size in a hybrid is due to combination of
favourablegrowth genes by crossing two inbred races.
 In other words, the hybrid vigour is a result of action and interaction of
dominant or fitness factors or cumulative (polygenic) effect of dominant
genes.
Dominance Hypothesis has Assumptions
(a) Dominant genes are beneficial and recessive genes aredeleterious.
(b) The loci show addition effects, non-allelic interactions are absent.
(c) No recombination barrier between the genes.

13. Example of Dominance hypothesis of heterosis
 In a cross between Inbred A (AAbbCCdd) with Inbred B (AAbbCCdd),
there will be no heterosis in F1 hybrid, there is no masking of recessive
gene in hybrid. But in another cross, Inbred A (AAbbCCdd) is crossed
with Inbred D (aaBBccDD), where the F1 hybrid is (AaBbCcDd) with
all the genes having dominant allele.
 As a result the harmful effects of a, b, c, d are hidden by the dominant
alleles A, B, C and D. Thus, some parents produce heterotic progeny
while others do not.
 Generally, parents of diverse or different origin are more likely to
produce heterotic progeny than those of similar origin.

14. Example of Dominance hypothesis of heterosis

15. Objections of Dominance hypothesis of heterosis
1. Failure in Isolation of Inbreds as Vigorous as Hybrids:
According to dominance hypothesis it should be possible to get the inbred line with all
the dominant genes. Such inbreds should be as vigorous as the F1 hybrids, but such
inbreds have not been isolated.
2. Symmetrical Distribution in F2:
According to dominance hypothesis, the quantitative characters should not show
symmetrical distribution as because dominant and recessive alleles should segregate in
the proportion of 3: 1, but generally the F2 shows symmetrical distribution.
Above two objections can be explained by linked genes. Many of the quantitative
characters are governed by linked genes together, so to get the inbred line with all domi-
nant genes require several precisely placed crossovers. In another explanation it can be
showed that if the number of genes governing the quantitative characters is large, sym-
metrical distribution would be obtained even without linkage.

16. B: Over dominance hypothesis of heterosis
considered that there is a physiological stimulus to
development that increases with the diversity of the
uniting gametes.
The over dominance hypothesis is variouslyknown as
single gene heterosis, cumulative action of
divergentalleles, or stimulation of divergent alleles. Fisher
(1930) called it superdominance.

17. Over dominance hypothesis of
heterosis
Example:

18. Objections of Over Dominance Hypothesis of
heterosis
1. There are many examples where the superiority is due to the
epistatic affect of several non-allelic genes, not due to over-
dominance (which is the interaction between allelic genes).
2. There is another objection against over-dominance hypothesis
that there are many examples where the homozygotes are superior
to the heterozygotes.

19. Types of heterosis
There are three main types of heterosis:
1. Individual heterosis:
Heterosis is retained in the breeding of crossbred animal and is
related to the probability of alleles from different parental lines
joining together.
2. Maternal heterosis:
The offspring of a F1 female will benefit from maternal
heterosis
3. Paternal heterosis:

20. Types of heterosis
A. On the basis of origin and nature have 2 types:
1. Euheterosis or true heterosis
a) Mutational heterosis
b)Balanced heterosis
2. Pseudoheterosis:
B. On the basis of types of estimation :
1: Average or Relative heterosis
2: Heterobeltiosis
3: Useful or standard or Economic heterosis

21. Euheterosis or true heterosis
a) Mutational Heterosis
Lethal (mostly), recessive, adaptively unfavorable mutants are
either eliminated or sheltered by their non-lethal, dominant and
adaptively superior alleles in cross pollinated crops. This is
termed as mutational heterosis
b) Balanced Heterosis
Well balanced gene combinations which are more adaptive to
environmental conditions and useful from the agriculture point
of view result in balanced heterosis

22. Pseudoheterosis
Also termed as luxuriance. Progeny possess superiority
over parents is in vegetative growth, but not in yield and
adaptation, usually sterile or poorly fertile. This concept
cannot be utilized in hybrid varieties production.

23. Types of heterosis On the basis of
types of estimation
a): Average or Relative Heterosis:
When heterosis is estimated over mid parental value i.e.
average of two parents it is referred as average or relative heterosis.
b):Heterobeltiosis
When heterosis is estimated over better parent it is
called as heterobeltiosi,
c): Standard Heterosis
When heterosis is estimated over standard commercial hybrid it
is called as standard heterosis. It has practical importance in
plant breeding. It is also referred as useful or economicheterosis.

24. Factos Affecting Heterosis
1) Geographical and Genetic Diversity:
2) Agronomic Performance:
3) Adaptability:
4) Genetic Base:

25. Factos Affecting Heterosis
1) Geographical and Genetic Diversity:
In upland cotton, a close relationship is observed
between the genetic diversity of parental varieties and
performance of their hybrids for lint yield. In intra and
interspecific hybrids of cotton, the highest heterosis is
observed in the cross combinations involving ecologically
distant parents.High heterosis is observed in crosses
involving local x exotic lines.

26. Factos Affecting Heterosis
2) Agronomic Performance:
High heterosis can be obtained from the crosses of
two low yielded inbreds but absolute yield of such hybrids is
lower than the adapted varieties. To produce good hybrids,
varieties with high per se performance must be chosen.
There is a high positive correlation between parental
performance and hybrid vigour. The performance of any
trait in a hybrid is dependent upon the relative performance
of its parental varieties.

27. Factos Affecting Heterosis
3) Adaptability:
A close association is observed between the
adaptability the hybrids and their parents. In India, several
hybrids have been developed at intra and intersepcific levels
especially in tetraploid cotton. Highly heterotic
combinations involves at least one of the parents with wider
adaptability.

28. Factos Affecting Heterosis
4) Genetic Base:
Genetic base of parental lines is found to play important
role in the manifestation of heterosis in their hybrids as
there is a close relationship between the genetic base and
adaptability of varieties. Hybrids with high levels of
heterosis involves at least one of the parents with broad
genetic base.

29.  Increasing yield and nutritive value of crop plants
 Increasing the growth rate
 Production of new combination of colours and scents in
 garden flowers
 Producing particular characteristics in breeds
 Producing pest resistant and disease resistant varieties of
plants.
 Producing plants that tolerate wide ranges of
environmental changes

30. Heterosis or hybrid vigour have been commercially utilised in both cross
pollinated and in some self-pollinated species. In most of the cases the
utilisation of this heterosis phenomenon is not successful because of
difficulty in production of large quantities of hybrid seeds. This is
particularly difficult in self-pollinated species.