Hello, everyone! I am Abhishek Singh, a passionate scholar in the field of genetics and plant breeding. With a profound love for plants and a curiosity about their genetic makeup, I embarked on a journey into the world of science and agriculture. Currently pursuing my studies in genetics and plant breeding, I am dedicated to unraveling the mysteries of plant genetics and contributing to the development of sustainable agricultural practices.

1. z
HETEROSIS AND
INBREEDING
DEPRESSION
ABHISHEK SINGH
DEPARTMET OF PLANT BREEDING AND GENETICS

2. z
INTRODUCTION
1. Population of cross pollinated and asexually species are highly
heterozygous and usually show severe reduction in fertility and vigor
when they are subjected to inbreeding.
2. Conversely, the hybridization between the unrelated
population/lines/varieties generally leads to increase the vigor and
fertility.
3. Therefore most of the cultivated varieties of such crops are either hybrids,
synthetic, composite or open pollinated varieties since they avoid
inbreeding and exploit hybrid vigor to variable extent.

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INBREEDING DEPRESSION
1. Mating between individuals related by descent or ancestry is termed as
inbreeding or consanguineous mating. Example: brother sister mating or
sib mating.
2. The highest degree of inbreeding is achieved in selfing.
3. Its chief effect leads to increase in homozygosity in the progeny.
4. The degree of inbreeding of an individual is expressed as inbreeding
coefficient (F).

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History
1. The Hindu society, perhaps, present the extreme example where marriages
between individuals related by ancestry.
2. The systematic observations on the effects on inbreeding began around
1700 A.D. when inbreeding became a common practice in cattle breeding .
3. In 1976 Darwin published his book and concluded that progeny produced
by self fertilization were weaker than those produced from outbreeding.
4. Shull 1908 consider a ground breaking which provide a basic idea related
to hybrid and inbreeding development that have influenced in the research
in this area.

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Effects of Inbreeding
 Inbreeding is generally accompanied with a general reduction in
vigor, reproductive capacity i.e., fertility size of various plant parts
and yields.
1. Appearance of lethal and sub-lethal alleles.
2. Reduced vigor
3. Reduced reproductive ability
4. Separation of the population into distinct lines
5. Increase in homozygosity
6. Reduction in yield

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EXPLANATION FOR DIFFERENTIAL
RESPONSE TO INBREEDING
 Cross-pollinated crops are highly heterozygous, and carry many lethal,
sub-lethal and other unfavourable recessive alleles, which are of little
immediate value to the species.
 The sum of effects of the unfavourable alleles maintained in a population
constitutes genetic load of the population. The deleterious alleles can arise
from mutational changes, including single nucleotide polymorphism (SNP),
In-Del (insertion-deletion of up to 50 nucleotides), transposon insertion
and/or imprecise excision (this produces addition of few bases at the
excision site), presence-absence variation (PAV), copy number variation
(CNV) and epigenetic changes.

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HETEROSIS
• The term heterosis was first used by Shull in 1914.
• Heterosis is sometimes called true heterosis or euheterosis to distinguish it from
luxuriance.
• Heterosis may be defined as the superiority of an F, hybrid over both its parents
in terms of yield or some other character.
• Generally, heterosis is manifested as an increase in vigour, size, growth rate, yield
or some other characteristic.
• But in some cases, the hybrid may be inferior to the weaker parent; this is also
regarded as heterosis.
• The expression of heterosis is governed by the nuclear gene. In some cases,
heterosis result due to interaction between nuclear and cytoplasm.

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• Heterosis has a positive association with the specific combining ability
(SCA) variance. The SCA is a measure of dominance variance and existence
of significant amount of dominance varience is essential for undertaking
the heterosis breeding program.
• The dominance effect are expected to be maximum in cross pollinated
species and minimum in self-pollinated species. For this reason, occurrence
of heterosis is more in cross pollinated.

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Heterosis and Hybrid Vigour
• Hybrid vigour is generally used as synonym of heterosis. It is generally
agreed that hybrid vigour describes only the superiority of hybrids over
their parents, while heterosis describes other situations as well.
• But a vast majority of cases of heterosis are cases of superiority of hybrids
over their parents. The few cases where F, hybrids are inferior to their
parents may also be regarded as cases of hybrid vigour in the negative
direction.

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Distant Heterosis and Luxuriance
 Expression of heterosis in crosses between two subspecies or between two species is
called distant heterosis. Although crosses between the two subspecies of Oryza
sativa, viz, indica and japonica, exhibit increased vegetative growth, actual grain yields
are low due to poor seed set resulting from hybrid sterility.

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HISTORY
Scientist Contributions
Shull 1914 Heterosis Term
Jones 1917 Double cross hybrid
Davis 1927 Top cross hybrid
Davenport Dominance theory
Bruce Mathematical model of heterosis
Jones & Collins Clear the objections of dominance theory of
heterosis
Fischer Overdominance theory
Hull Overdominance term
Goven Epistatic theory
Mather & Jinks Biometrical bases of heterosis
Power 1944 The term heterosis should be used only when
hybrid is either superior or inferior to both the
parents

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Manifestations of Heterosis
1. Increased Yield: Heterosis is generally expressed as increased yield of
grain, fruit, seed, leaf, tubers or biomass of hybrids.
2. Increased Reproductive Ability: Hybrids exhibiting heterosis show
increased fertility or reproductive ability.
3. Increased Size and General Vigour: Hybrids are generally more vigorous,
i.e., healthier and faster growing and larger in size, than their parents.
4. Better Quality: In many cases, hybrids show improved quality, which may
or may not be accompanied by higher yields.
5. Earlier Flowering and Maturity: In many cases, hybrids are earlier in
flowering and maturity than their parents.

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Some Significant Features of Heterosis
1. The extent of heterosis varies from one species to the other: it is the
highest in maize and the lowest in some self-pollinated crops species.
2. The magnitude of heterosis varies among different traits of the same
cross and among different crosses of the same crop. This would suggest
that heterosis is not the result of a single locus, overall heterozygosity or
some other 'unifying, systems-wide mechanism’.
3. In a heterotic cross, neither all the traits are affected nor the levels of
heterosis for the affected traits show ay relationship. Thus, the magnitude
of heterosis is trait specific.

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Heterosis Index
 A ratio that either indicates the presence of heterosis or estimates the
magnitude of heterosis is known as heterosis index. Several different
indices have been proposed to detect heterosis and/or estimate the extent
of heterosis in single cross hybrids, viz., potence ratio (Hpr), better parent
heterosis index (Hp), worse parent heterosis index (Hw) and mid-parent
heterosis (HMp).

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GENETIC BASIS OF HETEROSIS AND
INBREEDING DEPRESSION
Dominance Hypothesis:
• The theory was proposed by Davenport (1908), Bruce(1910) and Keeble
and pellew(1910).
• According to this theory, heterosis is the result of the superiority of
dominant alleles when recessive alleles are deleterious.
• The deleterious recessive genes of one parent are hidden by the dominant
genes of another parent and the hybrid exhibit heterosis.

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Objections
1. It should be possible to isolate completely homozygous inbred lines
carrying all the dominant alleles which would be as vigorous as F1 hybrid;
such lines couldn’t be developed
Jones 1917 – He suggested that since quantitative characters are governed by
many genes, these genes may linked with the deleterious recessive alleles;
hence all the dominant alleles in single line are difficult to isolate since since it
would require several precisely crossover.

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2. In F2, dominant and recessive characters segregate in the ratio of 3:1.
Hence the distribution should have been skewed; but in practical normal or
symmetrical distribution was invariably observed.
Collins – He suggested that the occurrence of normal distribution in F2 is
because large number of genes governing the character coupled with the
genotype x environment on the expression of phenotype.

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3. The per se performance of inbred lines has been considerably improved
over the decades. If dominance were the cause of heterosis, the magnitude of
heterosis of such generation should have declined.
In contrast the level increases – possibly due to the selection of right set of
loci that combine to enhance the level of heterosis in F1 hybrids.
4. Faster inbreeding depression in tetraploids rather than diploids.

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Overdominance Hypothesis:
• According to this theory; heterozygotes atleast in some of the loci are
superior to both the relevant homozygotes.
• The Aa would be superior to both the homozygotes AA and aa.
• Heterozygotes for the divergent alleles (A1A4) will be more heterotic than
those of less divergent (A1A2).

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Evidences of Overdominance
 The evidences shows that the concept of pseudooverdominance i.e.,
overdominance mimicked by the repulsion phase linkage between two
dominant genes.
 The superiority of these heterozygotes need not be the result of
overdominance; it could be due to linkage in the repulsion phase or
epistatic effects.

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COMPARSION
DOMINANCE HYPOTHESIS OVERDOMINANCE HYPOTHESIS
Heterosis due to masking of
harmful effects of recessive
alleles by their dominant
alleles.
Heterosis due to heterozygosity
itself
Inbreds as vigorous can be
isolated
Cannot be isolated
Phenotype similar to
dominant homozygotes
Phenotype superior to both the
homozygotes

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Epistasis Hypothesis:
• The influence of one locus on the expression of another locus may be
involved in heterosis when some conditions are involved.
• The first condition is the epistasis should be predominantly of
complementary type i.e., the estimates of h (dominance effect) and l
(dominance x dominance) have the same sign so that they don’t cancel
each other.
• The second condition is the interaction of pair of genes should be
dispersed in both parents.

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ESTIMATION OF HETEROSIS
Average heterosis or Relative heterosis: The superiority of F1 is estimated
over the average of the two parents or mid parent.
Average heterosis = [(F1-MP)/MP] x 100
Heterobeltosis: When the heterosis is estimated over the better parent. It is
worked out as follows.
Heterobeltosis = [(F1-BP)/BP] x 100

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Useful heterosis: The term useful heterosis was used by Meredith and
Bridge (1927), and it is refer to the superiority of F1 over the standard
commercial check variety. It is also called as the economic heterosis.
Useful heterosis = [(F1-CC)/CC] x 100

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BIOMETRICAL GENETIC EXPLANATION
FOR HETEROSIS
• According to the additive-dominance model the heterosis results when sum of
dominance effects at all the k loci i.e., ∑h is positive and greater than the sum of
additive effect at these loci i.e., ∑d.
• The extent of heterosis will be reduced by bidirectional dominance i.e., some
dominant alleles have positive, while other have negative effect on the trait, since
this will lead to cancellation of the dominance effects of at least some of the
genes.
• Heterosis will be maximum with the unidirectional dominance fir the genes and
the dispersion (rd) among the parents is zero which means both the parents
contains equal number genes but different dominant alleles affecting the traits.

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Heterosis can be observed when
1. Unidirectional dominance with rd < 1
2. Existence of over dominance ∑h > ∑d
The cause of heterosis from the above two phenomena can be estimated by
dominance ratio.
Dominance ratio = √H/D

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HETEROSIS AT POPULATION LEVEL
(Labaroo et al.)
At population level heterosis may be partitioned into
1. Panmictic-mid parent heterosis
2. Inbred mid parent heterosis
3. Baseline heterosis

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1. Panmictic-mid parent: A base population is divided into two or more
subpopulation, there would be some amount of inbreeding in the two
subpopulation as compared to base population. When these subpopulation are
random mated the resulting F1 population exhibit panmictic mid parent
heterosis.
2. Inbred mid parent: The difference between the performance of recreated base
population and average performance of all the inbred.
3. Baseline heterosis: The difference between the performance of a subpopulation
and the average performance of all the inbred derived from that subpopulation.

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The arrows denotes the crossing and the line denotes the selfing

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PHYSIOLOGICAL BASIS OF
HETEROSIS
 Net assimilation rate: The increased photosynthesis efficiency associated with
heterosis is a result of higher CO2 exchange rates and elevated nitrogen content
in the tissue.
 Leaf area index (LAI): The crop hybrid demonstrate significant advantages over
its parent during the early growth phase, producing a greater leaf area per square
meter.
 Root Growth: Root growth hydrolysis is directly influenced by the shoot growth
pathway. The roots act as a sink until the economic yield, triggering increased
photosynthetic production in hybrids with high leaf area and variant indices,
potentially resulting in longer root systems.

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 Hormone Balance: Heterosis may consequence of a superior hormone
balance in hybrid as compared to those their parents.
 Metabolic concept: In breeding an unbalanced metabolic system may
occur where specific enzymes are present in the limiting concentrations.
Hybrids can complement these limitations obey time leading to a
contrasting beneficial cross.

32. z
MOLECULAR BASIS OF HETEROSIS
 Intermediate amount of single gene product: Neurospora crassa gene pab
responsible for production of p-aminobenzoic acid. The heterozygote produces
an intermediate amount of gene product, which may lead to an increased vigour
and size.
 Separate gene product: Sickle cell anaemia in humans; where herterozygotes Ss
is more resistant to malaria than normal homozygote.
 Combined gene production: The dominant or recessive homozygotes product
protein which are made of two identical polypeptide homodimer or
homopolymer; whereas protein produced from heterozygotes alleles of the gene
will combine to produce a protein molecules with different properties known as
hybrid substance or hybrid product.

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 Effects in two different tissues: In maize AdhF produces higher activity of the
enzyme alcohol dehydrogenase in scutellar tissues of seeds; while the other
allele AdhS produces higher activity in pollen grains. The hybrid exhibits
intermediate expression of the enzymes in both the tissues.
 Expression of a greater number of genes: The level of expression in certain
loci is greater in the hybrid than their parents. There is considerably higher
number of genes expressed in hybrid than their parents particularly in heterotic
hybrid. It may be due to the following causes:
• DNA Methylation
• Regulatory gene
• Indels

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 Epigenetic Changes: It is the reversible alternation of DNA and histone
that affects the gene expression without change in the base sequence.
DNA modification includes DNA methylation / acetylation. It seems that
selfing leads to a gradual increase in DNA methylation, thus DNA
methylation could lead decreased gene expression in inbreds and
increased gene expression in hybrids.

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FIXATION OF HETEROSIS
1. Use of a technique to ensure that the heterotic gene combination present
in heterotic hybrid remains in the subsequent generations constitute
fixation of heterosis.
2. It can be done through the following methods:
 Vegetative Propagation
 Apomixis
 Balance lethal system