1.
HETEROSIS BREEDING
Dr. K. Vanangamudi
Formerly Dean (Agriculture),
Dean Adhiparasakthi Agricultural College,
Professor & Head,
Seed Science & Technology, TNAU, Coimbatore.
HETEROSIS BREEDING
 Koelreuter (1763) was the first to report hybrid vigour in the hybrids of tobacco,
Datura etc.
 Mendel (1865) observed this in pea crosses.
 Darwin (1876) also reported that inbreeding in plants results in deterioration of
vigour and the crossing in hybrid vigour.
 Beal (1877-1882) concluded that F1 hybrids yield as much as 40 percent more of
the parental varieties.
 G.H. ShuII (1914) proposed the term heterosis (heteros = different;
osis = condition) in maize
Heterosis or hybrid vigour or outbreeding enhancement
 Increased function of any biological quality in a hybrid offspring.
 Superiority of F1 hybrids over both parents in terms of yield and vigour
 Occurrence of a genetically superior offspring from mixing the genes of its
parents.
 Manifestation of an increase in vigour, size, growth rate, yield or some other
characteristic

2.
Inbreeding depression
 Reduced fitness and vigour with decreased heterozygosity as a result of breeding
of related individuals.
 Self and often cross pollinated crops show little or no loss in vigour and yield due
to inbreeding.
 Inbreeding depression is high and high hybrid vigour in Cross pollinated crops.
 Cross pollinated crops are best suited for hybrid development.
 Effects of inbreeding
 Appearance of lethal and sub lethal genes.
 Reduction in vigour: Appearance of dwarf plants.
 Reduction in reproductive ability - Less seed set, sterility.
 Segregation of population in distinct lines.
 Increase in homozygosity.
 Reduction in yield.
Types of heterosis

3.
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.
Balanced heterosis
 Well balanced gene combinations which are more adaptive to environmental
conditions and useful from the agriculture point of view.
 It has application in hybrid production.
Pseudoheterosis
 Progeny possess superiority over parents in vegetative growth, but not in yield
and adaptation, usually sterile or poorly fertile.
 This concept cannot be utilized in hybrid varieties production.
Average or relative heterosis
 Estimated over mid parental value i.e. average of two parents.
AV = [(F1 - MP) / MP] x 100
Where, F1 = Value of F1
MP = Mean value of two parents
Heterobeltiosis
 Estimated over better parent.
Heterobeltiosis =[F1 – BP] / BP x 100
Where, F1 = Value of F1,
BP = Value of better parent
Standard or economic heterosis
 Estimated over standard commercial hybrid.
Standard heterosis = [(F1 - SH)/ SH] x 100.
Where, F1 = Value of F1
SH = Value of standard hybrid
Genetic basis of heterosis
Dominance hypothesis
 First proposed by Davenport in 1908. It was later on expanded by Bruce and
coworkers.
 Superiority of hybrids to the suppression of undesirable (deleterious) recessive
alleles from one parent by dominant alleles from the other.
 Due to poor performance of inbred strains to the loss of genetic diversity, with
the strains becoming purely homozygous deleterious alleles at many loci.
Overdominance hypothesis
 This hypothesis was independently proposed by East and Shull in 1908.
 Also known as single gene heterosis or super dominance theory.
 According to this hypothesis, heterozygotes or at least some of the loci are
superior to both the homozygotes.
 Thus, heterozygote Aa would be superior to AA and aa

4.
Application of heterosis
 Increased yield: Measured in terms of grain, fruit, seed, leaf, tubers or the whole
plant.
 Increased reproductive ability: More number of flowers/fruits/seeds.
 Increase in size and general vigour: More vigorous, healthier and faster growing
and larger in size than their parents.
 Better quality: Shows improved quality. Hybrids in onion show better keeping
quality, than open-pollinated varieties.
 Earlier flowering and maturity: Earliness is highly desirable in many situations
particularly in vegetables.
 Greater resistance to disease and pest
 Greater adoptability: Hybrids are generally more adopted to environmental
changes than inbreds.
 Increase in the number of plant parts: An increase in the number of nodes,
leaves and other plant parts.
Factors affecting heterosis
 Mode of pollination: Magnitude of heterosis is generally higher in cross
pollinated species than in self-pollinated species.
 Genetic diversity of parents: In alfalfa and cotton, greater heterosis was
associated with greater parental diversity.
 Genetic base of parents: Higher heterosis is associated with broad genetic base of
the parents.
 Adaptability of parents: Heterosis is associated with wider adaptability of the
parents, because there is a close association between adaptability and genetic
base.
HYBRIDS
 Any offspring resulting from the mating of two distinct, dissimilar, homozygous
individuals
Attributes of F1 hybrids
 Maximum performance under optimal condition
 Stability of performance under stress
 Proprietary control of parents
 Often, reduced time to cultivar development
 Joint improvement of traits
Steps in hybrid breeding
 Development of inbred homozygous lines
 Evaluation and selection of productive inbred lines
 Production of hybrid seeds

5.
1. Development of inbreds
 Inbred: An inbred is a nearly homozygous line obtained through continuous
inbreeding (self pollination) of a cross pollinating species and followed by
selection.
 Procedure
 Isolation of lines: Superiors lines are isolated from open pollinated variety
population.
 Continues self fertilization of a cross-pollinated species
o Purpose of inbreeding is to fix the desirable characters in
homozygous condition in order to maintain them without any
genetic change.
o Inbreeding of an OPV leads to many deficiencies like
o Loss of vigour
o Reduction plant height
o Plants become susceptible to lodging, insects and pests and
many other undesirable characters appear.
 After each selfing, desirable plants are selected and self-pollinated or sib
pollinated.
 Repeat this steps for 6-7 generations to attain homozygosity.
 Then further, an inbred line can be maintained by selfing or sibbing.
In India, maize inbred lines are released through co-ordinated maize improvement
scheme
 CM (Co-ordinated maize)
 CM-100-199 - Yellow flint corn
 CM-200-299 - Yellow dent corn
 CM-300-399 - White flint corn
 CM-400-499 - White dent corn
 CM-500-599 - Yellow flint corn
 CM-600-699 - White dent corn
 Developed inbred line is crossed with other inbreds and its productiveness
in single and double cross combination is evaluated.
2. Evaluation of inbred lines
 Combining ability: Ability of an inbred to transmit desirable performance to its
hybrid progenies
o General combining ability (gca): Average performance of an inbred line
in a series of crosses with other inbred lines.
o gca is the characteristics of parents
o Specific combining ability (sca): Excessive performance of a cross over
and above the excepted performance based on gca of the parents.
o sca is characteristic of crosses or hybrids.

6.
 Inbreds are evaluated by
1. Phenotypic evaluation
o Based on phenotypic performance of inbreds themselves.
o Effective for characters, which are highly heritable i.e. high gca.
o Performance of inbreds is tested in replicated yield trials and the inbreds
showing poor performance are discarded.
2. Top cross test
o Inbreds, which are selected on phenotypic evaluation, are crossed to a
tester with wide genetic base
 E.g.. An OPV, a synthetic variety or a double cross.
o Plant alternate rows of the tester and the inbred line and the inbred line
has to be detasselled.
3. Single cross evaluation
o Outstanding single cross combinations can be identified only by testing
the performance of single cross.
o Remaining inbred lines after top cross test are crossed in diallel or line x
tester mating design to test for sca.
 Number of single crosses with reciprocals = n (n-1)
 Number of single crosses without reciprocals = n (n-1)/2
3. Production of hybrids
 A x B = F
Types of hybrids
 Inter varietal hybridization
o Crossing of two parents from the same species (Two varieties, strains or
races of same species)
o Eg. CSH 5 & 9, K Tall (Sorghum), JKHy – 1, Suguna (Cotton).
 Single cross hybrid
 Cross between two different homozygous lines to produce a F1 hybrid (F1
= Filial 1; meaning "first offspring").
 F1 is heterozygous having two alleles, contributed by each parent and one
is dominant and other recessive (MSms).

7.
A x B = F1
 Eg. Maize - COH1 (UMI 29 x UMI 51).
 Double cross hybrid is cross between two different F1 hybrids.
 (A x B) x (C x D) = Double cross
 Eg. Deccan maize (CM 104 x CM 105) x (CM 202 x CM 201)
 Three-way cross hybrid is cross between F1 hybrid and an inbred.
 (A x B) x C = Three-way hybrid
 Eg. Maize – Ganga (CM 202 x CM 111) x CM 500
 Triple cross hybrid is crossing of two different three-way cross hybrids.
 Top cross hybrid is cross between inbred line and OPV.

8.
Distant hybridization (Population hybrids)
 Inter generic hybridization is crossing between parents from the two different
genera
 Triticale – Wheat x Rye
 (Triticum aestivm x Secale cereal)
 Inter specific hybridization is crossing between parents from entirely two
different species
 Eg. Cotton - Varalakshmi
 Laxmi x SB 289 E
 (G. hirsutum) x (G. barbadense)
 DCH 32 or Jayalakshmi
 D.S. 28 x SB (YF) 425
 (G. hirsutum) x (G. barbadense)
 Tomato (Pusa Red Pulp)
 L. esculentum x L. pimpinellifolium