Heterosis breeding Heterosis or hybrid vigour or outbreeding enhancement Types of heterosis Genetic basis of heterosis HYBRIDS Development of inbreds Combining ability Types of hybrids Single cross hybrid Double cross hybrid Triple cross hybrid Top cross hybrid

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