Genetical and physiological basis of heterosis and inbreeding

1. Genetical and physiological basis of heterosis and
inbreeding
Dev Hingra
Ph.D Scholar
Mail Id- mail2devhingra@gmail.com
Department of Plant Breeding and Genetics
RAJASTHAN COLLEGE OFAGRICULTURE
MPUAT,UDAIPUR (RAJASTHAN)

2. INTRODUCTION
DEFINATION
GENETIC BASIS OF HETEROSIS &
INBREEDING
DOMINANCE HYPOTHESIS
OVER DOMINANCE HYPOTHESIS
PHYSISOLOGICAL BASIS OF HETEROSIS
AND INBREEDING
STAGES OF PHYSOLOGICAL HETEROSIS
INBREEDING DEPERSSION
COMMERCIAL APPLICATION

3. GENETICALAND PHYSIOLOGICAL
BASIS OF HETEROSIS AND
INBREEDING
INTRODUCTION
Development of Heterosis Concept
1907 Simpson – Rejuvenation by hybridization
1908 Davenport - Dominance theory
1908 East and Shull - Overdominance theory
1912 East and Hayes – Stimulus of heterozygosis
1914 Shull – Coined the term heterosis
1918 Jones – Practical utilization of heterosis

4. Definitions
Heterosis refers to the 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. The former refers to increase of F1 over parents
in general vigour, yield and adaptation, whereas latter refers
to increase of F1 over parents in vegetative growth but not
in
yield and adaptation .
The term heterosis was first used by shull in 1914.

5. Features of heterosis
Superiority over parents:- heterosis leads to superiority in
adaptation , yield ,quality, disease resistance, maturity and
general vigor over its parents.
Confined to F1:-heterosis confined only to the F1 generation of
a cross.it declines and disappears in F2and subsequent
generations of a cross as a consequence of segregation and
recombinations. Thus heterosis is related to F1generation only.
Genetic control:- the expression of heterosis is governed by
nuclear genes.
Reproducidle:-heterosis once identified can be easily reproduced
in a definite environment.
Effect of heterozygosity:-the magnitude of heterosis is
associated with heterozygosity,because the dominance
variance is associated with heterozygosity.

6. Heterosis and inbreeding depression are closely related
phenomena in fact they may be regarded as two opposite
sides of the same coin. The genetic theories that explain
heterosis also explain inbreeding depression. There are two
main theories to explain heterosis and consequently
inbreeding's depression:
Genetic basis of heterosis and inbreeding

7. Dominance hypothesis
The hypothesis, first proposed in 1908 by Davenport and later in
1910 supported independently by Bruce and by Keeble and
Pellew.
Heterosis due to superiority of dominant alleles over recessive
alleles. heterosis is directly proportional to the number of
dominant genes contributed by each parent.
AABBccdd aabbCCDD AaBbCcDd
parent 1 parent 2 hybrid with 4
with 2 dominant with 2 dominant dominant genes
genes genes

8. Dominance Hypothesis (Davenport, 1908)
1. Heterosis is not due to heterozygosity per se
2. Superiority of dominant alleles over recessive alleles
3. Heterosis is due to masking of deleterious recessive
alleles by dominant alleles
4. Heterosis is directly proportional to the number of dominant
genes contributed by each parent

9. Genetic Basis of Heterosis dominance hypothesis
Explanations
• Dominance of linked gene hypothesis (Jones, 1917)
• Smooth and symmetrical as yield is governed by
• polygene's (Collins, 1921)
Objections
• True breeding homozygous individuals for all dominant
genes in F 2
• F 2 Curve should be skewed towards dominant gene

10. Overdominance hypothesis
The overdominance hypothesis was proposed in 1908
by Shull and East, independently. It is assumed here that there is
a physiological stimulus to development that increases with the
diversity of the uniting gametes. In other words, using Mendelian
terms, it means that there are loci at which the heterozygote is
superior to either homozygote and that vigour increases in
proportion to the amount of heterozygosity. Various workers
have given different names to this hypothesis such as 'single-gene
heterosis','cumulative action of divergen alleles' and 'stimulations
of divergent alleles'. Fisher (1930) called it superdominance.

11. Genetic Basis of Heterosis
Overdominance Hypothesis (Shull & East, 1908)
1 Heterozygosity per se is the cause of Heterosis
2 Heterozygote is superior to better parent
3 Increase in vigour is directly proportional to diversity of
combining alleles
4 Superiority of heterozygote may arise due
● production of superior hybrid substance in heterozygote
● cumulative action of divergent alleles
5 More divergent alleles will exhibit more heterosis than less
divergent ones

12. dominance and overdominance hypothesis -
An illustration

13. Overdominance or single locus heterosis
Evidence
They are not many clear cut cases where the heterozygote is
superior to the two homozygote ; in fact over dominance has
not been demonstrated unequivocally for any polygomic
trait: in case in maize gene ma affects maturity. The
heterozygote's Ma ma is more vigorous and later in anthesis
and maturity than the homozygote's Ma Ma
Objections
Evidences of single locus heterosis may not hold good for
quantitative characters like yield.
Quantitative estimation of over dominance is difficult due to
linkage basis.

15. Physiological Basis of Heterosis and Inbreeding
1 Heterozygosity results from the greater initial weight of
the embryo
2 Hybrid vigour is nothing more than the maintenance of
initial advantage in embryo size
3 Hybrids do not differ from their parents in relative
growth rate

16. Stages of physiological manifestation of heterosis
1. Seed and Embryo development
*High positive correlation was recorded between hybrid vigour
and embryo weight
* Copland noted that hybrid embryo exhibited greater vigour in
the earlier stages of development
* They are found significant differences between embryo weight
of reciprocal crosses
* Early differences in embryo size in favour of hybrids resulted
from rapid growth rate of multi cellular zygote

17. 2. Early seedling growth
*Hybrid shows higher growth rate than inbred immediately after
germination but not later.
*Hybrids do have a larger maximum growth rate period.
*They are says embryo size is more significant than reserve food
material.

18. 3. Later Seedling Growth
* Hoffer found root dry matter content of hybrids to be
intermediate between parents
*Hybrids found to absorb and utilize more amount of N ,P
than the inbred lines
* Catalase activity was higher in heterosis hybrids

19. Inbreeding depression
Inbreeding depression refers to decrease in fitness and vigour due to
inbreeding. The degree of inbreeding is measured by the inbreeding
coefficient. The main differences between inbreeding and heterosis
are as follows:
1. Inbreeding result from mattings between closely related
individuals, whereas heterosis results from crossing between
unrelated stains.
2. Inbreeding depression is the decline in fitness and vigour with
decreased heterozygosity,whereas heterosis is the increase in
fitness and vigour.
3. Inbreeding results due to fixation of unfavorable recessive
genesF2 while in case of heterosis the unfavorable recessive
genes of one line are covered by favorable dominant genes of
other parents.

20. Commercially application
Crop species A . Asexually propagated species
1 . Cross pollinated species: maize(Ganga2,Ganga5,Ganga11),pearl
millet(HB3,BJ104,MH179) sunflower(BSH1,KBSH1,KBSH11)
castor(GCH2,GCH3,GCH4).
2 . Self pollinated crop: rice (by directorate of rice research),
pigeon pea (ICPH8), jute(Hybrid C).
B.Vegetable crops :- tomato(hybrid tomato),
brinjal(vijay, pusa kranti) watermelon(pusa sanyog).
C.Fruits trees:- in almost all the fruit
trees.

21. REFERENCES
Singh, B.P. 2000. plant breeding principles and methods 2nd edn.
Kalyani Publishers, New Delhi. 186-195.
Singh, P. 2001.Essentials of Plant Breeding 2nd edn. Kalyani
Publishers, New Delhi. 146-150