1. HETEROSIS
• HETEROSIS- REFERS TO SUPERIORITY OF F1, HYBRIDS IN ONE OR MORE
CHARACTERS OVER ITS PARENTS.
• HETEROSIS- AS INCREASE OF F1, OVER PARENTS IN GENERAL VIGOR, YIELD
AND ADAPTATION.
• THE TERM HYBRID VIGOR IS USED AS SYNONYM FOR HETEROSIS.
2. Hybrid vigour:
According to Shull,
Hybrid vigour: (used as a synonym of heterosis)
The developed superiority of the hybrids is the 'hybrid vigour.
Heterosis:
The mechanism by which this superiority is developed
3.
4.
5. GENETIC BASIS OF HETEROSIS
The phenomenon of heterosis can be explained on the basis of these Genetic
causes.
There are two most commonly known theories-
(1) The Dominance Hypothesis
(2) The Overdominance Hypothesis
A third theory has also been proposed named
(3) Epistasis
6. (1) Dominance hypothesis:
This theory was proposed by Davenport, Bruce and Keable and Pellew in the
year 1910.
This is the most widely accepted theory.
Heterosis in F1, hybrids arises mainly from an accumulation of maximal
number of dominant favorable alleles.
(contributed and complemented by either of the parents)
Alternatively, this hypothesis predict that recessive alleles are harmful or
injurious to their carrier.
According to this theory, genes that are favorable for vigour and growth are
dominant.
7. Assume that dominant genotypes contribute 2 units to the phenotype
while a recessive genotype contributes 1 unit.
A cross between two inbred parents could produce the following outcome
P1 X P2
parents (AAbbCCdd) (aaBBccDD)
phenotypic value 2+1+2+1=6 1+2+1+2=6
F1
(AaBbCcDd)
2+2+2+2=8
8. (2) Overdominance hypothesis:
It was independently proposed by Shull and East in 1910 and
supported by Hull in 1945.
The theory indicates that there is something inherent in the
heterozygous state that causes heterosis.
This hypothesis is the result of superiority of heterozygote over its
both homozygous parents.
This theory is called by various names such as stimulation of
heterozygosis, single gene heterosis, super dominance etc.
9. (3) Epistasis
Epistasis refers to the interaction between alleles of two or
more different location.
It is also known as non-allelic interaction.
They are of many types, for example dominance X dominance
etc.
Epistasis particularly that involves dominance effects may
contribute to heterosis.
This has been observed in cotton and maize. It can be
estimated or detected by various biometrical models.
10. EFFECTS OF HETEROSIS
Increase yield
Increased reproductive ability
Increase in size and general vigor
Better quality
Earlier flowering and maturity
Greater resistance to disease and pests
Greater adaptability
Faster growth rate
Increase in number of plant parts
11. USE OF HETEROSIS IN PLANT BREEDING
Heterosis is exploited through the development of hybrid.
It is commercially exploited in seed production of cross
pollinated crops like jowar, maize, bajra, onion and cucurbit.
It has been also used in some self-pollinated crop species such
such as rice, wheat, tomato and brinjal etc.
