Charles Darwin observed that crossed plants of Linaria vulgaris were taller and more vigorous than self-fertilized plants of the same species. Heterosis, coined by Shull in 1952, refers to the increased performance of F1 hybrid plants compared to the average of their inbred parental lines, in traits like biomass, size, yield and resistance. There are several hypotheses for heterosis, including dominance, overdominance and epistasis models. The dominance model proposes that superior performance is due to dominant alleles masking recessive alleles, while the overdominance model suggests heterozygosity itself provides benefits over either homozygote.

3. ‘‘I raised close together two large beds of self-
fertilized and crossed seedlings from the same plant
of Linaria vulgaris. To my surprise, the crossed plants
when fully grown were plainly taller and more vigorous
than the self-fertilized ones.’’ - Charles Darwin

4. ‘Heterosis’ coined by “SHULL”
in (1952) as “ stimulation of
heterozygosity”.

5. Superior performance of heterozygous F1 hybrid
plants in terms of increased biomass, size, yield,
speed of development, fertility, resistance to
disease and insect pest, or to climatic rigors of
any kind compared to the average of their
homozygous parental inbred lines. (Shull, 1952)

6. AA BB
Parents

7. AA BB
Parents
Mid-parent

8. AA BB AB(case1)
OffspringParents
Mid –parent
heterosis
Mid-parent

9. AA BB
Parents
Best-parent

10. AA BB AB(case2)
OffspringParents
Best- parent
heterosis
Best-parent

11. AA BB AB(case1) AB(case2)
OffspringParents
Mid –parent
heterosis
Best- parent
heterosis
Best-parent
Mid-parent

12. Refers to the phenomenon in which
hybrid progeny of two inbred
varieties exhibits enhanced growth
or agronomic performance.

13. 1. DOMINANCE MODEL
2. OVERDOMINANCE MODEL
3. PSEUDO-OVERDOMINANCE
4. EPISTASIS MODEL

14. Dominance hypothesis (Davenport, 1907)

15. • Superiority of dominant alleles over
recessive alleles.
• Masking of recessive alleles by dominant
alleles.
• Heterosis is directly proportional to the
number of dominant genes contributed by
each parent.

16. AA aa Aa
AA BB
OffspringParents
bb
ee
dd
CC
EE
dd
cc
BB
Ee
dd
Cc
Bb
Dominance hypothesis

17. AA aa Aa
AA BB
OffspringParents
bb
ee
dd
CC
EE
dd
cc
BB
Ee
dd
Cc
Bb
AA = Aa > aa
2
1
1
2
1
1
2
1
1
2
2
2
1
2
2
7 7
9

18. OVER-DOMINANCE (East and Shull 1908)

19. • Super dominance
• Heterozygosity is essential and cause of
heterosis.
• Heterozygosity is superior to either of the
homozygotes.
• Increase in vigour is directly proportional to
amount of heterozygosis.

20. AA aa Aa
AA BB
OffspringParents
bb
ee
dd
CC
EE
dd
cc
BB
Ee
dd
Cc
Bb
Over Dominance
hypothesis

21. AA aa Aa
AA BB
OffspringParents
bb
ee
dd
CC
EE
dd
cc
BB
Ee
dd
Cc
Bb
1.5
1
1
1.5
1
1
1.5
1
1
1.5
2
2
1
2
2
6 6
9
Aa > AA or aa

22. PSEUDO-OVERDOMINANCE

23. • The genetic intermediate of dominance and
over dominance is Pseudo-Over dominance.
(Stuber et. al., 1992; Graham et. al.,
1997)
• Heterosis associated with this can dissipate
in selfing progeny due to recombination.
• This can arise from numerous alleles of
recombination suppression regions.

24. EPISTASIS

25. • Epistasis refers to interaction between
alleles of two or more different loci.
• It is also known as non allelic interaction.
• Epistasis, particularly that involves dominance
effects (dominance X dominance) may
contribute to heterosis.
• This has been observed in cotton and maize.

27. VENKATESH R
B.Sc. AGRICULTURE
ANBIL DHARMALINGAM AGRICULTURAL
COLLEGE AND RESEARCH INSTITUTE
TRICHY-9