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
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)
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
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
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.
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.