Study in brief about transgressive segregation

1. TRANSGRESSIVE SEGREGATION

2. Transgressive segregaTion
It
is the formation of extreme phenotypes , or transgressive phenotypes ,
observed in segregated hybrid populations compared to phenotypes observed
in the parental lines.
Hybrid offspring generally possess traits or characteristics seen in
ancestral species, which are expected to be subdued or diluted when
compared to the original species.
Transgressive segregation attempts to explain situations when the converse
is true, hybrid offspring that appear to have overstated traits when
compared to the parental line.

3. Transgressive segregaTion
Transgressive
segregation may be a major source of novel adaptations in
hybrids.
Transgressive segregation results when progeny plants contain new
combinations of multiple genes with more positive effects, or more negative
effects, for a quantitative character than were present in either parent.
It is due to quantitative inheritance .

4. Quantitative inheritance:
Quantitative characters:
Inheritance controlled by several to many genes ( Poly genes) at different
loci, that express major phenotypic effects,
Inheritance unlike simple Mendelian traits
Segregating population result in continuous classes
Expression under strong influence of environment
Quantitative Inheritance, is concerned with inheritance of multigenic
traits.

5. causes
There are many causes of transgressive segregation in hybrids such as:
Recombination of additive alleles
An elevated mutation rate
Reduced developmental stability
Epistatic effects between alleles
Overdominance caused by heterozygosity at specific loci or chromosome
number variation

6. Possible Predictions:
Transgressive
segregation will be most frequent in crosses between selfing
lineages because genetic and phenotypic variation will occur among lineages
rather than within them.
The frequency of transgression will be positively correlated with the
genetic divergence of the parental lines.
The more similar the phenotype of the parents, the greater the likelihood
transgressive segregation will be observed in the F2
Traits with a history of directional selection are less likely to exhibit
transgressive segregation than those with a history of genetic drift or
stabilizing selection

7. Transgressive segregaTionexample:
Two plants with genotypes R1R1R2R2
(Dark red) r1r1r2r2 (White) were crossed.
R1R1R2R2
x
r1r1r2r2
R1R2
R1R2
R1r2
R1 R2
R 1R 2
R1 r1R2 r2
R1r2
r1R2
R1 r1R2 r2 (Medium Red)
F1
F2
r1r2
r1R2
r1r2
x
R1 r1R2 r2
R1R2
R1r2
r1r2
r1R2
r1r2
R1r2
r1R2
r1r2
R1R1R2R2 R1R1R2r2
R1r1R2R2
R1r1R2r2
Dark Red
Medium
Dark Red
Medium
Dark Red
R1R1R2r2
R1R1r2r2
R1r1R2r2
R1r1r2r2
Medium
Dark Red
Medium
Red
Medium
Red
Light Red
R1r1R2R2
R1r1R2r2
r1r1R2R2
r1r1R2r2
Medium
Dark Red
Medium
Red
Medium
Red
Light Red
R1r1R2r2
R1r1r2r2
r1r1R2r2
r1r1r2r2
Medium
Red
Light Red
Light Red
White
Medium
Red

8. Transgressive segregaTionexample:
All
the progenies produced in the F1 are of same phenotypic character, ie
medium red.
The progenies of F2 generation show a variation in the phenotypic
expression.
The phenotypic trend shown in F2 is as follows:
Dark Red
1
Medium
Dark Red
4
Medium
Red
6
Light Red
4
White
1