This document discusses several hypotheses for heterosis, or hybrid vigor. It summarizes the dominance hypothesis, which proposes that heterosis results from the superiority of dominant alleles over recessive alleles. It also summarizes the overdominance hypothesis, which suggests heterosis occurs when a heterozygote is superior to either homozygous parent due to production of superior hybrid substances or greater buffering capacity. The document also briefly discusses the epistasis hypothesis, which proposes non-allelic interaction between loci can contribute to heterosis, particularly dominance by dominance epistasis.

1. Heterosis
Dr. Kaushik Kumar Panigrahi

2. h

6. Objections and Explanation-
There are two objections to dominant gene hypothesis.
i) If it is true it is should be possible to obtain pure heterotic
individuals in F2 which are homozygous for all the dominant
genes. Jones (1917) provided explanation for this. He
suggested that there may be linkage between some favorable
dominant genes and some unfavorable recessive genes and as a
result. It is not possible to obtain true breeding homozygous
individual for all dominant genes to F2 generation. He
proposed dominance of linked gene hypothesis to explain
heterosis.
ii) If the heterosis is due to dominance, the F2 curve should be
skews towards dominant genes, but the curve of F2 is found
always smooth and symmetrical not skewed. Collins (1921)
provided explanation for the objections. He suggested that trial
like yield is governed by large number of genes or poly genes,
which exhibit continuous variation resulting in symmetrical
distribution of genes.

9. EXPLANATION-
According to this hypothesis is the result of superiority of
heterozygote over its both homozygous parents. Thus heterosis
is directly proportional to the heterozygosis. The superiority of
heterozygote over both homozygotes may arise either due to if
1) Production of superior hybrid substances in heterozygote is
completely different from either of the homozygous products
or due to
2) Greater buffering capacity in the heterozygote resulting from
cumulative action of divergent alleles of stimulation of
divergent alleles.East in 1936 further elaborated this theory by
proposing a series of alleles a1,a2,a3,a4 ----- of gradually
increasing divergence in function. Thus a combination of more
divergent alleles will exhibit higher heterosis than less divergent
combinations. For example, combination of a1, a4 will exhibit
higher heterosis as compared to combination as a1, a2, a3 and
a4.

10. EPESTSIS HYPOTHESIS-
Epistasis refers to interaction between alleles of two
or more different loci. It is also known as non-allelic
interaction. The non-allelic interaction is of three
type’s viz. additive X additive, dominance X
dominance and heterosis has positive association
with the presence and magnitude of non allelic
interaction. Epistasis, particularly that involves
dominance effects (dominance X dominance) may
contribute to heterosis. This has been observed in
cotton and maize. Epistasis can be detected or
estimated by various biometrical models.
.

11. DOMINANCE
HYPOTHESIS-
*This theory was proposed
by
Davenport,bruce,keeble,and
pellow.
*According to this
hypothesis hetorisis is the
result of superiority of
dominant alleles when
recessive alleles are
deleterious.
*Here heterosis is directly
proportional to the no of
dominant gene contributed
to each gene.
*most widely accepted
hypothesis.
OVERDOMINACE
HYPOTHESIS-
*This theory was proposed by
sheull and east and supported
by east and hull.
*According to this hypothesis
the result of superiority of
heterozygous over its both
homozygous parents.
*Here heterosis is directly
proportional to heterozygosis.
*not so widely accepted
hypothesis.

17. SUBMITTED TO-
DR.KOUSIK KUMAR
PANIGRAHI .
SUBMITTED BY-
KIRAN KUMAR MAJHI
ADM. NO.-28 HO/16