This document discusses duplicate dominant epistasis, where two different genes at separate loci produce the same dominant phenotype. Both dominant alleles (A and B) must be present for the triangular seed shape phenotype, resulting in a 15:1 phenotypic ratio. Only the double recessive genotype (aabb) produces the rounded seed shape phenotype. An example in maize is provided, where genes at two loci control seed shape, with triangular being dominant to rounded at both loci. A test cross demonstrates the 15:1 phenotypic ratio.

1. DUPLIC
ATE
DOMINA
NT
EPISTAS
Presented by Waiton Sherekete
B1542258

2. WHAT IS EPISTASIS
Epistasis is Greek word meaning standing over.
It was first used in 1909 by Bateson to describe a masking effect.
The phenotypic expression at one locus depends on the
genotype at a different locus.
Or simply is when two different kinds of genes at different locus
interact in producing one trait (phenotype)

3. DUPLICATE DOMINANT EPISTASISThis is a situation when two different genes on different locus
producing a same trait are all dominant to each other .
The dominant alleles of both the genes produce the same
phenotypic effect giving the ratio 15:1.
At least one of the dominant allele is necessary for the
phenotypic effect. e.g. AABB, AaBb, Aabb, aaBB, aaBbgive one
phenotype.
In the absence of all the dominant genes (only in case of aabb),
the recessive phenotype will be expressed.
The duplicate genes are also called pseudo alleles

4. EXAMPLE
In maize :a gene on the on a certain locus affect the shape of a
maize seed ,the shapes coded for are tri angular shaped and rounded
shape .
On another locus a gene controlling the secretion of an enzyme
that also controls seed shape ,the shapes are also tri angular shape
and rounded shape .
When there is a situation like this all the genes work together to
produce a certain phenotype.
The genotypes of these genes are :for enzyme secretion (A)
produces tri angular shapes and is dominant to (a) rounded shape.
 On the other locus gene (B) also produces tri angular shape and is
dominant to (b) which produces rounded seeds .
The crosses overleaf shows these type of gene interaction .

5. P : AABB × aabb
(triangular) (rounded )
meiosis
F1 : AaBb
(all triangular)
self fertilization AaBb X AaBb
AB Ab aB ab
AB
AABB
(triangular)
AABb
(triangular)
AaBB
(triangular)
AaBb
(triangular)
Ab
AABb
(triangular)
AAbb
(triangular)
AaBb
(triangular)
Aabb
(triangular)
aB
AaBB
(triangular)
AaBb
(triangular)
aaBB
(triangular)
aaBb
(triangular)
ab
AaBb
(triangular)
Aabb
(triangular)
aaBb
(Triangular)
aabb
(rounded)

6. RATIOS
The phenotypic ratio of these crosses is 15 tri angular :1 rounded
=15:1
The genotypes produces are:
AABB(1) AABb(2) AaBB(2)
Aabb(2) aaBb(2) AaBb(4)
Aabb(1) aaBB(1)

7. CONCLUSION
We conclude that in order to get a triangular shaped seed either 1 of
the genes on either locus must be dominant .
The rounded seed is only produced when all genes available on the
different locus are recessive as shown on the cross above .

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Thank you