subject genitics

1. Over Dominance
Definition:
Over Dominance is the interaction between
genes that are alleles and result in the
heterozygous individuals being superior to
either of their homozygotes.
When A1 and A2 Gene occurs together:
Genotypes are: A1A1, A1A2 and A2A2
Example: A particular blood type in rabbits

2. Over Dominance
• Genotype A1A1 produces Antigen 1
• Genotype A2A2 produces Antigen 2
• Genotype A1A2
Not only produces Antigen 1 and 2
But also produce Antigen 3
Example in Human haptoglobinus, a protein
produced by genes with a specific property of
binding hemoglobines.H1H1,H1H2,H2H2
Over dominance affects traits of fitness/ vigor

3. EPISTASIS
• Epistasis is another form of non additive gene
action.
• Epistasis involves two or more pairs of genes
which are not alleles.
• In over dominance the action between genes
of a particular pair (A1 & A2) as
heterozygotes give different phenotypic
expression than when these genes are
homozygous pairs.

4. EPISTASIS
• In Epistasis the action is between entirely
different pair of genes and not between
members of the same gene pair.
• In epistasis the two or more different pairs of
genes may be on the same chromosome or
on different chromosomes.
 In Horses, several coat colors are
determined by epistasis.

5. EPISTASIS
• Black and chestnut are two basic coat colors in
horses and these colors are affected by two
alleles.
• The dominant gene B= Black.
• The recessive gene b= Chestnut.
• These two genes (B and b) segregates in the
gamete, recombine in zygote and express there
phenotypes accordingly.
• Some horses posses white color , this is referred
as dominant gene W (white gene)

6. • BBww Black color
• Bbww Black color
• bbww Chestnut color
 Dominant White gene will produce
• BBWw White color
• BbWw White color
• bbWw White color
• The dominant white gene W inhibits the
expression of either the black B or chestnut
gene b.
• Thus W is the epistasis to these two genes.

7. • aaBBww black color
• aaBbww black color
• AaBBww bay color
• AaBbww bay color
• AABBww bay color
• AABbww bay color

8. • AAbbCCww chestnut color
• AabbCCww chestnut color
• AAbbC ͨͬ Cww palomino
• AabbC ͨͬ Cww palomino
• AAbbC ͨͬC ͨͬCww type A, pseudo albino
• AabbC ͨͬC ͨͬCww type A, pseudo albino