Over DominanceDefinition:Over Dominance is the interaction betweengenes that are alleles and result in theheterozygous individuals being superior toeither of their homozygotes.When A1 and A2 Gene occurs together:Genotypes are: A1A1, A1A2 and A2A2Example: A particular blood type in rabbits
Over Dominance• Genotype A1A1 produces Antigen 1• Genotype A2A2 produces Antigen 2• Genotype A1A2Not only produces Antigen 1 and 2But also produce Antigen 3Example in Human haptoglobinus, a proteinproduced by genes with a specific property ofbinding hemoglobines.H1H1,H1H2,H2H2Over dominance affects traits of fitness/ vigor
EPISTASIS• Epistasis is another form of non additive geneaction.• Epistasis involves two or more pairs of geneswhich are not alleles.• In over dominance the action between genesof a particular pair (A1 & A2) asheterozygotes give different phenotypicexpression than when these genes arehomozygous pairs.
EPISTASIS• In Epistasis the action is between entirelydifferent pair of genes and not betweenmembers of the same gene pair.• In epistasis the two or more different pairs ofgenes may be on the same chromosome oron different chromosomes. In Horses, several coat colors aredetermined by epistasis.
EPISTASIS• Black and chestnut are two basic coat colors inhorses and these colors are affected by twoalleles.• The dominant gene B= Black.• The recessive gene b= Chestnut.• These two genes (B and b) segregates in thegamete, recombine in zygote and express therephenotypes accordingly.• Some horses posses white color , this is referredas dominant gene W (white gene)
• 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 theexpression of either the black B or chestnutgene b.• Thus W is the epistasis to these two genes.
• aaBBww black color• aaBbww black color• AaBBww bay color• AaBbww bay color• AABBww bay color• AABbww bay color
• 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