Recessive masking epistasis
Recessive masking epistasis – In recessive masking epistasis, a homozygous recessive genotype will mask the expression of another gene. – Example: Labrador Retriever coat colour o Two genes involved, three possible phenotypes B_E_ = black lab bbE_ = chocolate lab _ _ ee = yellow lab o When homozygous ee present, coat colour will be yellow regardless of what the alleles on the other gene are o The homozygous ee genotype is epistatic to the coat colour “B” gene Cross 1: BBEE x bbee be be be be BE BbEe BbEe BbEe BbEe BE BbEe BbEe BbEe BbEe BE BbEe BbEe BbEe BbEe BE BbEe BbEe BbEe BbEe All offspring (100%) would be heterozygous for both traits and puppies would have a black coat. Cross 2: BbEe x BbEe BE Be bE be BE BBEE BBEe BbEE BbEe Be BBEe BBee BbEe Bbee bE BbEE BbEe bbEE bbEe be BbEe Bbee bbEe bbee 9/16 puppies would be black 4/16 puppies would be yellow 3/16 puppies would be chocolate Dominant masking epistasis – In dominant masking epistasis, the presence of at least one dominant allele in one gene (homozygous or heterozygous) will mask the expression of another gene. – Example: Horse coat colour o Two genes involved, three possible phenotypes G_ _ _ = gray horse at maturity gg_ _ = horse is not gray ggE _ = black horse ggee = chestnut horse o When the dominant “G” allele is present, the horse will be gray at maturity because the expression of this dominant allele is masking the phenotypic expression of the second gene. When the genotype is homozygous, gg, the phenotype (coat colour) of the second gene can be expressed, and the horse will not be gray o The “E” genotype varies based on the dominant or recessive combination of alleles o The dominant allele in the “G” genotype is epistatic to the “E” gene Cross: GGEE x ggee ge ge ge Ge GE GgEe GgEe GgEe GgEe GE GgEe GgEe GgEe GgEe GE GgEe GgEe GgEe GgEe GE GgEe GgEe GgEe GgEe All offspring (100%) would be heterozygous for both traits and all horses gray at maturity. Cross: GgEe x GgEe GE Ge gE ge GE GGEE GGEe GgEE GgEe Ge GGEe GGee GgEe Ggee gE GgEE GgEe ggEE ggEe ge GgEe Ggee ggEe ggee 12 horses would be gray at maturity 3 horses would be black 1 horse would be chestnut Modifying epistasis – In modifying epistasis, the expression of a gene at one locus (location) modifies or changes the expression of the phenotype of another gene – Example: Doberman coat colour o o o Two genes involved, four possible phenotypes B_D_ = black dog bbD_ = red dog B_dd = blue (faded black) dog Bbdd = fawn (faded red) dog When the “D” genotype is homozygous recessive, dd, it acts to modify the phenotypic expression of the “B” gene by fading the colour, creating two new phenotypes. The homozygous recessive “D” genotype is epistatic to the “B” gene. Cross: BBDD x bbdd bd bd bd bd BD BbDd BbDd BbDd BbDd BD BbDd BbDd BbDd BbDd BD BbDd BbDd BbDd BbDd BD BbDd BbDd BbDd BbDd All offspring (100%) would be heterozygous for both traits and would have a black coat. Cross: BbDd x BbDd BD Bd bD bd BD BBDD BBDd BbDD BbDd Bd BBDd BBdd BbDd Bbdd bD BbDD BdDd bbDD bbDd bd BbDd Bbdd bbDd bbdd 9 dogs would be black 3 dogs would be red 3 dogs would be blue (faded black) 1 dog would be fawn (faded red)
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