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Recessive Epistasis Complete dominance at both loci, but homozygous recessive condition at one of the two loci is epistatic to the other. Flower Colour in blue-eyed Mary (Collinsia parviflora) wwMM x White Magenta WwMm Blue F1 F2 WWmm WM Wm wM wm WM WWMM WWMm WwMM WwMm Wm WWMm WWmm WwMm Wwmm wM WwMM WwMm wwMM wwMm wm WwMm Wwmm wwMm wwmm 9 Blue : 3 Magenta : 4 White Also Agouti : Black : Albino coat colour in mice Recessive Epistasis Complete dominance at both loci, but homozygous recessive condition at one of the two loci is epistatic to the other. Enzyme 1 (w) is epistatic to Enzyme 2 (m) Duplicate Dominant Epistasis Example: Seed capsule shape in Shepherd's purse (Capsella bursa-pastoris) Plant 1 has a heart–shaped seed capsule and Plant 2 has a narrow capsule Crossing the two produces an F1 with a heart-shaped capsule x Single Dominant Gene? Selfing the F1 produces the following F2 ratio: 15 heart to 1 narrow fruit Duplicate Dominant Epistasis Example: Seed capsule shape in Shepherd's purse (Capsella bursa-pastoris) AABB Heart Shape aabb Narrow Shape AaBb Heart Shape F1 F2 x AB Ab aB ab AB AABB AABb AaBB AaBb Ab AABb AAbb AaBb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb A Dominant allele at either of the two loci produces a heart-shaped fruit A is epistatic to B or b and B is epistatic to A or a to give a 15:1 ratio Dominant Suppression Epistasis Example: Malvidin production in Primula (anthocyanin giving blue flower) Plants 1 and 2 lack blue pigment in their flowers. When crossed together, the F1 also lacks pigment Selfing the F1 produces blue flowered segregants Phenotypic ratio: 13:3; Not 3:1 Dominant Suppression Epistasis Example: Malvidin production in Primula (anthocyanin giving blue flower) KKDD Non blue kkdd Non blue KkDd Non Blue F1 F2 x KD Kd kD kd KD KKDD KKDd KkDD KkDd Kd KKDd KKdd KkDd Kkdd kD KkDD KkDd kkDD kkDd kd KkDd Kkdd kkDd kkdd Malvidin production controlled by dominant allele at K locus but the pathway is blocked by a dominant allele at the suppressor locus D Produces a 13 : 3 ration, like feather colour in chickens Bell Pepper Colour Genetics prepared by Dr. Paul Kusolwa Two genes without Epistasis - Additive Example: Fruit colour in Bell Pepper CCrr Yellow ccRR Brown CcRr Red F1 F2 x CR Cr cR cr CR CCRR CCRr CcRR CcRr Cr CCRr CCrr CcRr Ccrr cR CcRR CcRr ccRR ccRr cr CcRr Ccrr ccRr ccrr A Dominant allele at either of the two loci produces red fruit Dominant alleles at the C locus and no dominant alleles at R give yellow fruit Dominant alleles at the R locus and no dominant alleles at C give brown fruit Double recessive gives green fruit 9:3:3:1 Fruit shapes in squash Di locus Dominant to spherical or pyriform Duplicate genes with cumulative effects When Di is present together with Spherical S locus Di is dominant = Disc fruits 9 Discs Di_S_ When Di present with recessive s = spherical fruits When didi/ss = long or pyriform fruits Modified Ratio = 9: 6: 1 6 Spherical = Di/s & di/S_ 1 Pyriform di/s Duplicate Recessive - Squash Pathway involving two genes Wt = warty fruits Dominant to non warty wt Hr = hard rind hr = intermediate texture 9:7 B_ Hr_ Wt_ Y_ More Epistasis in Squash Bicolor fruits = locus B pleiotropic for fruits and leaves For yellow and green color patches BB or Bb Extent of yellow or green: Model with 2 incompletely dominant additive loci, Ep1 and Ep 2, proposed for enlarging the yellow patches Genotypes Bb with a dosage of 0 to 1 Ep alleles = bicolor green and yellow fruits Dosage of 2-4 dominant Ep alleles extends the yellow coloration Genotypes 9 B_Ep_ Extended yellow 3 B_epep Yellow narrow 3 bbEp_ Green extended 1bbepep green Tri-Genic Interactions…. Three or more genes interactions in ornamental gourds Gb = green bands; gb for no bands Gr/G = green rind (gr/g buff skin) L-2 = color intensity (yellow /orange) Gr Gb L-2 Ep-1/2
