PB Ch 13. Gene Interaction and Epistatics

Gene interaction refers to the situation where two or more genes influence the same character. When genes at different loci interact such that one gene masks or modifies the expression of another, it is called epistasis. The term was first used by Bateson in 1909.

Key terminology:

• Epistatic gene: The gene that masks or suppresses the expression of another gene
• Hypostatic gene: The gene whose expression is being masked

Epistasis differs from dominance: Dominance involves the masking of one allele by another allele at the same locus (intra-allelic interaction). Epistasis involves masking between genes at different loci (inter-genic interaction).

The classical 9:3:3:1 dihybrid ratio becomes modified by epistasis into various combinations. The main types are described below.

Non-Epistatic Intergenic Interaction — Bateson and Punnett's Fowl Comb Experiment (9:3:3:1)

This is the classic starting point that first demonstrated intergenic interaction. Bateson and Punnett studied comb shape in fowls. Each breed shows a characteristic comb type that breeds true:

When Rose comb (RRpp) is crossed with Pea comb (rrPP), the F1 shows a new phenotype — Walnut comb (RrPp). This is a new phenotype not present in either parent, arising from the simultaneous presence of both dominant genes R and P.

Dihybrid F2 — Fowl Comb (RrPp × RrPp)

W=Walnut, R=Rose, P=Pea, S=Single | Phenotypic Ratio: 9 Walnut : 3 Rose : 3 Pea : 1 Single

Dominant Epistasis (12:3:1 Ratio)

• When a dominant allele at one locus masks (is epistatic to) the expression of alleles at a second locus, it is called dominant epistasis. A dominant allele at locus A suppresses the expression of both dominant and recessive alleles at locus B.
• Example — Sorghum grain colour: Gene Z produces pearly grain (shining, translucent); gene z produces chalky grain. A separate gene R produces red colour; r produces no red colour (white). When a dominant Z allele is present, it masks the expression of the R/r gene, converting all pigmentation to pearly/chalky regardless of the R allele.
• Cross: Pearly White (ZZrr) × Chalky Red (zzRR). F1: ZzRr (Pearly Red). F2 Ratio: 12 Pearly : 3 Red : 1 Chalky.
• The 12 pearly class consists of 9 (Z_R_) + 3 (Z_rr) — the dominant Z at locus 1 is epistatic to the colour gene at locus 2, masking red expression in all Z_ individuals.

Recessive Epistasis (9:3:4 Ratio)

• When a recessive allele at one locus in homozygous condition masks the expression of another gene at a different locus, it is called recessive epistasis.
• Example — Sorghum leaf sheath colour: Gene P produces blackish purple colour; gene p produces brown colour. A supplementary gene Q adds to the blackish purple, converting it to reddish purple. However, when the locus 1 is homozygous recessive (pp), Q cannot express itself — the phenotype is brown regardless of the Q allele.
• Cross:
  • Blackish Purple (PPqq) × Brown (ppQQ). 
  • F1: PpQq (Reddish Purple).
  • F2 Ratio: 9 Reddish Purple : 3 Blackish Purple : 4 Brown.

The 4 brown class = 3 (ppQ_) + 1 (ppqq) — recessive pp masks Q expression, so all pp individuals are brown.

Duplicate Dominant Epistasis (15:1 Ratio)

• When a dominant allele at either of two loci can mask the expression of recessive alleles at both loci, it is called duplicate dominant epistasis.
• Either dominant gene alone is sufficient to produce the dominant phenotype. Only the double recessive (aabb) shows the alternative phenotype.
• Example — Sorghum grain starchiness: Two independent dominant genes W1 and W2 both produce starchy grains. Only when both are absent (w1w1w2w2) are the grains waxy.
• Cross:
  • Starchy (W1W1w2w2) × Starchy (w1w1W2W2).
  • F1: W1w1W2w2 (Starchy).
  • F2 Ratio: 15 Starchy : 1 Waxy.

These two genes are called duplicate genes: two independently located dominant genes with identical or similar phenotypic effects.

Duplicate Recessive Epistasis / Complementary Genes (9:7 Ratio)

• When two dominant alleles at different loci must both be present to produce one phenotype, and the recessive allele at either locus (in homozygous form) independently masks the effect of the other locus, it is called complementary gene interaction.
• The two genes are called complementary genes.
• Classic example — Sweet Pea flower colour (Bateson and Punnett): Purple flower colour requires the simultaneous presence of two dominant genes A and B.
• When either gene is absent (in homozygous recessive form), the flower is white. Neither A alone nor B alone produces purple.
• Complementary Genes — Sweet Pea (AaBb × AaBb)

Pur=Purple (9 with A_B_), Wh=White (7 with aabb, A_bb, or aaB_) | Ratio: 9 Purple : 7 White

Example from Sorghum (grain colour): Brown colour requires two complementary genes B1 and B2. Plants with B1B1b2b2 are white (lack B2); plants with b1b1B2B2 are also white (lack B1). Crossing the two white parents gives brown F1 (B1b1B2b2) because both complementary genes are now present. F2 gives 9 Brown : 7 White.

Dominant and Recessive Epistasis (13:3 Ratio)

• When a dominant allele at one locus is epistatic (inhibitory), and a recessive homozygote at another locus is also epistatic — acting independently to suppress a third class — the result is a 13:3 ratio.
• This is also called inhibitory gene action because one dominant gene inhibits the expression of another dominant gene. In this case, the epistatic dominant allele converts the 9:3:3:1 ratio into 13:3 by combining the 9 (A_B_) + 3 (A_bb) + 1 (aabb) classes into a single masked phenotype.

Polymeric Gene Interaction / Additive Epistasis (9:6:1 Ratio)

• When two different dominant alleles at two loci each produce the same phenotype, but when both dominant alleles are present together they produce a more extreme (additive) phenotype, the result is a 9:6:1 ratio. The genes are called additive or polymeric genes.
• The 9:6:1 ratio = 9 (A_B_ — enhanced effect) : 6 (A_bb + aaB_ — single dominant) : 1 (aabb — double recessive).

Summary of Modified Dihybrid Ratios