Deviations from Mendelian ratios
Contents: Allelic interactions and deviations from Mendelian monohybrid ratio; Examples of
allelic interactions; Non-allelic interactions and deviations from Mendelian dihybrid ratio;
Examples of non-allelic interactions; Suggested reading.
Allelic interactions and deviations from Mendelian monohybrid ratio
The interactions of genes or alleles located on the same or different chromosomes of an
organism are referred to as gene interactions. According to the definition, therefore, gene
interactions could be categorized into either (a) allelic or (b) non-allelic.
We know that the classical monohybrid ratio is 3: 1, where T is completely dominant over t
(i.e. T > t). Deviations from monohybrid ratio are due to allelic interactions, which are of the
following three main types:
1. Incomplete dominance (T is not > t): The ratio is 1: 2: 1;
2. Codominance (T = t): The ratio is also 1: 2: 1; and
3. Lethal gene (homozygotes TT or tt die): The ratio is 2: 1.
Examples of allelic interactions
1. Incomplete dominance: The condition in heterozygotes where the phenotype is a
blending i.e. intermediate between the two homozygotes. For example, pink flower
progenies from the red and white flower parents.
2. Codominance: The condition in heterozygotes where both members of an allelic pair
contribute to a phenotype, resulting in a mixture of both homozygotes. For example,
roan offspring possess both red and white hairs of their parents.
3. Lethal gene: A gene whose phenotypic effect is sufficiently drastic to kill the bearer.
The gene may be dominant, incompletely dominant or recessive.
Examples of the aforesaid allelic interactions are described in the following paragraphs.
Incomplete dominance in the Snapdragon flower, Antirrhinum majus
P: Red flower (r1r1) × White flower (r2r2)
F1: r1r2 (all pink flowers; blending to a single colour)
F1×F1: r1r2 × r1r2
F2: r1r1 r1r2, r1r2 r2r2
F2: ¼ Red: ½ Pink: ¼ White
Phenotypic ratio 1 Red: 2 Pink: 1 White
Genotypic ratio 1: 2: 1
Lecture 3_Deviations from Mendelian ratios 1
, Fig. 3.1 Incomplete dominance for flower colour in the Snapdragon, Antirrhinum majus
Codominance in the Shorthorn cattle, Bos taurus
P: Red ox (RR) × White cow (rr)
F1: Rr (all roan cattle)
Roan= reddish grey; presence of both red and white hair side by side
F1×F1: Rr × Rr
F2: RR Rr, Rr rr
F2: ¼ Red: ½ Roan: ¼ White
Phenotypic ratio 1 Red: 2 Roan: 1 White
Genotypic ratio 1: 2: 1
Fig. 3.2 Codominance for coat colour in the Shorthorn cattle
Example of lethal gene in maize/Indian corn, Zea mays
P: Pale green (Gg) × Pale green (Gg)
F1: gg Gg, Gg GG
¼ white (dies): ½ Pale green: ¼ Deep green
Phenotypic ratio 2 Pale green: 1Deep green
Genotypic ratio 2: 1
Lecture 3_Deviations from Mendelian ratios 2
Contents: Allelic interactions and deviations from Mendelian monohybrid ratio; Examples of
allelic interactions; Non-allelic interactions and deviations from Mendelian dihybrid ratio;
Examples of non-allelic interactions; Suggested reading.
Allelic interactions and deviations from Mendelian monohybrid ratio
The interactions of genes or alleles located on the same or different chromosomes of an
organism are referred to as gene interactions. According to the definition, therefore, gene
interactions could be categorized into either (a) allelic or (b) non-allelic.
We know that the classical monohybrid ratio is 3: 1, where T is completely dominant over t
(i.e. T > t). Deviations from monohybrid ratio are due to allelic interactions, which are of the
following three main types:
1. Incomplete dominance (T is not > t): The ratio is 1: 2: 1;
2. Codominance (T = t): The ratio is also 1: 2: 1; and
3. Lethal gene (homozygotes TT or tt die): The ratio is 2: 1.
Examples of allelic interactions
1. Incomplete dominance: The condition in heterozygotes where the phenotype is a
blending i.e. intermediate between the two homozygotes. For example, pink flower
progenies from the red and white flower parents.
2. Codominance: The condition in heterozygotes where both members of an allelic pair
contribute to a phenotype, resulting in a mixture of both homozygotes. For example,
roan offspring possess both red and white hairs of their parents.
3. Lethal gene: A gene whose phenotypic effect is sufficiently drastic to kill the bearer.
The gene may be dominant, incompletely dominant or recessive.
Examples of the aforesaid allelic interactions are described in the following paragraphs.
Incomplete dominance in the Snapdragon flower, Antirrhinum majus
P: Red flower (r1r1) × White flower (r2r2)
F1: r1r2 (all pink flowers; blending to a single colour)
F1×F1: r1r2 × r1r2
F2: r1r1 r1r2, r1r2 r2r2
F2: ¼ Red: ½ Pink: ¼ White
Phenotypic ratio 1 Red: 2 Pink: 1 White
Genotypic ratio 1: 2: 1
Lecture 3_Deviations from Mendelian ratios 1
, Fig. 3.1 Incomplete dominance for flower colour in the Snapdragon, Antirrhinum majus
Codominance in the Shorthorn cattle, Bos taurus
P: Red ox (RR) × White cow (rr)
F1: Rr (all roan cattle)
Roan= reddish grey; presence of both red and white hair side by side
F1×F1: Rr × Rr
F2: RR Rr, Rr rr
F2: ¼ Red: ½ Roan: ¼ White
Phenotypic ratio 1 Red: 2 Roan: 1 White
Genotypic ratio 1: 2: 1
Fig. 3.2 Codominance for coat colour in the Shorthorn cattle
Example of lethal gene in maize/Indian corn, Zea mays
P: Pale green (Gg) × Pale green (Gg)
F1: gg Gg, Gg GG
¼ white (dies): ½ Pale green: ¼ Deep green
Phenotypic ratio 2 Pale green: 1Deep green
Genotypic ratio 2: 1
Lecture 3_Deviations from Mendelian ratios 2
