👨‍👧 Dominance

Dominance

• Dominance is the phenomenon of expression of only one character or allele in heterozygous state.
• It is of four types:
  • Co-dominance or No-dominance
  • Incomplete or Partial dominance
  • Complete dominance
  • Over dominance

a) Co-dominance/No-dominance

• In such condition both alleles of a gene express themselves in heterozygous condition.
• E.g. human blood group (ABO). AB blood group possess both the antigens A & B.
• The genotypic ratio in F2 is 1 : 2 : 1.

b) Partial/Incomplete dominance

• In such condition a dominant allele does not suppress completely the phenotypic expression of recessive allele.
• E.g. the cross between red flower variety of Snapdragon (Antirrhinum mazus) with its white flower variety produces pink flower in all the plants in F1 generation and in F2 three types red, pink and white are obtained in the ratio of 1 : 2 : 1.

c) Complete dominance

• In this condition one allele completely suppresses the expression of other.
• E.g. round seed shape in pea is completely dominant over wrinkled seed shape and the ratio of monohybrid in F2 is 3 : 1.

d) Over dominance

• Here the intensity of character is greater in heterozygotes than in the concerned homozygotes.
• E.g. in maize, the heterozygote is more pigmented than either homozygote in presence of certain R alleles.

Multiple alleles

• Generally a gene has two alternative forms called alleles. Usually one of them is dominant over the other. The two alleles of a gene determine the two contrasting forms of a single character. Ex. Tall (T) and dwarf (t) plant height in garden pea.
• But in many cases, several alleles of a single gene are known to exist and each one of them governs a distinct form of the concerned character or trait. Such a situation is known as multiple allelism and all the alleles of a single gene are called multiple alleles. Many genes in both animals and plants exhibit multiple alleles.
• Characteristic features of multiple alleles
  • Multiple alleles always belong to the same locus in a chromosome.
  • One allele is present at a locus at a time in a chromosome
  • Multiple alleles always control the same character of an individual. However, the phenotypic expression of the character will differ depending on the alleles present.
  • There is no crossing over in a multiple allelic series.
  • In a multiple allelic series, wild type is almost always dominant over the mutant type.
  • A cross between two strains homozygous for mutant alleles will always produce a mutant phenotype and never a wild phenotype. In other words, multiple alleles do not show complementation.
  • Further, F2 generations from such crosses show typical monohybrid ratio for the concerned trait.
• Ex:
  • Blood group in human beings
  • Fur colour / coat colour in rabbits
  • Self-incompatibility alleles in plants

Blood groups in human beings

• On the basis of presence / absence of certain antigens, four blood groups in human beings have been established by Karl Landsteiner in 1900.
• The blood group system in human beings is believed to be controlled by a single gene generally designated as “I”.
• The gene “I” has three alleles. – I^A, I^B and i.
• Allele I^A controls the production of antigen A, I^B controls the production of antigen B and i does not produce any antigen.

Fur or coat colour in rabbits

• The fur colour in rabbits is a well-known example of multiple alleles.
• In rabbits, the fur colour is of four types viz., agouti, chinchilla, himalayan and albino.
• It is due to multiple alleles of a single gene ‘C’.
  

  

Self-incompatibility alleles in plants

• A series of self-incompatibility alleles insures cross pollination in many plants.
• Such alleles were described first in toabacco and later they were found in several other plant species like Brassica, radish, tomato, potato etc.
• In these species, self-incompatibility is governed by a single gene ‘S’ which has multiple alleles viz., S₁, S₂, S₃, S₄ and so on.

Penetrance

• It refers to the statistical regularity with which a gene produces its effect when present in the requisite homozygous or heterozygous state.
• It means penetrance is the ability of a gene or a combination of genes to be expressed phenotypically to any degree. It may be either complete or incomplete.
• In complete penetrance genes always produce the expected phenotype whereas in incomplete penetrance the genes fail to produce complete phenotypic expression.
• E.g. In man the tendency to develop diabetes mellitus (excess sugar in blood) is controlled by certain genes. However not everyone carrying the genes for diabetes actually develop the condition. It means genes have incomplete penetrance.

Expressivity

• It is the degree of effect produced by a penetrant gene.
• E.g. in man polydactyly (more than five fingers) may be penetrant in the left hand (6 fingers) and not in the right hand (five fingers).
• The penetrance of a gene and expressivity are influenced by environmental factors.

Pleiotropism

• A single gene (allele) often influences more than one phenotypic trait; such multiple effect of a single gene is known as pleiotropism.
• E.g. the gene for a disease phenylketonuria in man has pleiotropic effect. It produces various abnormal phenotypic traits collectively called syndrome.

Lethality

• Lethality is such condition where death of a certain genotype occurs prematurely.
• E.g. In mice a dominant allele Y for yellow coat is lethal in homozygous state.

  

• Phenotypic ratio: 2 : 1 or 0 : 2 : 1
• Genotypic ratio: 2 : 1

Gene interaction

• Such phenomenon of two or more genes which affects the expression of each other in various ways in the development of a single character of an organism is called gene interaction. ^{UPPSC 2021}

Complementary genes

• Bateson & Punnet found flower coloured of sweet pea (Lathyrus odoratus) in the ratio of 9 : 7 (9 coloured & 7 white) instead of 9:3:3:1 in F2.

Epistasis

• Here one gene mask the effect of other.
• The dominance works at inter allelic and intragenic level while epistasis works at inter genic level.
• Interaction between alleles different loci.

a) Recessive epistasis

• 9 : 3 : 4 (in F2)
• E.g. cross between coloured mice & albino mice.
• Product will be Agouti: Coloured: Albino → 9 : 3 : 4

b) Dominant epistasis

• 12 : 3 : 1 (in F2)
• E.g. cross between two heterozygous white coated dogs.
• Product will be White : Black : Brown → 12 : 3 : 1

Inhibitory gene action

• 13 : 3 (in F2)
• Here one dominant inhibitory gene prevents the expression of another dominant gene and thus 9 : 3 : 3 : 1 ratio is modified in 13 : 3.
• E.g. seed colour in maize 13 white : 3 red.

Duplicate genes/Duplicate Epistasis

• 15 : 1 (in F2)
• A modified 15 : 1 ratio was observed by G.H. Shull when he studied shephered’s purse (capsella). ^{RRB-SO 2019}
• Phenotypic ratio 15 : 1 in F2 (15 triangular shaped capsule and 1 top shaped capsule).

Polymeric/Additive gene action

• 9 : 6 : 1 (in F2)
• In the coat colour of Duroc-Jersey pigs, the F2 phenotypic ratio: 9 red : 6 sandy : 1 white.