What is Dominance? – Definition
In the expression of characters genes play major role. Generally plants have two copies of each gene in their genome, one inherited from each parent.
The alternative forms of gene are known as allele. If phenotype of the organism is determined by one of the alleles by completely hiding the expression of an alternate allele, then that allele is said to be dominant.
The phenomenon is called as Dominance.
The other allele is recessive; its expression is suppressed in the presence of a dominant allele of organism.
Mechanism of Dominance
According to Mendel’s beliefs, certain characters are produced in progeny. The character cannot be present as such in gametes; the only link between progeny and the parent. Therefore there must be something that represents the character and is regulates its appearance. Mendel named this something as “Factor” (today’s gene).
The gene controls expression of character by the interaction with the other genes, the cytoplasm and the environment. A diploid cell contains two sets of chromosomes which come from two opposite sexes via gametes. Homologous chromosomes pair during meiotic cell division. Each character of a pair of contrasting character is represented by allele.
Thus, homozygous tall pea plant has two identical alleles TT and homozygous dwarf plant has tt alleles on both gene loci, of the homologous chromosome. During gamete formation, the homologous chromosomes with TT or tt genes get separated and each chromosome with T or t gene is passed to gamete. The gametes from both the parents unite during fertilization and produce a new individual (heterozygote or hybrid) containing allele for both tall (T) and dwarf (t) characters.
Because the hybrids of F1 have tall stems, the character of tallness is considered as dominant and dwarf character as recessive as it failed to express itself in F1 generation.
Types of Dominance or Variations in Dominance relation
Other than Simple or complete dominance other types of dominance are as follows. They are also considered as variations in dominance relation.
- Incomplete or Semidominance or Partial Dominance
- Over Dominance
- Dominant Negative
Incomplete or Semidominance or Partial Dominance
Discovered by Karl Correns.
Sometimes in a heterozygote, dominant allele does not completely masks or hide the phenotypic expression of the recessive allele; as a result of this heterozygote has intermediate phenotype. This is called as incomplete dominance.
Example – When pea plant with red flowers is crossed with that of white flowers, sometimes the F1 has intermediate phenotype for flower color i.e. pink.
Sometimes both alleles of a gene in a heterozygote lack the dominant recessive relationship, both alleles contribute to the phenotype of the heterozygote.
In codominance, heterozygote genotype gives rise to a phenotype distinctly different from either of the homozygous genotype.
Example – Coat color in the short horn breed of cattle or in horses. In a cross of red coat cattle with white coat cattle, progeny of roan coat is obtained. In roan coat, the red and white hairs occur in definite patches but no hair has intermediate color of red and white.
In this, progeny exceeds parental limits for any character.
Some loss-of-function mutations are dominant. These are referred as dominant negative
Visit Gregor Mendel who studied dominance while working on pea plant.
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