DOMINANCE

Dominance is a physiological effect of an allele over its partner allele on the same gene locus. In genetics, dominance relationship refers to how the alleles for a single locus interact to produce a phenotype. For example, flower color in sweet peas (Lathyrus odoratus) is controlled by a single gene with two alleles. The three genotypes are PP. Pp. and pp. The flower color for PP (purple) and pp (white) do not depend on the dominance relationship. However, the heterozygote Pp can have many different colors: purple, white or pink. The exact color it has reflects the dominance relationship.

The dominant/recessive relationship is made possible by the fact that higher organisms are diploid. Most of their cells have two copies of each chromosome — one copy from each parent. Polyploid organisms have more than two copies of each chromosome, and follow similar rules of dominance.

Humans each carry 46 chromosomes (23 pairs). They have single sex chromosome and 22 autosomes coming from each parent. Each chromosomal pair has the same genes. The specific variations are possible for a single gene. For example, a single eye-color gene contains a blue eye allele, a brown eye allele, a green eye allele. etc. Consequently, a child mat inherit a blue eye allele from their mother and a brown eye allele from their father. The dominance relationships between the alleles control which traits are and are not expressed.

There are three kinds of dominance relationships:

(a)   Simple dominance

(b)   Incomplete (partial) dominance

(c)   Co-dominance

Traits inherited in a dominant-recessive pattern follow Mendelian inheritance.

Simple Dominance

Consider the simple example in peas of flower color. It was first studied by Gregor Mendel. The dominant allele is purple and the recessive allele is white. In a given individual, the two corresponding alleles of the chromosome pair has one of three patterns:

  • both alleles purple
  • both alleles white
  • one allele purple and one allele %%Hie

If the two alleles are the same (homozygous), the trait will be expressed. But it’ the individual carries one of each allele (Heterozygous), only the dominart one will be expressed. The recessive allele will simply be suppressed.

Dominant traits are recognized by th fact that they do not skip generations like recessive. It is Merck) e possible two parents with purple flowers have a white flowers anion their progeny. But two such white offspring cannot have purple offspring. In this situation. the purple individuals in the first generation must have been heterozygous.

Human traits governed by situpie dominance

(a)   Examples of dominant genes include the tumor supressor genes BRCA I and BRCA2. Mutations in these genes lead to the development of breast cancer. It disables tumour-suppressing functions of the proteins.

(b)   Another example of an autosoma I dominant disorder is

Huntington’s disease. It is a neurological disorder. The mutant gene results in an abnormal protein. This protein contains a large number of amino acid glutamine. This defective protein is toxic to neural tissue. It causes characteristic symptoms of the disease.

Some genetic diseases carried by dominant and recessive alleles

. Disease                           Gene is…

Polydactyl ism                 dominant

Marfan syndrome             dominant

Tay-Sachs disease           recessive

Incomplete Dominance


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The dominance in which phenotype of the heterozygote is intermediate between the phenotypes the two homozygote called incomplete or partial dominance.

Incomplete dominance was studied by Carl Correns in 1899. He worked on flowering plant 4 0′ clock. He crossed the true breeding red flower with the white flower. The F, hybrids produced pink flowers. This new phenotype had a shade intermediate between both parent plants. Thus these flowers have an intermediate amount of pigment in petals. Then Correns self-fertilized F, pink. The F, showed all three phenotype of flowers. Their ratio was I red 2 pink I white flowers. Red was homozygous for red alleles, and white was homozygous for white alleles. Allele R and r were present in heterozygous condition. But none of them masked the effect of other. Rather these alleles showed incomplete dominance in the form of pink colour. There is no truly dominant allele. So the use of capital and small letter for dominant and recessive are not necessary. Both the alleles are represented by the same letter R. But they are numbered differently to distinguish white From red. Allele for red is designated as R, and the al!ele for white R. Punnett square. indicate that the phenotypic ratio is the same as the genotypic ratio. There is no need of a test cross in this case.12

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Application of Mendel’s principle on incomplete dominance There is blending of flower colours in F1 So it seems against the law of segregation of Mendel. But red and white flowers reappear in F, It indicates that blending does not occur at genetic level. The 1:2:1 ratio in F, shows that the law of segregation is applied on incomplete dominance.

Co-dominance

The dominance in which both alleles of a gene expressed in a heterozygous condition are called codominant. In co-dominance, neither phenotype is dominant. Instead, the individual expresses both phenotypes.

Example I: The most important example is in Landsteiner blood types. The gene for Moo d types has three alleles • A, H. and i. i causes 0 type and is recessive to both A and 13. When a person has both A and B, they have type Al) blood.

Example 2: Anode. example involves cattle. If a homozygous bull and homozygous cow mate (one being red and the other white), then the calves produced will be roan-colored, with a mix of red and white hairs.

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