Dominant allele

A dominant allele is a variation of a gene that will produce a certain phenotypeeven in the presence of other alleles. A dominant allele typically codes for a functional protein. The allele is dominant because one copy of the allele produces enough of the enzyme to supply a cell large amount of a given product. Some traits are based on the creation of a product, such as pigment molecules for hair colour, ion channels for proper cell function, and other traits that depend on a functional enzyme. Other traits are based on the lack of an enzyme or an enzyme that does not function efficiently. The lack of the enzyme product creates a completely different phenotype.

When one dominant allele is completely dominant over another allele, the other allele is known as recessive. However, there are also dominant and recessive situations. co-dominance incomplete. At incomplete dominancetwo dominant alleles can mix to create a third phenotype, an species of admixture between two phenotypes. Think of the alleles of a white and a red flower mixing in a heterozygous to produce a pink flower. In codominance, different alleles are expressed in different areas, creating a unique pattern. Think of codominance as the spots on a cow.

To describe an allele as a dominant allele, you must make reference to another allele. Dominant and recessive are just descriptions of a relationship between alleles. Some alleles of a gene compete differently with different alleles. They can be dominant for one allele and recessive for another. It all depends on the protein they create, how those proteins interact with each other, and how the whole system interacts with the environment. While most of the simple examples given are of purely dominant/recessive relationships, in reality the interactions are much more complex.

Dominant allele types

Dominance complete

In cases of complete dominance, a dominant allele completely masks the effects of a recessive allele. This can only be seen in heterozygous individuals. Homozygous dominant individuals have two dominant alleles, which produce the same enzyme. In homozygous recessive individuals, no dominant allele is present and the phenotype only reflects the action of the recessive alleles. In the heterozygous individual, the phenotype appears to be the same as in the homozygous dominant individual. This is the defining characteristic of complete dominance; the dominant allele completely hides the presence of the recessive allele from the observer.

This can be seen in pea plants, in the colour of their peas. The dominant allele produces a yellow pigment through the enzyme it encodes. One allele is sufficient to produce enough yellow pigment for the whole pea plant to produce yellow pigment. plant looks yellow. The green allele produces a non-functional enzyme and no yellow is created in a homozygous recessive plant. The yellow allele dominates the green allele and the recessive green allele dominates the yellow allele. The picture below shows the cross between a heterozygous individual on the left and a homozygous recessive individual on the top.

Incomplete dominance

Incomplete dominance is a case where dominant alleles produce different enzymes, but one allele does not outperform the other in phenotypic expression. Both alleles are considered dominant alleles, relative to each other, and produce the same amount of effect physically. In some flowers, this effect can be seen dramatically in heterozygous individuals. The Punnett square cross of two flowers showing incomplete dominance is shown below.

Red flowers have two copies of the red allele. This allows them to produce enough red pigment to make an intense red colour. The allele that produces white flowers creates a non-functional allele. Without its red colour, the flower appears white in individuals with two white alleles. However, the red allele is not dominant over the white allele because in the heterozygous individual the flowers are pink, not red. Incomplete dominance affects many different enzymes, not just the ones that control the things we see. Incomplete dominance is often responsible for different organisms producing high, medium and low levels of certain substances.

Codominance

In codominance, dominant alleles are expressed, but are segregated from each other in which areas they are expressed. This can be easily seen in cattle. In the picture below, cattle have two codominant alleles: red and white. A cow with two red alleles is only red, and a cow with only white alleles is only white. However, a cow with both alleles will show red and white spots all over her body. The picture is a bit misleading, because the distribution of the spots would be random across the cows, and each would look unique.

The gene that is activated is determined during development, and different alleles are activated in different areas according to chance and signals sent by the developing body. Co-dominance is responsible for the great variety and patterns seen in different animals, especially dogs and cats. Because dogs and cats have been artificially selected to produce a wide variety of coats, there are many alleles that exist in co-dominance with other alleles in the population.

• Codominance: when two dominant alleles are expressed separately, in different areas of a organism.
• Domain incompletewhen two dominant alleles are expressed equally, in each cell.
• Recessive allele: an allele that is completely phenotypically masked by a dominant allele.
• Genotype: In sexually reproducing organisms, the set of two alleles that determine the phenotype of an organism.