Incomplete dominance occurs when two different alleles in a single gene both show dominance in a resulting phenotype, creating a distinctive third phenotype. It is different from codominance, where two apparently dominant alleles each contribute a separate characteristic. In humans, incomplete dominance is seen in many traits, such as lip protrusion, tone of male voices, and hair type. Even diseases, such as Tay-Sachs disease, can be governed by incomplete mastery.
Incomplete dominance, or mixed inheritance, is a term used in genetics when two different alleles in a single gene both show dominance in a resulting phenotype, which is an observable trait or characteristic. It is not synonymous with codominance, where two apparently dominant alleles each contribute a separate characteristic. In incomplete dominance, the two allele genotypes fuse their phenotypes into a distinctive third.
Alleles are different versions of the same genes. They can be dominant, recessive, co-dominant or incompletely dominant. There are normally two alleles per gene, each parent contributing one to an offspring. Alleles determine the physical characteristics, or phenotypes, of living organisms.
Genes can be homozygous, meaning they carry one pair of identical alleles, or heterozygous, meaning they carry different alleles. Many alleles are either dominant or recessive, so if a gene has a dominant allele, the phenotype will show the dominant trait whether the gene is homozygous or heterozygous. Recessive genes will only be exhibited if the gene is homozygous for the recessive alleles. The alleles can also be neither dominant nor recessive as in the case of dominance and incomplete codominance. Incompleteness and codominance, however, are only seen in heterozygous genes as a gene must have two different alleles to share dominance between them.
For example, if a flower has a dominant allele for red petals and a recessive allele for white petals, the flower’s petals will be red. In alleles showing incomplete dominance, neither the red nor the white allele will be recessive or dominant, and instead, if a flower is heterozygous, the traits will fuse together, creating pink flower petals. This is different from codominance, which would create a flower with both red and white petals.
It is important to note, however, that the alleles themselves do not fuse together to create a third allele type, only the resulting phenotypes fuse. In cases such as color, geneticists believe these resulting phenotypes are caused by the production of pigments. If the red alleles always tell the plant to produce red pigments and the white alleles have no command to produce pigments, then flowers with two red alleles will be a darker color than those with only one.
In humans, incomplete dominance is seen in many traits, such as lip protrusion, tone of male voices, and hair type. For example, if one parent has completely straight hair and one parent has curly hair, the resulting child will have neither straight nor curly hair, but instead a blend of the two: wavy hair. Even diseases, such as Tay-Sachs disease, can be governed by incomplete mastery. The gene that produces Tay-Sachs antibodies generates only half as many antibodies in heterozygous individuals as it does in homozygous individuals, leaving heterozygous children susceptible to the disease.
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