Dominance and Phenotype
Extending Mendelian Genetics for a Single Gene
A spectrum of dominance indicates that alleles may show varying degrees of dominance and recessiveness.
The spectrum of dominance consists of the following:
Complete dominance is when an allele shows complete dominance, the phenotype of the heterozygote is the same as the phenotype of the dominant homozygote hence, indistinguishable. Seen in the F1 offspring. The F1 plants always looked like one of the two parental varieties.
Codominance is when the two alleles both affect the phenotype in separate, distinguishable ways. An organism displaying a heterozygous genotype displays the phenotype of both alleles of a single gene; neither allele is dominant or recessive to the other.
Ex. The human MN blood group is determined by codominant alleles for two specific molecules located on the surface of red blood cells, the M and N molecules. The phenotype of this blood group, MN is determined by one single gene locus that is able to house two allelic variations. The MN blood group shows that both M and N molecules are present on the red blood cells of individuals heterozygous for the M and N alleles, i.e. MN
The MN phenotype is not intermediate between the M and N phenotypes, rather both the M and N phenotypes are exhibited by the heterozygotes, since both molecules; M and N constitute the gene locus of the red blood cells.
Incomplete dominance is when alleles for some characters that fall in the middle of the spectrum of dominance signify that the phenotype that will be exhibited will be incomplete dominance.
The F1 hybrids will have a phenotype somewhere in between the phenotypes of the two parental varieties.
Ex. Lets consider the cross between red snapdragons and white snapdragons:
P: CRCR * CWCW
F1: CRCW
The F1 hybrids have pink flowers. This phenotype was observed because the heterozygote CRCW has less red pigment than the red homozygote as well as less white pigment than the white homozygote.
From incomplete dominance, evidence is provided for the blending hypothesis of inheritance. The blending hypothesis of inheritance if it existed, it would predict that the red or white trait can never be retrieved from the pink hybrids.
However, the blending hypothesis of inheritance failed when the F1 hybrids were interbred, for example, Interbreeding F1 hybrids produces F2 offspring with a phenotypic ratio of 1:2:1 for both genotype and phenotype; one red to two pink to one white.
In the F1 hybrid, in order to obtain the F2 generation the F1 hybrids are crossed with each other but before they are crossed the formation of their gametes takes place. It is the formation of gametes and the segregation of alleles into these gametes that further confirms that the alleles for characters are heritable factors maintain their identity in the hybrids.
Ex. The segregation of the red-flower and white-flower alleles in the gametes produced by the pink-flowered plants.
Even when alleles exhibit dominance or incomplete dominance at the phenotypic level, both alleles may be expressed at the molecular level.
For ex. A lethal disorder in which brain cells lack a critical enzyme to metabolize a type of lipid is known as Tay-Sachs disease. When this lipid is not metabolized by the enzyme it accumulates and damages the brain.
In a heterozygote, the Tay-Sachs allele is recessive at the organismal level. The organismal level involves if the organism will carry the disease or not; complete dominance. Therefore this shows that in this case the recessive allele has the overall dominance in the organismal level. At the biochemical level, the enzyme activity level is intermediate between both homozygotes; incomplete dominance.
At the molecular level, the alleles are codominant in that each produces its enzyme product, either normal or dysfunctional.
How common an allele is in a population does not depend on the type of allele (dominant or recessive).
The gene that determines human blood groups has three alleles. For example,
the alleles IA and IB are codominant with each other; they both code for an enzyme that attaches a carbohydrate to the surface of red blood cells
the allele I codes for an enzyme that attaches neither A nor B carbohydrate, and is thus recessive to IA and IB.
IAi and IBi
Note: Blood type is critical (significant, essential, important), in transfusions because, if the carbohydrate attached to the donor’s blood cells is foreign to the recipient, the recipient’s immune system will cause clumping of the donated blood cells.
The IA and IB are codominant with the fact that they both code for an ENZYME that attaches a carbohydrate. The carbohydrate attached, however, differs in the two alleles.
Pleiotropy is when a gene that influences many traits rather than just one is pleiotropic. Genes having multiple phenotypic effects in an individual creates a characteristic known as, Pleiotropy. A single gene that has multiple effects. Pleiortopic alleles are responsible for multiple symptoms associated with hereditary diseases in humans, like cystic fibrosis and sickle cell disease.
Epistasis comes from the Greek for stopping a gene at one locus alters the phenotypic expression of a gene at a second locus.
ex. F2 ratios that differ from the 9:3:3:1 ratio indicate epistasis.
Polygenic inheritance is when two or more genes have an additive effect on one character; each gene adds a small amount to the value of the phenotype. Is the opposite of pleiotropy, where a single gene affects several phenotypic characters.
Quantitative characters is due to polygenic inheritance characters that vary in the population along a continuum; traits that are not discrete.
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