User:Kirsten.bures/Dominance (genetics)

Addressing Common Misconceptions[edit][edit]
While it is often convenient to talk about a recessive allele or a dominant trait, dDominance is not inherent to either an allele or its phenotype. Dominance is a relationship between two alleles of a gene and their associated phenotypes. A "dominant" allele is dominant to a particular allele of the same gene that can be inferred from the context, but it may be recessive to a third allele, and codominant to a fourth. Similarly, a "recessive" trait is a trait associated with a particular recessive allele implied by the context, but that same trait may occur in a different context where it is due to some other gene and a dominant allele. .

'''Dominance relates to the relationship between two versions of a gene. A dominant trait is usually in correspondence to inheritance patterns that can be seen in Punnett Squares. If an individual has two versions of a gene, then the gene that is frequently observed in further generations is considered "dominant".'''

Dominance is unrelated to the nature of the phenotype itself, that is, whether it is regarded as "normal" or "abnormal," "standard" or "nonstandard," "healthy" or "diseased," "stronger" or "weaker," or more or less extreme. A dominant or recessive allele may account for any of these trait types.

'''In genetics, there are a few misconceptions that are fairly common. It is thought that a dominant trait is "stronger" and "overpowers" a recessive trait. Dominant traits are also assumed more likely to be inherited as well as more prevalent in a population. The idea of dominant traits being male or masculine is also a common assumption in the world of genetics. Any experienced biologist knows that these ideas are not factual, but to a beginner these ideas standout. But, if these ideas are not factual, why do they emerge at all? The differing ideas of dominance when it comes to genetics stems from commonly known definitions of the word dominance in a non-genetic setting. Dominance is thought to be controlling, strong, and powerful, giving genetic dominance this faulty stigma.'''

Dominance does not determine whether an allele is deleterious, neutral, or advantageous. However, selection must operate on genes indirectly through phenotypes and dominance affects the exposure of alleles in phenotypes, hence the rate of change in allele frequencies under selection. Deleterious recessive alleles may persist in a population at low frequencies, with most copies carried in heterozygotes, at no cost to those individuals. These rare recessives are the basis for many hereditary genetic disorders.

Dominance is also unrelated to the distribution of alleles in a population. Both dominant and recessive alleles can be extremely common or extremely rare.

Molecular mechanisms[edit][edit]
The molecular basis of dominance was unknown to Mendel. It is now understood that a gene locus includes a long series (hundreds to thousands) of bases or nucleotides of deoxyribonucleic acid (DNA) at a particular point on a chromosome. The central dogma of molecular biology states that "DNA makes RNA makes protein", that is, that DNA is transcribed to make an RNA copy, and RNA is translated to make a protein. In this process, d Different alleles at a locus may or may not be transcribed, and if transcribed may be translated to slightly into different versions of the same protein, called isoforms. These p P roteins often function as enzymes that catalyze chemical reactions in the cell, which directly or indirectly producing e phenotypes. In a ny diploid organism, the DNA sequences of the two alleles present at any gene locus may be identical (homozygous) or different (heterozygous). Even if the gene locus is heterozygous at the level of the DNA sequence, the proteins made by each allele may be identical. In the absence of any difference between the protein products, neither allele can be said to be dominant (see co-dominance, above). Even if the two protein products are slightly different (allozymes), it is likely that they produce the same phenotype with respect to enzyme action, and again neither allele can be said to be dominant.

Zygosity Loss of function and haplosufficiency[edit][edit]
Historically, Mendel's Law of Independent Assortment assumed that alleles will sort independently, with one allele being "dominant". However, the degree at which a gene's allele expression levels, Zygosity, might affect dominance. Within a diploid organism, these would be defined by the Haplotype interactions of the alleles. Gene haploidy may result in a single, functional allele making sufficient protein to produce a phenotype identical to that of the homozygote  An organism homozygous for the non-functional allele will generally show a distinctive phenotype, due to the absence of the protein product. Three general types of haplotype interactions are possible:
 * 1) Haplosufficiency. In a diploid, a functional allele of a haplosufficient gene would be considered dominant, while a non-functional allele would be considered recessive . For example, suppose the standard amount of enzyme produced in the functional homozygote is 100%, with the two functional alleles contributing 50% each. The single functional allele in the heterozygote produces 50% of the standard amount of enzyme, which is sufficient to produce the standard phenotype. If the heterozygote and the functional-allele homozygote have identical phenotypes, the functional allele is dominant to the non-functional allele. This occurs at the albino gene locus: the heterozygote produces sufficient enzyme to convert the pigment precursor to melanin, and the individual has standard pigmentation.  For example, in humans and other organisms, the unpigmented skin of the albino phenotype results when an individual is homozygous for an allele that encodes a non-functional version of an enzyme needed to produce the skin pigment melanin.
 * 2) Incomplete Haploinsufficiency. Less commonly, the presence of a single functional allele gives a phenotype that is not normal, but less severe, than that of the non-functional homozygote. This occurs when the functional allele is not haplo-sufficient thus the terms haplo-insufficiency and incomplete dominance are typically applied to these cases. The intermediate interaction occurs where the heterozygous genotype produces a phenotype intermediate between the two homozygotes. Depending on which of the two homozygotes the heterozygote most resembles, one allele is said to show incomplete dominance over the other. For example, in humans the Hb gene locus is responsible for the Beta-chain protein (HBB) that is one of the two globin proteins that make up the blood pigment hemoglobin. Many people are homozygous for an allele called HbA; some persons carry an alternative allele called HbS, either as homozygotes or heterozygotes. The hemoglobin molecules of HbS/HbS homozygotes undergo a change in shape that distorts the morphology of the red blood cells, and causes a severe, life-threatening form of anemia called sickle-cell anemia. Persons heterozygous HbA/HbS for this allele have a much less severe form of anemia called sickle-cell trait. Because the disease phenotype of HbA/HbS heterozygotes is more similar to but not identical to the HbA/HbA homozygote, the HbA allele is said to be incompletely dominant to the HbS allele.
 * 3) Complete Haploinsufficiency. Rarely, a A single functional allele in the heterozygote may produce insufficient gene product for any function of the gene, causing the usually non-functional alleles to become dominant. The phenotype will then resemble that of a homozygote with non-functional allele instead of the wild type. This is called complete haploinsufficiency.  is very unusual.  In these cases, tT he non-functional allele would be said to be dominant to the wild-type phenotype's functional allele. This situation may occur when the non-functional allele produces a defective protein that interferes with the proper function of the protein produced by the standard allele. The presence of the defective protein "dominates" the standard protein, and the disease phenotype of the heterozygote more closely resembles that of the homozygote for two defective alleles. The term "dominant" is often incorrectly applied to defective alleles whose homozygous phenotype has not been examined, but which cause a distinct phenotype when heterozygous with the normal allele. This phenomenon occurs in a number of trinucleotide repeat diseases, one example being Huntington's disease.  In Huntington's Disease, complete haploinsufficiency is seen in that a mutation causes a disease by a dominant effect of the mutant protein. Another example is Marfan syndrome, an inherited connective tissue disorder, caused by a mutation in the fibrillin-1 (FBN1) gene. One normal copy of the FBN1 gene is inherited from one parent while a dominant abnormal FBN1 gene copy in inherited by another parent .

(pasted from above - I don't know where exactly this would be placed) Even if the gene locus is heterozygous at the level of the DNA sequence, the proteins made by each allele may be identical. In the absence of any difference between the protein products, neither allele can be said to be dominant (see co-dominance, above). Even if the two protein products are slightly different (allozymes), it is likely that they produce the same phenotype with respect to enzyme action, and again neither allele can be said to be dominant.

An organism homozygous for the non-functional allele will generally show a distinctive phenotype, due to the absence of the protein product.