Dominant Genes
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Dominant refers to the relationship between two versions of a gene. Individuals receive two versions of each gene, known as alleles, from each parent. If the alleles of a gene are different, one allele will be expressed; it is the dominant gene. The effect of the other allele, called recessive, is masked.
Dominant refers to a relationship between two versions of a gene. If one is dominant, the other one must be not dominant. In that case, we call it recessive. A dominant gene, or a dominant version of a gene, is a particular variant of a gene, which for a variety of reasons, expresses itself more strongly all by itself than any other version of the gene which the person is carrying, and, in this case, the recessive. Now, it usually refers to inheritance patterns frequently used in conjunction with a Punnett square where, if an individual has two versions of a gene, and one is observed to frequently be transferred from one generation to another, then it is called dominant. Biochemically, what is going on in this case is that the genetic variation, for a variety of reasons, can either induce a function in a cell, which is either very advantageous or very detrimental, which the other version of the gene can't cover up or compensate for. In that case, you're going to have a dominant mutation, and that dominant mutation can be benign. It can refer to eye color of one sort or another; that can be can a dominant mutation. Or it can refer to a disease. Huntington's disease, for instance, is a dominant mutation where, if one is carrying that version of the Huntington gene, that mutation, that dominant mutation, will give the individual the disease regardless of what that person's other Huntington's disease gene allele is. That other Huntington's disease gene allele can be perfectly normal, but the person still has the disease because of that one copy of the Huntington's disease gene that is mutated. That is dominance.
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National Human Genome Research Institute
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In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome.[1][2] The first variant is termed dominant and the second recessive. This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new (de novo) or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child (see Sex linkage). Since there is only one copy of the Y chromosome, Y-linked traits cannot be dominant nor recessive. Additionally, there are other forms of dominance such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, and co-dominance, in which different variants on each chromosome both show their associated traits.
Dominance is not inherent to an allele or its traits (phenotype). It is a strictly relative effect between two alleles of a given gene of any function; one allele can be dominant over a second allele of the same gene, recessive to a third and co-dominant with a fourth. Additionally, one allele may be dominant for one trait but not others.
Dominance is a key concept in Mendelian inheritance and classical genetics. Letters and Punnett squares are used to demonstrate the principles of dominance in teaching, and the use of upper case letters for dominant alleles and lower case letters for recessive alleles is a widely followed convention. A classic example of dominance is the inheritance of seed shape in peas. Peas may be round, associated with allele R, or wrinkled, associated with allele r. In this case, three combinations of alleles (genotypes) are possible: RR, Rr, and rr. The RR (homozygous) individuals have round peas, and the rr (homozygous) individuals have wrinkled peas. In Rr (heterozygous) individuals, the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is dominant over allele r, and allele r is recessive to allele R.
Dominance differs from epistasis, the phenomenon of an allele of one gene masking the effect of alleles of a different gene.[3]
The concept of dominance was introduced by Gregor Johann Mendel. Though Mendel, "The Father of Genetics", first used the term in the 1860s, it was not widely known until the early twentieth century. Mendel observed that, for a variety of traits of garden peas having to do with the appearance of seeds, seed pods, and plants, there were two discrete phenotypes, such as round versus wrinkled seeds, yellow versus green seeds, red versus white flowers or tall versus short plants. When bred separately, the plants always produced the same phenotypes, generation after generation. However, when lines with different phenotypes were crossed (interbred), one and only one of the parental phenotypes showed up in the offspring (green, or round, or red, or tall). However, when these hybrid plants were crossed, the offspring plants showed the two original phenotypes, in a characteristic 3:1 ratio, the more common phenotype being that of the parental hybrid plants. Mendel reasoned that each parent in the first cross was a homozygote for different alleles (one parent AA and the other parent aa), that each contributed one allele to the offspring, with the result that all of these hybrids were heterozygotes (Aa), and that one of the two alleles in the hybrid cross dominated expression of the other: A masked a. The final cross between two heterozygotes (Aa X Aa) would produce AA, Aa, and aa offspring in a 1:2:1 genotype ratio with the first two classes showing the (A) phenotype, and the last showing the (a) phenotype, thereby producing the 3:1 phenotype ratio.
Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later. He did introduce the notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today.
In 1928, British population geneticist Ronald Fisher proposed that dominance acted based on natural selection through the contribution of modifier genes. In 1929, American geneticist Sewall Wright responded by stating that dominance is simply a physiological consequence of metabolic pathways and the relative necessity of the gene involved. Wright's explanation became an established fact in genetics, and the debate was largely ended. Some traits may have their dominance influenced by evolutionary mechanisms, however.[4][5][6]
Most animals and some plants have paired chromosomes, and are described as diploid. They have two versions of each chromosome, one contributed by the mother's ovum, and the other by the father's sperm, known as gametes, described as haploid, and created through meiosis. These gametes then fuse during fertilization during sexual reproduction, into a new single cell zygote, which divides multiple times, resulting in a new organism with the same number of pairs of chromosomes in each (non-gamete) cell as its parents.
Each chromosome of a matching (homologous) pair is structurally similar to the other, and has a very similar DNA sequence (loci, singular locus). The DNA in each chromosome functions as a series of discrete genes that influence various traits. Thus, each gene also has a corresponding homologue, which may exist in different versions called alleles. The alleles at the same locus on the two homologous chromosomes may be identical or different.
The blood type of a human is determined by a gene that creates an A, B, AB or O blood type and is located in the long arm of chromosome nine. There are three different alleles that could be present at this locus, but only two can be present in any individual, one inherited from their mother and one from their father.[7]
If two alleles of a given gene are identical, the organism is called a homozygote and is said to be homozygous with respect to that gene; if instead the two alleles are different, the organism is a heterozygote and is heterozygous. The genetic makeup of an organism, either at a single locus or over all its genes collectively, is called its genotype. The genotype of an organism, directly and indirectly, affects its molecular, physical, and other traits, which individually or collectively are called its phenotype. At heterozygous gene loci, the two alleles interact to produce the phenotype.
In complete dominance, the effect of one allele in a heterozygous genotype completely masks the effect of the other. The allele that masks the other is said to be dominant to the latter, and the allele that is masked is said to be recessive to the former.[8] Complete dominance, therefore, means that the phenotype of the heterozygote is indistinguishable from that of the dominant homozygote.
A classic example of dominance is the inheritance of seed shape (pea shape) in peas. Peas may be round (associated with allele R) or wrinkled (associated with allele r). In this case, three combinations of alleles (genotypes) are possible: RR and rr are homozygous and Rr is heterozygous. The RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is completely dominant to allele r, and allele r is recessive to allele R.
Incomplete dominance (also called partial dominance, semi-dominance or intermediate inheritance) occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. For example, the snapdragon flower color is homozygous for either red or white. When the red homozygous flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in the four o'clock plant wherein pink color is produced when true-bred parents of white and red flowers are crossed. In quantitative genetics, where phenotypes are measured and treated numerically, if a heterozygote's phenotype is exactly between (numerically) that of the two homozygotes, the phenotype is said to exhibit no dominance at all, i.e. dominance exists only when the heterozygote's phenotype measure lies closer to one homozygote than the other.
When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Pink:White).[9]
Co-dominance occurs when the contributions of both alleles are visible in the phenotype.
For example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (IA, IB) and dominant over the recessive i at the ABO locus. The IA and IB alleles produce different modifications. The enzyme coded for by IA adds an N-acetylgalactosamine to a membrane-bound H antigen. The IB enzyme adds a galactose. The i allele produces no modification. Thus the IA and IB alleles are each dominant to i (IAIA and IAi individuals both have type A blood, and IBIB and IBi individuals both have type B blood), but IAIB individuals have both modifications on their blood cells and thus have type AB blood, so the IA and IB alleles are said to be co-dominant.
Another example occurs at the locus for the beta-globin component of hemoglobin, where the three molecular phenotypes of HbA/HbA, HbA/HbS, and HbS/HbS are all distinguishable by protein electrophoresis. (The medical condition produced by the heterozygous genotype is called sickle-cell trait and is a milder condition distinguishable from sickle-cell anemia, thus the alleles show incomplete dominance with respect to anemia, see above). For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are transcribed into RNA.
Co-dominance, where allelic products co-exist in the phenotype, is different from incomplete dominance, where the quantitative interaction of allele products produces an intermediate phenotype. For example, in co-dominance, a red homozygous flower and a white homozygous flower will produce offspring that have red and white spots. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Spotted:White). These ratios are the same as those for incomplete dominance. Again, this classical terminology is inappropriate β in reality such cases should not be said to exhibit dominance at all.
While it is often convenient to talk about a recessive allele or a dominant trait, dominance 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 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.
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, and 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 the population. Both dominant and recessive alleles can be extremely common or extremely rare.
This section is about gene notations that identify dominance. For modern formal nomenclature, see Gene nomenclature.
In genetics, symbols began as algebraic placeholders. When one allele is dominant to another, the oldest convention is to symbolize the dominant allele with a capital letter. The recessive allele is assigned the same letter in lower case. In the pea example, once the dominance relationship between the two alleles is known, it is possible to designate the dominant allele that produces a round shape by a capital-letter symbol R, and the recessive allele that produces a wrinkled shape by a lower-case symbol r. The homozygous dominant, heterozygous, and homozygous recessive genotypes are then written RR, Rr, and rr, respectively. It would also be possible to designate the two alleles as W and w, and the three genotypes WW, Ww, and ww, the first two of which produced round peas and the third wrinkled peas. The choice of "R" or "W" as the symbol for the dominant allele does not pre-judge whether the allele causing the "round" or "wrinkled" phenotype when homozygous is the dominant one.
A gene may have several alleles. Each allele is symbolized by the locus symbol followed by a unique superscript. In many species, the most common allele in the wild population is designated the wild type allele. It is symbolized with a + character as a superscript. Other alleles are dominant or recessive to the wild type allele. For recessive alleles, the locus symbol is in lower case letters. For alleles with any degree of dominance to the wild type allele, the first letter of the locus symbol is in upper case. For example, here are some of the alleles at the a locus of the laboratory mouse, Mus musculus: Ay, dominant yellow; a+, wild type; and abt, black and tan. The abt allele is recessive to the wild type allele, and the Ay allele is codominant to the wild type allele. The Ay allele is also codominant to the abt allele, but showing that relationship is beyond the limits of the rules for mouse genetic nomenclature.
Rules of genetic nomenclature have evolved as genetics has become more complex. Committees have standardized the rules for some species, but not for all. Rules for one species may differ somewhat from the rules for a different species.[10][11]
Although any individual of a diploid organism has at most two different alleles at any one locus (barring aneuploidies), most genes exist in a large number of allelic versions in the population as a whole. If the alleles have different effects on the phenotype, sometimes their dominance relationships can be described as a series.
For example, coat color in domestic cats is affected by a series of alleles of the TYR gene (which encodes the enzyme tyrosinase). The alleles C, cb, cs, and ca (full colour, Burmese, Siamese, and albino, respectively) produce different levels of pigment and hence differ
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Dominant Genes
