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In genetics, the term dominant gene refers to the allele that causes a phenotype that is seen in a heterozygous genotype. Every person has two copies of every gene, one from mother and one from father. If a genetic trait is dominant, a person only needs to inherit one copy of the gene for the trait to be expressed.
Dominance/recessiveness refers to phenotype, not genotype. Consider sickle cell anemia as an example. The sickle cell genotype is caused by a single base pair change in the beta-globin gene. There are several phenotypes associated with the sickle genotype: 1) anemia (a recessive trait), 2) blood cell sickling (partially dominant), 3) altered beta-globin electrophoretic mobility (codominant), and 4) resistance to malaria (dominant). This example demonstrates that one can only refer to dominance/recessiveness with respect to individual phenotypes.
A dominant gene when written in a genotype is always written before the recessive gene in a heterozygous pair. A heterozygous genotype is written Aa, not aA.
- For other non-genetic uses of the term "dominance", see Dominance.
In genetics, dominance relationship refers to how the alleles for a single locus interact to produce a phenotype. For example, flower color in sweet peas (Lathyrus odoratus) is controlled by a single gene with two alleles. The three genotypes are PP, Pp, and pp. The flower color for PP (purple) and pp (white) do not depend on the dominance relationship. However, the heterozygote Pp could theoretically have many different colors: purple, white or pink. The exact color it has reflects the dominance relationship.
There are three kinds of dominance relationships:
Traits inherited in a dominant-recessive pattern are often said to "follow Mendelian inheritance".
The dominant/recessive relationship is made possible by the fact that most higher organisms are diploid: that is, most of their cells have two copies of each chromosome -- one copy from each parent. Polyploid organisms have more than two copies of each chromosome, and follow similar rules of dominance, but for simplicity will not be discussed here.
Humans, a diploid species, typically have 23 pairs of chromosomes, for a total of 46. In regular reproduction, half come from the mother, and half come from the father (see meiosis for further discussion of how this happens, and chromosome for less usual possibilities in humans).
Relationship to other genetics concepts
Although humans have only 23 homologous chromosome pairs (22 autosomal chromosomes and two distinct sex chromosomes, X and Y), it is estimated that they contain 20,000-25,000 genes, each of which is related to some biological trait of the organism. Many genes are strung together in a single chromosome.
Humans each carry 46 chromosomes (23 pairs), with a single sex chromosome and 22 autosomes coming from each parent. Each chromosomal pair has the same genes, although it is generally unlikely that homologous genes from each parent will be identical in sequence. The specific variations possible for a single gene are called alleles: for a single eye-color gene, there may be a blue eye allele, a brown eye allele, a green eye allele, etc. Consequently, a child may inherit a blue eye allele from their mother and a brown eye allele from their father. The dominance relationships between the alleles control which traits are and are not expressed.
Consider the simple example in peas of flower color, first studied by Gregor Mendel. The dominant allele is purple and the recessive allele is white. In a given individual, the two corresponding alleles of the chromosome pair fall into one of three patterns:
- both alleles purple
- both alleles white
- one allele purple and one allele white
If the two alleles are the same (homozygous), the trait they represent will be expressed. But if the individual carries one of each allele (heterozygous), only the dominant one will be expressed. The recessive allele will simply be suppressed.
Simple dominance in pedigrees
Dominant traits are recognisable by the fact that they do not skip generations, as recessive traits do. It is therefore quite possible for two parents with purple flowers to have a white flowers among their progeny, but two such white offspring could not have purple offspring (although very rarely, one might be produced by mutation). In this situation, the purple individuals in the first generation must have both been heterozygous (carrying one copy of each allele).
- Main article: Punnett square
The genetic combinations possible with simple dominance can be expressed by a diagram called a Punnett square. One parent's alleles are listed across the top and the other parent's alleles are listed down the left side. The interior squares represent possible offspring, in the ratio of their statistical probability. In this example, P represents the dominant purple-colored allele and p the recessive white-colored allele. If both parents are purple-colored and heterozygous, it would look like this:
|P||P P||P p|
|p||p P||p p|
In the PP and Pp cases, the offspring is purple colored due to the dominant P. Only in the pp case is there expression of the recessive white-colored phenotype.
Human traits governed by simple dominance
Examples of dominant genes include the tumor supressor genes BRCA1 and BRCA2. Mutations in these genes lead to the development of breast cancer, as the tumour-suppressing functions of the proteins encoded by the genes have been disabled.
Another example of an autosomal dominant disorder is Huntington's disease, which is a neurological disorder resulting in impaired motor function. The mutant gene results in an abnormal protein, containing large repeats of amino acid glutamine. This defective protein is toxic to neural tissue, resulting in the characteristic symptoms of the disease.
|Curled Up Nose||Roman Nose|||
|Clockwise Hair Whorl||Counter-clockwise Hair Whorl|||
|Can Roll Tongue||Can't Roll Tongue|||
|Widow's Peak||No Widow's Peak|||
|Facial Dimples||No Facial Dimples|||
|Able to taste PTC||Unable to taste PTC|||
|Earlobe hangs||Earlobe attaches at base|||
|Middigital hair (fingers)||No middigital hair|||
|No hitchhiker's thumb||Hitchhiker's thumb|||
|Tip of pinkie bends in||Pinkie straight|||
|Oval face||Square face|||
|Cleft chin||no cleft chin|||
|Broad eyebrow||Slender eyebrow|||
|Separated eyebrows||Joined eyebrows|||
|Long eyelashes||Short eyelashes|||
|Almond eyes||Round eyes|||
|Freckles||No freckles|||
|Wet-type earwax||Dry-type earwax|||
|Left thumb on top of interlocking fingers||Right thumb on top of interlocking fingers|||
Some genetic diseases carried by dominant and recessive alleles
- Main article: Genetic disorder
|Some types of Dwarfism||recessive|
Dominant alleles are not necessarily more common or more desirable.
Some genetic diseases inherited as autosomal dominant traits
- Achondroplasia (or Chondrodystrophy)(most common form of dwarfism without mental retardation)
- Centronuclear myopathy (but other forms are autosomal recessive or X-linked).
- Familial hypercholesterolemia
- Holt-Oram syndrome
- Nail-Patella syndrome
- Huntington's disease
- Hypertrophic cardiomyopathy
- Marfan syndrome
- Osteogenesis Imperfecta (type I,II and IV, type III is progressivly deforming and is usually inherited as Autosomal recessive the penetrance is variable on this type)
- Polycystic kidney disease(Autosomal dominant (adult) type)
- Von Hippel-Lindau disease
- Familial hemiplegic migraine
- Ehlers-Danlos Syndrome
- Gardner's syndrome
In incomplete dominance (sometimes called partial dominance), a heterozygous genotype creates an intermediate phenotype. In this case, only one allele (usually the wild type) at the single locus is expressed, creating an intermediate phenotype. A cross of two intermediate phenotypes (= monohybrid heterozygotes) will result in the reappearance of both parent phenotypes and the intermediate phenotype.
The classic example of this is the colors of carnations.
R is the allele for red pigment. R' is the allele for no pigment.
Thus, RR offspring make a lot of red pigment and appear red. R'R' offspring make no red pigment and appear white. RR' and R'R offspring make a little bit of red pigment and therefore appear pink.
In co-dominance, neither phenotype is dominant. Instead, the individual expresses both phenotypes. The most important example is in Landsteiner blood types. The gene for blood types has three alleles: A, B, and i. i causes O type and is recessive to both A and B. When a person has both A and B, they have type AB blood.
Another example involves cattle. If a homozygous bull and homozygous cow mate (one being red and the other white), then the calves produced will be roan-colored, with a mix of red and white hairs.
Example Punnett square for a father with A and i, and a mother with B and i:
Amongst the very few co-dominant genetic diseases in humans, one relatively common one is A1AD, in which the genotypes Pi00, PiZ0, PiZZ, and PiSZ all have their more-or-less characteristic clinical representations.
Most molecular markers are considered to be co-dominant.
Most loss-of-function mutations are recessive. However, some are dominant and are called "dominant negative" mutations. Typically, a dominant negative mutation results in a protein that is structurally similar to the wild-type protein, but which has lost the normal function. Such proteins may be competitive inhibitors of the normal protein functions.
Mechanisms of dominance
Many genes code for enzymes. Consider the case where someone is homozygous for some trait. Both alleles code for the same enzyme, which causes a trait. Only a small amount of that enzyme may be necessary for a given phenotype. The individual therefore has a surplus of the necessary enzyme. Let's call this case "normal". Individuals without any functional copies cannot produce the enzyme at all, and their phenotype reflects that. Consider a heterozygous individual. Since only a small amount of the normal enzyme is needed, there is still enough enzyme to show the phenotype. This is why some alleles are dominant over others.
In the case of incomplete dominance, the single dominant allele does not produce enough enzyme, so the heterozygotes shows some different phenotype. For example, fruit color in eggplants is inherited in this manner. A purple color is caused by two functional copies of the enzyme, with a white color resulting from two non-functional copies. With only one functional copy, there is not enough purple pigment, and the color of the fruit is a lighter shade, called violet.
Some non-normal alleles can be dominant. The mechanisms for this are varied, but one simple example is when the functional enzyme is composed of several subunits. In this case, if any of the subunits are nonfunctional, the entire enzyme is nonfunctional. In the case of a single subunit with a functional and nonfunctional allele (heterozygous individual), the concentration of functional enzymes is 50% of normal. If the enzyme has two identical subunits, the concentration of functional enzyme is 25% of normal. For four subunits, the concentration of functional enzyme is about 6% of normal. This may not be enough to produce the wild type phenotype. There are other mechanisms for dominant mutants.
It is important to note that most genetic traits are not simply controlled by a single set of alleles. Often many alleles, each with their own dominance relationships, contribute in varying ways to complex traits.
Some medical conditions may have multiple inheritance patterns, such as in centronuclear myopathy or myotubular myopathy, where the autosomal dominant form is on chromosome 19 but the sex-linked form is on the X chromosome.
Codominance and incomplete dominance
In certain cases, a "blend" of genes will occur because neither of the two genes of a genotype are dominant over the other. As an example, in blood cells, the trait for blood type has three different alleles: type A, type B, or type O, with O being recessive. If a father passes a gamete with the allele of type A and the mother passes on type B, then codominance results, with the offspring being type AB since neither allele type dominates the other.
Incomplete dominance occurs when certain of the recessive gene appears within the phenotype of the organism, causing a blend in between both the dominant and recessive gene.
A mutation whose gene product adversely affects the normal, wild-type gene product within the same cell. This usually occurs if the product can still interact with the same elements as the wild-type product, but block some aspect of its function.
1. A mutation in a transcription factor that removes the activation domain, but still contains the DNA binding domain. This product can then block the wild-type transcription factor from binding the DNA site leading to reduced levels of gene activation.
2. A protein that is functional as a dimer. A mutation that removes the functional domain, but retains the dimerization domain would cause a dominate negative phenotype, because some fraction of protein dimers would be missing one of the functional domains.
The development of phenotype
|Key concepts: Genotype-phenotype distinction | Norms of reaction | Gene-environment interaction | Heritability | Quantitative genetics|
|Genetic architecture: Dominance relationship | Epistasis | Polygenic inheritance | Pleiotropy | Plasticity | Canalisation | Fitness landscape|
|Non-genetic influences: Epigenetic inheritance | Epigenetics | Maternal effect | dual inheritance theory|
|Developmental architecture: Segmentation | Modularity|
|Evolution of genetic systems: Evolvability | Mutational robustness | Evolution of sex|
|Influential figures: C. H. Waddington | Richard Lewontin|
|Debates: Nature versus nurture|
|List of evolutionary biology topics|
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