Dominance relationship
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- 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, e.g., purple, white, or a light purple. The color of flowers produced by the heterozygous plants depends on the dominance relationship between the two alleles in question.
There are three main kinds of dominance relationships:
- Simple dominance (simple Mendelian inheritance)
- Incomplete (partial) dominance
- Co-dominance
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[edit] Chromosome redundancy
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. Similarly, organisms that are normally haploid do not show dominance relationships.
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).
[edit] Relationship to other genetics concepts
Humans have 23 homologous chromosome pairs (22 pairs of autosomal chromosomes and two distinct sex chromosomes, X and Y). It is estimated that the human genome contains 20,000-25,000 genes "[1]". 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.
An example of an autosomal dominant human 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 the amino acid glutamine. This defective protein is toxic to neural tissue, resulting in the characteristic symptoms of the disease.
A list of human traits that follow a simple inheritance pattern can be found in human genetics. Humans have several genetic diseases, often but not always caused by recessive genes.
[edit] Simple dominance
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.
[edit] Simple dominance in pedigrees
Dominant traits are recognizable 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).
[edit] 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 the previous example of flower color, P represents the dominant purple-colored allele and p the recessive white-colored allele. If both parents are purple-colored and heterozygous (Pp), the Punnet square for their offspring would be:
| P | p | |
| 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.
[edit] Incomplete dominance
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 | R' |
|---|---|
| R RR | RR' |
| R' RR' | R'R' |
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.
[edit] Codominance
In codominance, neither phenotype is completely dominant. Instead, the heterozygous individual expresses both phenotypes. A common example is the ABO blood group system. The gene for blood types has three alleles: A, B, and i. i causes O type and is recessive to both A and B. The A and B alleles are codominant with each other. When a person has both A and B, they have type AB blood.
Example Punnett square for a father with A and i, and a mother with B and i:
| A | i | |
| B | AB | B |
| i | A | O |
Amongst the very few codominant 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 codominant. A roan horse has codominant follicle genes, expressing individual red and white follicles.
[edit] Dominant negative
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.
[edit] 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 show 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.
[edit] Other factors
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.
[edit] See also
[edit] Notes
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| 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 | 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 | |
