Incomplete dominance, codominance & multiple alleles (article) | Khan Academy
Mendel worked on pea plants, but his principles apply to traits in plants of second-generation (F2) progeny with dominant or recessive traits. Since the Y (yellow) allele is dominant over the G (green) allele for pea plants, let us define "A" as being the dominant normal allele and "a" as the recessive. Genetic Dominance: Genotype-Phenotype Relationships These allelic interactions were not exclusively recessive or dominant, and they greatly enriched our understanding of how . Gregor Mendel and the Principles of Inheritance.
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. 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. Complete dominance[ edit ] 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. 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.Alleles and Genes
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[ edit ] This Punnett square illustrates incomplete dominance. In this example, the red petal trait associated with the R allele recombines with the white petal trait of the r allele. The plant incompletely expresses the dominant trait R causing plants with the Rr genotype to express flowers with less red pigment resulting in pink flowers.
The colors are not blended together, the dominant trait is just expressed less strongly. 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 geneticswhere 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. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1: This diagram shows co-dominance.
In this example a white bull WW mates with a red cow RRand their offspring exhibit co-dominance expressing both white and red hairs. Co-dominance occurs when the contributions of both alleles are visible in the phenotype.
For example, in the ABO blood group systemchemical 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 the membrane-bound H antigen.
The IB enzyme adds a galactose. The i allele produces no modification.
The medical condition produced by the heterozygous genotype is called sickle-cell trait and is a milder condition distinguishable from sickle-cell anemiathus 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. These ratios are the same as those for incomplete dominance. Again, note that this classical terminology is inappropriate — in reality such cases should not be said to exhibit dominance at all. Addressing common misconceptions[ edit ] 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.
genetics - Recessive is Dominant to Dominant? - Biology Stack Exchange
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. Some dominant alleles are extremely common, while others are extremely rare. The most common allele in a population may be recessive when combined with some rare variants.
Nomenclature[ edit ] 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.
Note that 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.
Sexually reproducing species, including people and other animals, have two copies of each gene. The two copies, called alleles, can be slightly different from each other. Proteins affect traits, so variations in protein activity or expression can produce different phenotypes. A dominant allele produces a dominant phenotype in individuals who have one copy of the allele, which can come from just one parent. For a recessive allele to produce a recessive phenotype, the individual must have two copies, one from each parent.
An individual with one dominant and one recessive allele for a gene will have the dominant phenotype. The terms are confusing and often misleading Dominant and recessive inheritance are useful concepts when it comes to predicting the probability of an individual inheriting certain phenotypes, especially genetic disorders. But the terms can be confusing when it comes to understanding how a gene specifies a trait.
This confusion comes about in part because people observed dominant and recessive inheritance patterns before anyone knew anything about DNA and genes, or how genes code for proteins that specify traits. The critical point to understand is that there is no universal mechanism by which dominant and recessive alleles act. Whether an allele is dominant or recessive depends on the particulars of the proteins they code for. The terms can also be subjective, which adds to the confusion.
The same allele can be considered dominant or recessive, depending on how you look at it. The sickle-cell allele, described below, is a great example. However, these patterns apply to few traits. The sickle-cell allele Inheritance patterns Sickle-cell disease is an inherited condition that causes pain and damage to organs and muscles.
Multiple alleles, incomplete dominance, and codominance
Instead of having flattened, round red blood cells, people with the disease have stiff, sickle-shaped cells. The long, pointy blood cells get caught in capillaries, where they block blood flow. The disease has a recessive pattern of inheritance: People with just one copy are healthy.
In addition to causing disease, the sickle-cell allele makes people who carry it resistant to malaria, a serious illness carried by mosquitos. Malaria resistance has a dominant inheritance pattern: This is the very same allele that, in a recessive inheritance pattern, causes sickle-cell disease! People with two copies of the sickle-cell allele have many sickled red blood cells. People with one sickle-cell allele and one normal allele have a small number of sickled cells, and their cells sickle more easily under certain conditions.
So we could say that red blood cell shape has a co-dominant inheritance pattern. That is, individuals with one copy of each allele have an in-between phenotype. So is the sickle cell allele dominant, recessive, or co-dominant? It depends on how you look at it. Protein function If we look at the proteins the two alleles code for, the picture becomes a little more clear.