Dominant alleles show their effect even if the individual only has one copy of the allele also known as being heterozygous. For example, the allele for brown eyes is dominant, therefore you only need one copy of the 'brown eye' allele to have brown eyes although, with two copies you will still have brown eyes. If both alleles are dominant, it is called codominance.
The resulting characteristic is due to both alleles being expressed equally. An example of this is the blood group AB which is the result of codominance of the A and B dominant alleles. Recessive alleles only show their effect if the individual has two copies of the allele also known as being homozygous.
For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the 'blue eye' allele.
Related Content:. What is a gene? What is inheritance? What is genetic variation? What are single gene disorders? What is a genetic disorder? 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. 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. Sickle-cell disease is an inherited condition that causes pain and damage to organs and muscles.
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: only individuals with two copies of the sickle-cell allele have the disease.
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: just one copy of the sickle cell allele is enough to protect against infection.
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. Although the offspring may show a variety of phenotypes, each one will lie along a continuum bracketed by the homozygous parental phenotypes. In Figure 1, for example, neither flower color red or white is fully dominant.
Thus, when homozygous red flowers A1A1 are crossed with homozygous white A2A2 , a variety of pink-shaded phenotypes result.
Note, however, that partial dominance is not the same as blending inheritance ; after all, when two F 1 pink flowers are crossed, both red and white flowers are found among the progeny. In other words, nothing is different about the way these alleles are inherited; the only difference is in the way the alleles determine phenotype when they are combined.
As opposed to partial dominance, codominance occurs when the phenotypes of both parents are simultaneously expressed in the same offspring organism. Indeed, "codominance" is the specific term for a system in which an allele from each homozygote parent combines in the offspring, and the offspring simultaneously demonstrates both phenotypes.
An example of codominance occurs in the human ABO blood group system. Many blood proteins contribute to blood type Stratton, , and the ABO protein system in particular defines which types of blood you can receive in a transfusion.
In a hospital setting, attention to the blood proteins present in an individual's blood cells can make the difference between improving health and causing severe illness. There are three common alleles in the ABO system. These alleles segregate and assort into six genotypes, as shown in Table 1.
As Table 1 indicates, only four phenotypes result from the six possible ABO genotypes. How does this happen? To understand why this occurs, first note that the A and B alleles code for proteins that exist on the surface of red blood cells; in contrast, the third allele, O, codes for no protein.
Thus, if one parent is homozygous for type A blood and the other is homozygous for type B, the offspring will have a new phenotype, type AB. In people with type AB blood, both A and B proteins are expressed on the surface of red blood cells equally. Therefore, this AB phenotype is not an intermediate of the two parental phenotypes, but rather is an entirely new phenotype that results from codominance of the A and B alleles. All of these heterozygote genotypes demonstrate the coexistence of two phenotypes within the same individual.
In some instances, offspring can demonstrate a phenotype that is outside the range defined by both parents. In particular, the phenomenon known as overdominance occurs when a heterozygote has a more extreme phenotype than that of either of its parents. A well-known example of overdominance occurs in the alleles that code for sickle-cell anemia.
Sickle-cell anemia is a debilitating disease of the red blood cells, wherein a single amino acid deletion causes a change in the conformation of a person's hemoglobin such that the person's red blood cells are elongated and somewhat curved, taking on a sickle shape.
This change in shape makes the sickle red blood cells less efficient at transporting oxygen through the bloodstream. The altered form of hemoglobin that causes sickle-cell anemia is inherited as a codominant trait. Specifically, heterozygous Ss individuals express both normal and sickle hemoglobin, so they have a mixture of normal and sickle red blood cells.
In most situations, individuals who are heterozygous for sickle-cell anemia are phenotypically normal. Under these circumstances, sickle-cell disease is a recessive trait. Individuals who are homozygous for the sickle-cell allele ss , however, may have sickling crises that require hospitalization.
In severe cases, this condition can be lethal. Producing altered hemoglobin can be beneficial for inhabitants of countries afflicted with falciparum malaria, an extremely deadly parasitic disease. Sickle blood cells "collapse" around the parasites and filter them out of the blood. Thus, people who carry the sickle-cell allele are more likely to recover from malarial infection. In terms of combating malaria, the Ss genotype has an advantage over both the SS genotype, because it results in malarial resistance, and the ss genotype, because it does not cause sickling crises.
Allelic dominance always depends on the relative influence of each allele for a specific phenotype under certain environmental conditions. For example, in the pea plant Pisum sativum , the timing of flowering follows a monohybrid single-gene inheritance pattern in certain genetic backgrounds. While there is some variation in the exact time of flowering within plants that have the same genotype, specific alleles at this locus Lf can exert temporal control of flowering in different backgrounds Murfet, Investigators have found evidence for four different alleles at this locus: Lf d , Lf , lf , and lf a.
Plants homozygous for the lf a allele flower the earliest, while Lf d plants flower the latest. A single allele causes the delayed flowering. Thus, the multiple alleles at the Lf locus represent an allelic series, with each allele being dominant over the next allele in the series.
Mendel's early work with pea plants provided the foundational knowledge for genetics, but Mendel's simple example of two alleles, one dominant and one recessive, for a given gene is a rarity.
In fact, dominance and recessiveness are not actually allelic properties. Rather, they are effects that can only be measured in relation to the effects of other alleles at the same locus. Furthermore, dominance may change according to the level of organization of the phenotype. Variations of dominance highlight the complexity of understanding genetic influences on phenotypes. Murfet, I. Flowering in Pisum : Multiple alleles at the Lf locus.
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