Blood group antigen proteins serve a variety of functions within the cell membrane of red blood cells. These protein functions include transporting other proteins and molecules into and out of the cell, maintaining cell structure, attaching to other cells and molecules, and participating in chemical reactions. There are 29 recognized blood groups, most involving only one gene. Variations polymorphisms within the genes that determine blood group give rise to the different antigens for a particular blood group protein.
The changes that occur in the genes that determine blood group typically affect only blood type and are not associated with adverse health conditions, although exceptions do occur.
The A and B alleles are codominant, which is similar to incomplete dominance in that heterozygotes have an intermediate phenotype. If both the A and B alleles are present, both will be seen in the phenotype. The O allele is recessive to both A and B.
Coat variation in the domestic dog is governed by variants in three genes. Morphologic and molecular characterization of two novel Krt71 Krtg mutations: Krt71rco12 and Krt71rco Mamm Genome.
OpenStax, Biology. OpenStax CNX. Figure 1: These pink flowers of a heterozygote snapdragon result from incomplete dominance. Eye color in Drosophila was one of the first X-linked traits to be identified. Thomas Hunt Morgan mapped this trait to the X chromosome in In flies, the wild-type eye color is red X W and it is dominant to white eye color X w Figure 4. Because of the location of the eye-color gene, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous , because they have only one allele for any X-linked characteristic.
Hemizygosity makes the descriptions of dominance and recessiveness irrelevant for XY males. In an X-linked cross, the genotypes of F 1 and F 2 offspring depend on whether the recessive trait was expressed by the male or the female in the P 0 generation.
With regard to Drosophila eye color, when the P 0 male expresses the white-eye phenotype and the female is homozygous red-eyed, all members of the F 1 generation exhibit red eyes. Now, consider a cross between a homozygous white-eyed female and a male with red eyes Figure 5. Figure 5. Punnett square analysis is used to determine the ratio of offspring from a cross between a red-eyed male fruit fly and a white-eyed female fruit fly.
What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color? Discoveries in fruit fly genetics can be applied to human genetics.
When a female parent is homozygous for a recessive X-linked trait, she will pass the trait on to percent of her offspring. In humans, the alleles for certain conditions some forms of color blindness, hemophilia, and muscular dystrophy are X-linked. Females who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects. These females will pass the disease to half of their sons and will pass carrier status to half of their daughters; therefore, recessive X-linked traits appear more frequently in males than females.
In some groups of organisms with sex chromosomes, the gender with the non-homologous sex chromosomes is the female rather than the male. This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in which they are hemizygous.
Improve this page Learn More. Skip to main content. However, sometimes the heterozygote displays a phenotype that is an intermediate between the phenotypes of both homozygote parents one of which is homozygous dominant, and the other of which is homozygous recessive. This intermediate phenotype is a demonstration of partial or incomplete dominance. When partial dominance occurs, a range of phenotypes is usually observed among the offspring.
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.
0コメント