One of Gregor Mendel's greatest contributions to the study of heredity was the concept of dominance. Mendel observed that a heterozygote offspring can show the same phenotype as the parent homozygote, so he concluded that there were some traits that dominated over other inherited traits. However, the relationship of genotype to phenotype is rarely as simple as the dominant and recessive patterns described by Mendel. As the study of inheritance expanded beyond the seven traits Mendel initially examined and also included organisms other than pea plants, biologists began to notice a variety of relationships between alleles that code for the same trait. These allelic interactions were not exclusively recessive or dominant, and they greatly enriched our understanding of how genotype leads to phenotype.
Complete versus Partial Dominance
A diagram shows a genetic cross between two flowering plants: the plant at left has red flowers and an A1A1 genotype, and the plant at right has white flowers and an A2A2 genotype. The top panel represents a complete dominance scenario, and shows illustrations of two possible progeny plants that may result from this genetic cross. The progeny plant at left is red, with the genotype A1A2 and a red dominant allele, and the progeny plant at right is white, with the genotype A1A2 and a white dominant allele. The lower panel shows an incomplete dominance scenario, and shows illustrations of three possible progeny plants that may result from this genetic cross. All three plants have the genotype A1A2; each plant is a different shade of pink.
View Full-Size ImageFigure 1
Figure Detail
Dominance affects the phenotype derived from an organism's genes, but it does not affect the way these genes are inherited. Complete dominance occurs when the heterozygote phenotype is indistinguishable from that of the homozygous parent. 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 F1 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.
Codominance
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, 1952), 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.
Answers & Comments
Explanation:
One of Gregor Mendel's greatest contributions to the study of heredity was the concept of dominance. Mendel observed that a heterozygote offspring can show the same phenotype as the parent homozygote, so he concluded that there were some traits that dominated over other inherited traits. However, the relationship of genotype to phenotype is rarely as simple as the dominant and recessive patterns described by Mendel. As the study of inheritance expanded beyond the seven traits Mendel initially examined and also included organisms other than pea plants, biologists began to notice a variety of relationships between alleles that code for the same trait. These allelic interactions were not exclusively recessive or dominant, and they greatly enriched our understanding of how genotype leads to phenotype.
Complete versus Partial Dominance
A diagram shows a genetic cross between two flowering plants: the plant at left has red flowers and an A1A1 genotype, and the plant at right has white flowers and an A2A2 genotype. The top panel represents a complete dominance scenario, and shows illustrations of two possible progeny plants that may result from this genetic cross. The progeny plant at left is red, with the genotype A1A2 and a red dominant allele, and the progeny plant at right is white, with the genotype A1A2 and a white dominant allele. The lower panel shows an incomplete dominance scenario, and shows illustrations of three possible progeny plants that may result from this genetic cross. All three plants have the genotype A1A2; each plant is a different shade of pink.
View Full-Size ImageFigure 1
Figure Detail
Dominance affects the phenotype derived from an organism's genes, but it does not affect the way these genes are inherited. Complete dominance occurs when the heterozygote phenotype is indistinguishable from that of the homozygous parent. 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 F1 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.
Codominance
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, 1952), 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.
Table 1: Possible ABO Genotypes
Blood Type Related Genotype(s)
A AA or AO
B BB or BO
AB AB
O OO