Other Allelic Relationships for Genetics Help
Alleles that lack complete dominant and recessive relationships and are both observed phenotypically are called codominant. This means that the phenotypic effect of each allele is observable in the heterozygous condition. Hence, the heterozygous genotype gives rise to a phenotype distinctly different from either of the homozygous genotypes, but possesses characteristics of each. For codominant alleles, all uppercase base symbols with different superscripts are used. The uppercase letters call attention to the fact that each allele can be detected phenotypically to some degree even when in the presence of its alternative allele (heterozygous).
EXAMPLE 2.12 The alleles governing the M-N blood group system in humans are codominant and may be represented by the symbols LM and LN, the base letter (L) being assigned in honor of its codiscoverers (Landsteiner and Levine). Two antisera (anti-M and anti-N) are used to distinguish three genotypes and their corresponding phenotypes (blood groups). Agglutination of red blood cells results from a reaction between the antisera and a specific protein antigen (i.e., anti-M reacts only with protein M) and is represented by +; nonagglutination (0) occurs when the specific antigen is not present (i.e., anti-M will not agglutinate cells with protein N if M is absent). Similarly, cells possessing the N antigen will only agglutinate with the anti-N antibodies.
Alleles that lack dominance relationships and result in heterozygotes that have an intermediate phenotype that is distinct from either homozygous parent are called incompletely or partially dominant alleles. The phenotype may appear to be a "blend" in heterozygotes, but each allele maintains its individual identity and alleles will segregate from each other in the formation of gametes.
EXAMPLE 2.13 Flower color in flowering plants, such as snapdragons, is a good example of incomplete dominance. A cross between pure-breeding red-flowered plants (R1R1) and pure-breeding white flowered plants (R2R2) results in pink plants (R1R2). The F2 offspring produce red, pink and white progeny in the ratio 1 : 2 : 1, respectively.
The phenotypic manifestation of some genes is the death of the individual organism prior to sexual maturity. The factors that cause such a manifestation are called lethal alleles. A fully dominant lethal allele (i.e., one that kills in both the homozygous and heterozygous conditions) occasionally arises by mutation from a wild-type allele. Individuals with a dominant lethal allele die before they can reproduce. Therefore, the mutant dominant lethal is removed from the population in the same generation in which it arose. Lethals that kill only when homozygous may be of two kinds: (1) one that has no obvious phenotypic effect in heterozygotes, and (2) one that exhibits a distinctive phenotype when heterozygous.
EXAMPLE 2.14 The following phenotypes are associated with the possible genotypes that involve a completely recessive lethal (l) allele.
EXAMPLE 2.15 The amount of chlorophyll in snapdragons is controlled by a pair of alleles C1 and C2, one of which, C2, exhibits a lethal effect when homozygous and a distinctive color phenotype when heterozygous. Thus, with regard to color, these alleles are incompletely dominant. However, with regard to viability, the C2 allele is fully recessive; i.e., the C2 allele only causes death when C1 is absent. Note that these genes have at least two phenotypic manifestations (color and viability). This phenomenon of a single gene producing more than one phenotype manifestation is called pleiotropism.
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