Other Allelic Relationships for Genetics Help (page 2)
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.
Penetrance and Expressivity
Differences in environmental conditions or in genetic backgrounds may cause individuals that are genetically identical at a particular locus to exhibit different phenotypes. The percentage of individuals in a population with a particular gene combination that exhibit the corresponding character to any degree represents the penetrance of the trait.
EXAMPLE 2.16 One type of polydactyly (extra fingers and/or toes) in humans can be produced by a dominant gene (P). The wild-type condition with five digits on each limb is produced by the recessive genotype (pp). However, some heterozygous individuals (Pp) are not polydactylous. If 20% of Pp individuals do not show polydactyly (i.e., are wild type), the gene has a penetrance of 80%.
A trait, although penetrant, may be quite variable in its level of expression. The degree of effect produced by a penetrant genotype is termed expressivity.
EXAMPLE 2.17 The polydactylous condition may be penetrant in the left hand (six digits) and not in the right (five digits), or it may be penetrant in the feet but not in the hands
A recessive lethal allele that lacks complete penetrance and expressivity will kill less than 100% of the homozygotes before sexual maturity. The terms semilethal, sublethal, or subvital apply to such genes. The effects that various kinds of lethals have on the reproduction of the next generation form a broad spectrum from complete lethality to sterility in completely viable genotypes. Problems in this book, however, will consider only those lethals that become completely penetrant, usually during the embryonic stage. Genes other than lethals will likewise be assumed completely penetrant.
The genetic systems proposed thus far have been limited to a single pair of alleles. The maximum number of alleles at a gene locus that any individual possesses is two, with one on each of the homologous chromosomes. But since a gene can be changed to alternative forms by the process of mutation, a large number of alleles is theoretically possible in a population of individuals. When evermore than two alleles are identified at a gene locus in a population, we have a multiple allelic series. The dominance hierarchy should be defined at the beginning of each problem involving multiple alleles. A capital letter is commonly used to designate the allele that is dominant to all others in the series. The corresponding lowercase letter designates the allele that is recessive to all others in the series. Other alleles, intermediate in their degree of dominance between these two extremes, are usually assigned the lowercase letter with some suitable superscript.
EXAMPLE 2.18 The color of Drosophila eyes is governed by a series of alleles that cause the hue to vary from red or wild type (w+ or W) through coral (wco), blood (wbl), eosin (we), cherry (wch), apricot (wa), honey (wh), buff (wbf), tinged (wt), pearl (wp), and ivory (wi) to white (w). Each allele in the system except w can be considered to produce pigment, but successively less is produced by alleles as we proceed down the hierarchy: w+ > wco > wbl > we > wch > wa > wh > wbf > wt > wp > wi > w. The wild-type allele (w+) is completely dominant and w is completely recessive to all other alleles in the series. Compounds are heterozygotes that contain unlike members of a multiple allelic series. The compounds of this series that involve alleles other than w+ tend to be phenotypically intermediate between the eye colors of the parental homozygotes.
EXAMPLE 2.19 A classical example of multiple alleles is found in the ABO blood group system of humans, where the allele IA for the A antigen is codominant with the allele IB for the B antigen. Both IA and IBare completely dominant to the allele i, which fails to specify any detectable antigenic structure. The hierarchy of dominance relationships is symbolized as (IA = IB) > i. Two antisera (anti-A and anti-B) are required for the detection of four phenotypes.
EXAMPLE 2.20 A slightly different kind of multiple allelic system is encountered in the coat colors of rabbits: C allows full color to be produced (typical gray rabbit); cch, when homozygous, removes yellow pigment from the fur, making a silver-gray color called chinchilla; cch, when heterozygous with alleles lower in the dominance hierarchy, produces light-gray fur; ch produces a white rabbit with black extremities, called ''Himalayan''; c fails to produce pigment, resulting in albino. The dominance hierarchy may be symbolized as follows: C > cch > ch > c.
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