Other Allelic Relationships for Genetics Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 21, 2011

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.

Multiple Alleles

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.

Multiple Alleles

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.

Multiple Alleles

Practice problems for these concepts can be found at: 

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