Variation of Inherited Characteristics
Although there are characteristics whose inheritance adheres strictly to Mendel's predictions, more often than not, that is not the case. Many different mechanisms can alter the phenotypic expression of an inherited genotype, complicating attempts to predict offspring phenotypes.
One such case is called incomplete penetrance. If a phenotype is determined by a dominant allele, it is expected that all individuals bearing the dominant allele will show the phenotype. Sometimes, the phenotype is not expressed in some individuals, or the phenotype seems to skip generations, only to reappear later. If all individuals in a population express the phenotype, its allele is said to have complete penetrance. The degree of penetrance is described on the level of population--in what percentage of individuals having the dominant allele the phenotype will be expressed. An example of a gene with incomplete penetrance is that causing polydactyly (extra fingers or toes).
Polydactyly serves also as an example of variable expressivity of a gene, or the degree to which a gene is expressed within an individual. Low expressivity of the polydactyly gene might result in an individual's having one extra finger, whereas high expressivity might result in extra digits on each hand and foot.
Another variation in the expression of inherited characteristics involves the individual's sex. Some characteristics, like pattern baldness, are sex-influenced. Depending on the sex of the individual, a given genotype may be expressed or not. In the baldness example, an individual of either sex who is homozygous dominant for the baldness gene will demonstrate hair loss, although it is more extreme and earlier in onset in males. In individual who is homozygous recessive for this gene will escape baldness. Males, but not females, who are heterozygous for the gene will lose their hair. This difference appears to be caused by the background of the individual's sex hormones. Females having adrenal gland tumors that cause overproduction of male hormones lose their hair if they are heterozygous for the baldness gene; but it grows back if the condition is corrected by surgical removal of the tumor.
Sex-limited traits can be passed through generations from one sex to another (from mother to son, for example), but are expressed in only one sex. These include breast development in females and beard growth in males.
In an interaction called epistasis, a gene at one locus can alter the phenotypic expression of a gene at another locus. For example, black fur is dominant over brown fur in mice, and inheritance of coat color in mice follows Mendelian predictions. However, there is another gene that determines whether any pigment at all will be deposited in the fur. If a mouse is recessive for this deposition gene, it will be albino, regardless of its genotype for the black/brown gene. Thus, the deposition gene is said to be epistatic to the color gene.
Certain genes, called suppressor genes, nullify the effect of other genes. In Drosophila, a mutation called hairy-winged shows in the phenotype of flies recessive for this gene. If they have the suppressor hairy-winged allele as well, however, their normal-winged phenotype is restored.
The position of a gene on a chromosome may also affect its expression. If a gene is moved from its original location due to a translocation or an inversion, it may not be expressed. This is true particularly if the gene is relocated to an area near heterochromatin, which appears to inhibit the expression of adjacent genes.
Temperature can effect a gene's expression. A well-known example is in the Siamese cat, whose nose, ears, and paws are more darkly colored than the rest of its body. A temperature-sensitive allele for pigment production works better at the slightly lower temperatures in the extremities than in the rest of the body, yielding the characteristic color pattern.
A phenomenon called genetic anticipation explains the observation that some conditions become more severe as they pass from one generation to the next. In anticipation, segments of DNA, such as trinucleotide repeats, are unstable and increase in number during replication; therefore, the number of repeats increases with each generation. The higher the number of repeats, the more severe the disease will be, and the earlier its onset. Anticipation has been observed in Huntington disease, fragile-X syndrome, and myotonic dystrophy.
In cases of genomic imprinting, phenotypic expression varies depending on the parental origin of the chromosome carrying a particular allele. Some unidentified chemical imprint, perhaps a methyl group, is added to chromsomes before or during gamete formation, leading to differentially imprinted genes or chromosome regions, the imprint indentifying the chromosome as being from the mother or the father. This imprint can be reversed from one generation to the next, as when a chromosome is passed from father to daughter. Probably the best-known example of imprinting in humans occurs in the q1 region of the 15th chromosome. A deletion in the paternal chromosome in this region causes Prader-Willis syndrome, whereas a corresponding deletion in the maternal chromosome causes Angelman syndrome. These two disorders are phenotypically distinct.
This is the complete article, containing 823 words
(approx. 3 pages at 300 words per page).
