Dominant, genetic trait

Taste. Of the fundamental tastes, bitter is unique in showing human genetic differences in sensitivity. Six decades ago, it was reported tiiat phenylthiocarbamide (PTC) tasted extremely bitter to some individuals while being almost tasteless to others (45). Tlie ability to taste PTC was found to be a dominant genetic trait which occurs across gender, age and culture, with 70% of the American pulation carrying the dominant trait (46). Sensitivity to PTC and propylthiouracil (PROP) are correlated with sensitivities to otiier bitter tasting compounds, such as caffeine, saccharin (after-taste) and salts of i tassium cations and benzoate anions (47,48,49 0). However, in a reexamination of the sensitivity to NaQ and KQ, no differences were found between tasters and nontasters to non-PTC type compounds, and the statistical methods that showed differences were questioned (51). Individuals who do not respond to PTC are not necessarily insensitive to quinine, another intensely bitter compound (49,50,52). 

The genetic trait for MH is not sex-linked and both men and women can inherit MH. Inheritance in humans appears to be autosomal dominant with variable penetrance. Studies of large families have documented an autosomal dominant pattern. McPherson and Taylor (1982) studied 93 families in whom MH occurred. Even though various patterns of inheritance did emerge in the study we should assume that 50% of children are at risk in MH susceptible families. Kalow and Britt (1992) suggested that, in some families, at least two different non-allelic genes are likely to be present, one of which is probably autosomal dominant but rare, and the other autosomal recessive but common. 

The most common forms of ichthyosis — autosomal dominant ichthyosis vulgaris (IV) and X-linked recessive ichthyosis (XRI) — in many countries occur at frequencies a high as 1/300 and 1/2500, respectively. In fact, the genetic traits for IV and XRI are so frequent that the two diseases sometimes coexist in one and the same family, which may cause confusion as to the inheritance pattern. Although the incidence of IV and XRI is probably similar around the world, climate differences in particular will affect the severity of the disease, and hence its notification by the health care system. 

Genetic disorders produced by a single causal gene, such as our example of dystrophin mutations leading to Duchenne muscular dystrophy, are termed Mendelian disorders, after the Austrian monk Gregor Mendel (1822-1884), who discovered unitary inheritance in pea plants. There are three primary types of Mendelian inheritance. First, dominant genetic variants or alleles produce a trait or disease if one copy of the allele is sufficient to cause the disease (e.g. Huntington disease). Sec-... 

The [PSP] element is inherited in an orderly, reproducible way but one that is different from most genetic traits (Fig. 1) (Cox, 1965). When a haploid [PSP] strain is mated to a haploid [psi strain, the resulting diploid has a suppression phenotype that is, [PSP] is dominant. On sporulation, however, none of the haploid progeny are [psir] as would be expected for a nuclear determinant Instead, [PSP] is transmitted to all haploid progeny. (The capital letters in [PSP] signify dominance the brackets signify nonchromosomal inheritance.) This unusual pattern of inheritance was partly explained by later experiments that localized the [PS7+] determinant to the cytoplasm (Cox et al, 1980 Fink and Conde, 1976). Surprisingly, however, the [PSP] phenotype could not be linked to any of tbe known cytoplasmic nucleic acids in yeast (Cox et al, 1988 Serio and Lindquist, 1999). 

Explain why the dominant-recessive genetic model is much simpler than most genetic trait determinations. 

Needless to say, the range of phenotypes of the dominantly inherited traits includes almost the totality of human phenotypic variability, and the range of severity includes traits that are so mild that they can be detected only by special laboratory tests, as well as those that are so severe in their effects on viability and fertility that it is difficult to establish that they are in fact inherited. There is the additional class that leads to early death, pre-or postnatal, and which cannot ordinarily be proven to be genetic at all. 

The molecular genetic picture that has emerged to describe grain hardness can be summarized as follows. Endosperm softness is the dominantly inherited trait (i.e., the soft phenotype is the wild type). When both puroindoline a and puroindoline b are expressed, they work together to impart softness to the endosperm. When both are absent (as in durums), the endosperm is very hard. When only one is present, an intermediate degree of hardness is found. Another way in which an intermediate degree of hardness is obtained is when one of the puroindolines is present and the other has been altered as a result of mutation. 

His observations also led him to coin two terms that still persist in present-day genetics dominance for a trait that shows up in an offspring, and recessiveness for a trait masked by a dominant gene. 

R. R., and Johnson, M.L. (2002) A Mutation in the LDL Receptor Related Protein 5 Gene Results in the Autosomal Dominant High Bone Mass Trait. American Journal of Human Genetics 70, 11-19. 

Gene—Part of the genetic material that directs production of a particular protein, and thus determines the presence or absence of a particular trait. Genes may be dominant or recessive. 

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