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Mendel's 3(dominant):1(recessive) trait ratio does not always occur. Mendel's notation for a dominant trait, a hybrid and a recessive trait were (B), (Bb) and (b) respectively. For reasons given in the preceding paper [1], the hybrid is represented in Figure 3 by (H). When graded changes are made in any one enzyme in a metabolic pathway the response of the flux through that pathway is always non-linear (non-additive) but not always quasi-hyperbolic (Figure 2). Consequently when the enzyme activity at one metabolic locus is decreased in the heterozygote to (say) 50% of wild-type, the trait displayed by the hybrid (H) is now distinguishable from the trait (B) displayed by the wild type cell or organism and from the trait (b) displayed by the homozygously mutant cell or organism. Mendel's 3(dominant):1(recessive trait ratio will not be observed. The explanation is consistent with the explanation for the observation of the 3:1 trait ratio in Figure 2 and achieves what the currently favoured explanation of Mendel's observations cannot achieve [1].
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Figure 3: Mendel's 3(dominant):1(recessive) trait ratio does not always occur. Mendel's notation for a dominant trait, a hybrid and a recessive trait were (B), (Bb) and (b) respectively. For reasons given in the preceding paper [1], the hybrid is represented in Figure 3 by (H). When graded changes are made in any one enzyme in a metabolic pathway the response of the flux through that pathway is always non-linear (non-additive) but not always quasi-hyperbolic (Figure 2). Consequently when the enzyme activity at one metabolic locus is decreased in the heterozygote to (say) 50% of wild-type, the trait displayed by the hybrid (H) is now distinguishable from the trait (B) displayed by the wild type cell or organism and from the trait (b) displayed by the homozygously mutant cell or organism. Mendel's 3(dominant):1(recessive trait ratio will not be observed. The explanation is consistent with the explanation for the observation of the 3:1 trait ratio in Figure 2 and achieves what the currently favoured explanation of Mendel's observations cannot achieve [1].

Mentions: Figure 2 illustrates one of a family of regularly non-linear (non-additive) response plots which exhibit various degrees of asymmetry [3]. Is the flux response always sufficiently asymmetric for the 3:1 trait ratio to be observed? It is not. A flux response was observed in one particular (diploid) metabolic system (Reference [3], Figure 1f) that was still clearly non-linear (non-additive) but not as asymmetric as that shown in Figure 2. As in Figure 2, so in Figure 3, a recessive trait (b) can be clearly distinguished from the dominant trait (B) because the concentrations of the molecular components of this trait were sufficiently different when one enzyme activity in the metabolic system is decreased to a sufficient extent. The trait displayed by the hybrid (H) is now distinguishable (rather than indistinguishable) from the dominant trait (B) expressed in a genetically related normal cell or organism when, as in Figure 2, the enzyme activity is decreased to an arbitrarily chosen 50% of the normal activity. The 3(dominant):1(recessive) trait ratio will not then be observed (Figure 2). A blend of traits (B) and (b) is possible in the hybrid (H), for example when traits (B) and (b) are distinguished by colour differences.

A rational treatment of Mendelian genetics

Porteous JW - Theor Biol Med Model (2004)

Bottom Line: The key to a rational treatment of elementary Mendelian genetics, specifically to an understanding of the origin of dominant and recessive traits, lies in the facts that: (1) alleles of genes encode polypeptides; (2) most polypeptides are catalysts, i.e. enzymes or translocators; (3) the molecular components of all traits in all cells are the products of systems of enzymes, i.e. of fluxing metabolic pathways; (4) any flux to the molecular components of a trait responds non-linearly (non-additively) to graded mutations in the activity of any one of the enzymes at a catalytic locus in a metabolic system; (5) as the flux responds to graded changes in the activity of an enzyme, the concentrations of the molecular components of a trait also change.It is then possible to account rationally, and without misrepresenting Mendel, for: the origin of dominant and recessive traits; the occurrence of Mendel's 3(dominant):1(recessive) trait ratio; deviations from this ratio; the absence of dominant and recessive traits in some circumstances, the occurrence of a blending of traits in others; the frequent occurrence of pleiotropy and epistasis.

Affiliation: Department of Molecular and Cell Biology, Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, Scotland, UK. j.w.porteous@abdn.ac.uk

ABSTRACT

Background: The key to a rational treatment of elementary Mendelian genetics, specifically to an understanding of the origin of dominant and recessive traits, lies in the facts that: (1) alleles of genes encode polypeptides; (2) most polypeptides are catalysts, i.e. enzymes or translocators; (3) the molecular components of all traits in all cells are the products of systems of enzymes, i.e. of fluxing metabolic pathways; (4) any flux to the molecular components of a trait responds non-linearly (non-additively) to graded mutations in the activity of any one of the enzymes at a catalytic locus in a metabolic system; (5) as the flux responds to graded changes in the activity of an enzyme, the concentrations of the molecular components of a trait also change.

Conclusions: It is then possible to account rationally, and without misrepresenting Mendel, for: the origin of dominant and recessive traits; the occurrence of Mendel's 3(dominant):1(recessive) trait ratio; deviations from this ratio; the absence of dominant and recessive traits in some circumstances, the occurrence of a blending of traits in others; the frequent occurrence of pleiotropy and epistasis.

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