Extensions of Mendelian Genetics

Alleles and Their Effects on Phenotype

Alleles are alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. The wild-type allele is the most common allele in a population and serves as the standard for comparison. Mutations are the source of new alleles, which can alter phenotypes in various ways:

• Loss-of-function (LOF) mutations: Reduce or eliminate the function of the gene product. A null allele results when the loss is complete.

• Gain-of-function (GOF) mutations: Enhance or confer new activity on the gene product.

Alleles are symbolized using italicized letters, with uppercase for dominant and lowercase for recessive. If no dominance exists, superscripts are used (e.g., R1, R2).

Hemoglobin Structure and Sickle-Cell Mutation

Hemoglobin is a quaternary protein composed of two alpha and two beta chains, each with a heme group that binds oxygen. A single amino acid substitution (Glu to Val) in the beta chain causes sickle-cell hemoglobin, leading to abnormal aggregation and reduced oxygen-carrying capacity.

Degrees of Dominance

Inheritance patterns can deviate from simple Mendelian genetics in several ways:

• Complete dominance: Heterozygote phenotype is identical to the dominant homozygote.

• Incomplete dominance: Heterozygote phenotype is intermediate between the two homozygotes.

• Codominance: Both alleles in a heterozygote are fully expressed, producing a distinct phenotype.

In incomplete dominance, the phenotypic ratio matches the genotypic ratio (1:2:1 in F2 generation).

Multiple Alleles and Codominance

More than two alleles can exist for a gene in a population, though any individual carries only two. The ABO blood group system is a classic example, with three alleles (IA, IB, i) showing both dominance and codominance. The IA and IB alleles are codominant, while both are dominant over i.

Gene Interaction and Epistasis

Many phenotypes are influenced by more than one gene. Epistasis occurs when one gene masks or modifies the effect of another gene. For example, in mice, one gene determines pigment color (B = black, b = brown), and another gene (C = color, c = no color) determines whether pigment is deposited. The interaction produces a 9:3:4 phenotypic ratio in the F2 generation.

The Bombay Phenotype

The Bombay phenotype is an example of epistasis in humans. Individuals with the hh genotype at the FUT1 locus cannot produce the H substance, a precursor for A and B antigens, resulting in a functional blood type O regardless of their ABO genotype.

Lethal Alleles

Lethal alleles are those that cause death when present in a certain genotype. Recessive lethal alleles are only fatal in the homozygous state, while dominant lethal alleles (e.g., Huntington disease) are fatal even in heterozygotes, often after reproductive age. Huntington disease is caused by expansion of CAG repeats in the HD gene, leading to neurodegeneration.

Polygenic Inheritance

Polygenic traits are controlled by two or more genes, resulting in continuous variation (quantitative traits). Human skin color is a classic example, likely controlled by three or four genes, each with additive effects.

Pleiotropy

Pleiotropy occurs when a single gene influences multiple phenotypic traits. Marfan syndrome is an example, caused by mutations in the fibrillin gene, affecting connective tissue in the eyes, skeleton, and cardiovascular system.

Genotypic Background and Environmental Effects

The expression of genetic traits can be modified by other genes (genetic background) and environmental factors (e.g., diet, exposure to toxins). Two important concepts are:

• Penetrance: The percentage of individuals with a given genotype who express the expected phenotype.

• Expressivity: The degree or range of expression of a phenotype among individuals with the same genotype.

Reduced penetrance means some individuals with the genotype do not show the phenotype (e.g., retinoblastoma). Variable expressivity is seen in disorders like neurofibromatosis type 1, where severity varies among individuals.

Nutritional and Environmental Effects

Some genetic disorders are influenced by diet. For example, individuals with phenylketonuria (PKU) cannot metabolize phenylalanine and must avoid high-phenylalanine foods to prevent symptoms.

Sex-Linked, Sex-Limited, and Sex-Influenced Inheritance

Genes on the X chromosome exhibit unique inheritance patterns due to dosage differences between males (XY) and females (XX). Dosage compensation in mammals is achieved by X-inactivation, forming a Barr body in females. X-linked traits, such as color blindness and hemophilia, show distinct pedigree patterns.

X-Linkage in Drosophila and Humans

Thomas Hunt Morgan's studies of eye color in Drosophila established the concept of X-linkage. In humans, X-linked recessive disorders are more common in males, as they have only one X chromosome.

Sex-Limited and Sex-Influenced Traits

Sex-limited traits are expressed in only one sex (e.g., feather type in chickens), while sex-influenced traits are affected by the individual's sex hormones (e.g., pattern baldness in humans).

Summary Table: Extensions of Mendelian Genetics