Extensions of Mendelian Genetics

Alleles and Their Effects on Phenotypes

Alleles are alternative forms of a gene, and mutations are the primary source of new alleles. The wild-type allele is the most common allele in a population and serves as the standard for comparison. Alleles can alter phenotypes in various ways, including loss-of-function (LOF) mutations, which reduce or eliminate the function of the gene product, and gain-of-function (GOF) mutations, which enhance or change the function of the gene product. A null allele results from a complete loss of function.

• Dominant alleles are typically represented by italic uppercase letters (e.g., D).

• Recessive alleles are represented by italic lowercase letters (e.g., d).

• If no dominance exists, superscripts are used (e.g., R1, R2).

Example: In fruit flies, the ebony mutant phenotype is indicated by e, while the wild-type gray is e+.

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 in the beta chain (glutamic acid to valine) causes sickle-cell hemoglobin, leading to reduced oxygen-carrying capacity and altered red blood cell shape.

Degrees of Dominance

Complete, Incomplete, and Codominance

The relationship between genotype and phenotype can vary:

• 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, resulting in a phenotype that shows both traits distinctly.

Example: In snapdragons, crossing red (CRCR) and white (CWCW) flowers produces pink (CRCW) offspring, demonstrating incomplete dominance.

Codominance Example: MN Blood Group

In the MN blood group system, both M and N antigens are expressed in heterozygotes (LMLN), resulting in the MN phenotype. This is a classic example of codominance.

Multiple Alleles and Blood Groups

ABO Blood Group System

Multiple alleles exist for the ABO blood group gene: IA, IB, and i. Each individual can have only two alleles, but the population may have more. IA and IB are codominant, while i is recessive. The combination of these alleles determines the blood type (A, B, AB, or O).

• Genotypes and Phenotypes:

  • IAIA or IAi: Type A

  • IBIB or IBi: Type B

  • IAIB: Type AB

  • ii: Type O

Epistasis and the Bombay Phenotype

Epistasis occurs when one gene masks or modifies the expression of another gene. The Bombay phenotype is an example where a mutation in the FUT1 gene prevents the production of the H antigen, so A and B antigens cannot be formed, resulting in a functional type O blood group even if the genotype suggests otherwise.

Gene Interactions and Epistasis

Coat Color in Mice

Coat color in mice is determined by two genes: one controls pigment color (B = black, b = brown), and the other controls pigment deposition (C = color, c = no color). The interaction between these genes produces a 9:3:4 phenotypic ratio in the F2 generation, illustrating recessive epistasis.

Modified Dihybrid Ratios

Gene interactions can modify the classic 9:3:3:1 dihybrid ratio. For example, complementary gene action or epistasis can produce ratios such as 9:7 or 9:3:4, depending on the interaction.

Lethal Alleles

Recessive and Dominant Lethal Alleles

Lethal alleles cause death when present in certain genotypes. Recessive lethal alleles are only fatal in homozygotes, while dominant lethal alleles are fatal even in heterozygotes. Huntington disease is an example of a dominant lethal allele, caused by expansion of CAG repeats in the HD gene, leading to neurodegeneration.

Polygenic Inheritance

Quantitative Traits

Polygenic traits are controlled by multiple genes, each contributing additively to the phenotype. These traits show continuous variation, such as human skin color, which is influenced by several genes with additive effects.

Pleiotropy

Single Gene, Multiple Effects

Pleiotropy occurs when a single gene influences multiple phenotypic traits. Marfan syndrome is a classic example, where mutations in the fibrillin gene affect the skeleton, eyes, and cardiovascular system.

Genotypic Background and Environmental Effects

Penetrance and Expressivity

Penetrance is the percentage of individuals with a particular genotype who express the expected phenotype. Expressivity refers to the degree or range of expression of a phenotype among individuals with the same genotype. Reduced penetrance and variable expressivity can complicate genetic analysis.

Variable Expression

Variable expression is seen in disorders like Neurofibromatosis Type 1, where the severity of symptoms can range from mild to severe among individuals with the same mutation.

Nutritional and Environmental Effects

Some genetic disorders, such as phenylketonuria (PKU), can be managed by dietary modifications. Individuals with PKU must avoid foods high in phenylalanine to prevent intellectual disability and other symptoms.

Modification of Mendelian Ratios: Sex Chromosomes and Dosage Compensation

X-Inactivation and Dosage Compensation

In mammals, dosage compensation is achieved by X-inactivation, where one X chromosome in females is randomly inactivated, forming a Barr body. This ensures equal expression of X-linked genes in males and females.

X-Linked Inheritance

X-linked genes show unique inheritance patterns due to the presence of only one X chromosome in males (hemizygosity). Classic studies in Drosophila by Thomas Hunt Morgan demonstrated X-linkage using eye color mutations.

Sex-Limited and Sex-Influenced Inheritance

Sex-Limited Inheritance

Sex-limited inheritance occurs when a trait is expressed in only one sex, even though both sexes carry the genes. An example is feather type in domestic fowl, where only males can express the cock-feathered phenotype.

Sex-Influenced Inheritance

Sex-influenced inheritance occurs when the expression of a trait is affected by the sex of the individual, often due to hormonal differences. Pattern baldness in humans is an example, where the phenotype is more commonly expressed in males.

Summary Table: Extensions of Mendelian Genetics