Essential Concepts in Genetics and Heredity

Core Terminology in Classical Genetics

• Character (Trait): An inherited attribute of an individual.
• Gene: The fundamental unit of heredity responsible for a character.
• Alleles: The different variations or forms that a gene can take.
• Homozygous: An individual possessing two identical alleles for a specific gene.
• Heterozygous: An individual possessing two different alleles for a specific gene.
• Genotype: The specific set of genes an individual possesses.
• Phenotype: The set of observable, hereditary traits expressed by an individual.

Allelic Expression and Inheritance Patterns

• Dominant Factor (Allele): The allele that is phenotypically expressed in both homozygous and heterozygous individuals.
• Recessive Factor (Allele): The allele whose phenotypic expression is masked by a dominant allele.
• Intermediate Inheritance: A pattern where each allele partially dominates the other, resulting in a blended phenotype.
• Codominant Inheritance: A pattern where two alleles are equally and simultaneously expressed in the phenotype.

Chromosomal Basis of Inheritance

Thomas H. Morgan's Discoveries

Thomas H. Morgan observed in the fruit fly (Drosophila) that multiple characters were always inherited together, leading to the concept of linked genes.

Key Aspects of Chromosome Theory

• Genes are located linearly along chromosomes.
• Each gene occupies a specific position on the chromosome, known as a locus.
• Genetic variation occurs through homologous recombination (crossing over) between chromatids.

Linked Genes

Linked genes are two or more genes located on the same chromosome that tend to be transmitted together. Linked genes can be separated if crossing over (recombination) occurs between them.

Specific Genetic Conditions and Interactions

• Hemophilia: A deficiency in blood coagulation (clotting).
• Color Blindness: The inability to distinguish certain colors, typically green from red.

Gene Interactions

Epistasis

A phenomenon where the allele of one gene masks the phenotypic expression of alleles of a different gene. The individual does not display the characters determined by the latter gene.

Polygenic Inheritance

Characteristics of individuals, such as size or weight, that are the result of the cumulative action of many genes.

Genotype-Phenotype Relationship Metrics

Penetrance

Defined as the proportion of individuals with a specific genotype who actually exhibit the corresponding phenotype.

Expressivity

The degree or intensity to which a particular genotype is expressed in the phenotype.

Molecular Genetics: Transcription and Gene Regulation

Coding Region

A segment of DNA from which the messenger RNA (mRNA) molecule is transcribed.

Termination Signal

The group of nucleotides that determines the final point of transcription.

The Operon Model

An operon is a set of genes that encode proteins with related functions, along with the regulatory sequences necessary for their expression. An operon typically includes:

• Promoter: The DNA sequence that acts as the start signal for RNA polymerase.
• Operator: A regulatory region, often adjacent to the promoter, where repressor proteins can bind.
• Structural Genes: The genes responsible for the synthesis of functional RNA and subsequent proteins.
• Regulatory Genes: Genes that synthesize proteins (like repressors or activators) that control the expression of the structural genes.