Introduction to Genetics

Genes

• Definition: Units of heredity that are responsible for the transmission of traits from parents to offspring.

• Nature: Genes occur in pairs (alleles) and are located on chromosomes, which are found in the nucleus of cells.

Alleles

• Definition: Different forms of the same gene, distinguished by variations in DNA sequence.

• Example: Wild type (normal allele) versus mutant allele that may cause disease.

Homozygous and Heterozygous

• Homozygous: Organisms having two identical alleles for a given trait (e.g., AA or aa).

• Heterozygous: Organisms possessing two different alleles for a trait (e.g., Aa).

Dominance in Alleles

• Dominant Alleles: Alleles that express their phenotype even in the presence of a different allele (heterozygous state).

• Recessive Alleles: Alleles that only manifest their characteristics when present in a homozygous state (e.g., aa).

Genotype vs. Phenotype

• Genotype: The genetic constitution of an individual, encompassing all alleles.

• Phenotype: The observable physical and physiological traits of an individual, influenced by genotype and environmental factors.

Patterns of Inheritance

1. Autosomal Dominant

• Key Characteristics:

  • Heterozygotes exhibit the disease phenotype.

  • Equal proportions of affected males and females.

  • Each affected person has a 50% chance to pass the trait to offspring.

  • Unaffected individuals cannot transmit the trait, ensuring that unaffected parents cannot have affected children.

2. Autosomal Recessive

• Key Characteristics:

  • Only individuals with two copies of the recessive allele (homozygotes) display the disease phenotype.

  • Equal proportions of affected males and females.

  • Parents of affected individuals are often carriers (heterozygotes) and may not show symptoms themselves.

  • Frequently, affected individuals emerge with no prior family history of the condition due to the carrier status of parents.

3. X-linked Diseases

• Key Characteristics:

  • No transmission of the trait from father to son.

  • Both genders can be affected; the ratio may vary depending on whether the condition is dominant or recessive.

  • In recessive X-linked conditions, males are predominantly affected (having only one X chromosome), whereas females may be carriers or unaffected.

  • In dominant X-linked traits, affected males transmit the condition to all daughters but none to sons.

Understanding Pedigrees

• Symbols Used in Pedigrees: Specific symbols denote identical twins, fraternal twins, sexes, marriages, and affected individuals.

• Generational Representation: Generations are represented by Roman numerals (I, II, III), while individuals within generations are denoted by Arabic numbers (1, 2, 3).

Genetic Markers and Polymorphisms

• Polymorphisms: Variations in DNA sequences among individuals in a population.

• Mutations: Changes in DNA that can lead to defective proteins and diseases; some mutations are neutral and have no effect on function.

  • Single Nucleotide Polymorphisms (SNPs): Variations at a single nucleotide position in the genome.

  • Restriction Fragment Length Polymorphisms (RFLPs): Variations that alter restriction enzyme recognition sites, impacting DNA fragment size when cut.

Restriction Enzymes Basics

• Function: Enzymes that cut DNA at specific nucleotide sequences, generating fragments of varying lengths for analysis.

• Fragment Ends: Depending on the type of restriction enzyme, the resultant DNA fragments may have blunt ends or protruding (sticky) ends, influencing how they can be used in cloning and analysis.

Types of Mutations

1. Point Mutations

• Silent Mutations: Do not change the amino acid sequence of a protein.

• Missense Mutations: Result in the substitution of one amino acid for another, which may alter protein function depending on the role of the altered amino acid.

  • Example: Sickle cell disease caused by a single amino acid change in hemoglobin.

• Nonsense Mutations: Introduce a premature stop codon, truncating the protein and typically rendering it nonfunctional.

  • Example: Beta-thalassemia.

2. Insertions/Deletions

• Frameshift Mutations: Alter the reading frame of the genetic code, resulting in widespread changes to the amino acid sequence following the mutation.

  • Example: Haemophilia A, caused by an insertion mutation that disrupts the coagulation factor gene.

Impact of Mutations on Restriction Enzyme Cutting

• Significance: Mutations can affect whether restriction enzymes can cut DNA at specific sites, altering fragment sizes and patterns seen on agarose gels during electrophoresis, which is essential for genetic analysis.

Techniques for DNA Detection

• Restriction Enzyme Digestion: Dismantles DNA into fragments of various sizes, crucial for subsequent analysis.

• Gel Electrophoresis: Enables the separation of DNA fragments based on size; smaller fragments move faster through the gel compared to larger ones.

• Southern Blotting: Technique combining gel electrophoresis with membrane transfer, allowing the detection of specific DNA sequences using labeled probes.

Charcot-Marie-Tooth Disease (CMT)

• Overview: A hereditary demyelinating disease, resulting in motor and sensory neuropathies affecting limb strength and sensation.

• Prevalence: Occurs in approximately 1 in 2500 individuals; manifests variably across affected individuals.

• Genetic Causes: May arise due to autosomal dominant, autosomal recessive, or X-linked inheritance patterns (CMTX). Common genetic causes involve mutations in genes such as PMP-22 (in 60-70% of cases), mutations in myelin protein zero (P0), or connexin 32 (CMTX).

Role of Connexin 32 (Cx32)

• Function: Found in Schwann cells, Cx32 forms gap junctions that are critical for nutrient transfer and communication in the myelin sheath surrounding peripheral axons.

• Pathology: Loss-of-function mutations in Cx32 lead to degeneration of inner myelin layers and loss of axons, contributing to the disease phenotype seen in affected individuals.

Conclusion

• Summary: Understanding the complexities of genetics, patterns of inheritance, and the mechanisms behind specific diseases is essential for the identification, research, and management of genetic disorders.