Genes and Genetic Diseases Study Notes

Deoxyribonucleic Acid (DNA) Structure and Function

• Nucleotide Structure
  • DNA is composed of basic subunits called nucleotides.
  • Each nucleotide consists of three distinct components:
    • Deoxyribose: A five-carbon sugar molecule.
    • Phosphate molecule: Forms the sugar-phosphate backbone of the DNA chain.
    • Nitrogenous base: Divided into two chemical categories:
      • Pyrimidines: Cytosine and thymine.
      • Purines: Adenine and guanine.
• Double Helix Model
  • DNA is structured as a double-stranded helix consisting of two complementary anti-parallel strands.
• Protein Synthesis Direction
  • DNA contains the genetic code that directs the synthesis of proteins within the cell.
  • Proteins are composed of one or more polypeptides.
  • Polypeptides are made up of sequences of amino acids.
  • There are $20$ standard amino acids utilized in protein construction.
  • The sequence of amino acids in a polypeptide is specified by sequences of three nitrogenous bases known as codons.

DNA Replication

• Unwinding Process
  • DNA replication begins with the untwisting and unzipping of the double-stranded DNA molecule.
  • Each separated single strand serves as a template for the synthesis of a new complementary strand.
• Complementary Base Pairing
  • Enzymatic synthesis is mediated by DNA polymerase.
  • Base pairing rules dictate specific complementary pairing:
    • Adenine pairs exclusively with thymine ($A-T$).
    • Cytosine pairs exclusively with guanine ($C-G$).
• Cellular Location
  • DNA replication occurs strictly within the cell nucleus, not in the cytoplasm.

Genetic Mutations

• Definition
  • A mutation is defined as any inherited alteration of genetic material (DNA).
• Types of Chromosomal and Base Mutations
  • Base Pair Substitution:
    • Occurs when one base pair is substituted for another.
    • May alter a single amino acid or result in a silent change.
  • Frameshift Mutation:
    • Occurs due to the insertion or deletion of one or more base pairs.
    • Alters the entire reading frame of the gene, radically changing the downstream amino acid sequence.
• Spontaneous Mutations and Hot Spots
  • Spontaneous Mutation: A genetic mutation that occurs in the absence of exposure to known mutagens.
  • Mutational Hot Spots: Specific areas along chromosomes that exhibit abnormally high rates of mutation.
• Mutagens
  • Mutagens are physical or chemical agents known to increase the frequency of genetic mutations.
  • Radiation: Ionizing radiation capable of damaging DNA strands.
  • Chemical Mutagens: Include substances such as:
    • Nitrogen mustard
    • Vinyl chloride
    • Alkylating agents
    • Formaldehyde
    • Sodium nitrite

Protein Synthesis: Transcription, Gene Splicing, and Translation

• Transcription
  • Process in which messenger RNA (mRNA) is synthesized from a DNA template.
  • RNA Polymerase Activity:
    • RNA polymerase binds to a specific promoter site on the DNA strand.
    • RNA polymerase synthesizes pre-mRNA until it reaches a termination sequence, at which point it detaches.
  • RNA Base Differences:
    • RNA is single-stranded.
    • RNA contains uracil instead of thymine; uracil is structurally similar to thymine and pairs with adenine.
  • Migration: Following synthesis and processing, mRNA leaves the cell nucleus and enters the cytoplasm.
• Gene Splicing
  • Introns: Non-coding sequences within the initial pre-mRNA transcript that are removed.
  • Exons: Coding sequences that remain and are spliced back together.
  • Splicing occurs prior to the migration of mature mRNA into the cytoplasm.
• Translation
  • Process by which RNA directs the synthesis of a polypeptide in interaction with transfer RNA (tRNA).
  • Ribosome: The primary cellular site of protein synthesis, consisting of small and large ribosomal subunits.
  • tRNA Structure and Function:
    • Each tRNA molecule carries a specific amino acid.
    • tRNA contains a three-nucleotide sequence called an anticodon, which is complementary to a specific three-nucleotide codon on the mRNA strand.
  • Elongation and Termination:
    • The ribosome moves sequentially along the mRNA strand, matching codons with anticodons and catalyzing peptide bond formation between amino acids.
    • Translation continues until a termination signal (stop codon) is encountered.
    • Upon termination, the completed polypeptide chain is released into the cytoplasm.

Chromosomal Structure and Organization

• Somatic Cells
  • Contain $46$ chromosomes arranged in $23$ pairs.
  • Are diploid cells ($2n$).
  • Replicate and divide via mitosis.
• Gametes
  • Contain $23$ individual chromosomes.
  • Are haploid cells ($n$), possessing only one member of each chromosome pair.
  • Formed through meiosis.
• Autosomes vs. Sex Chromosomes
  • Autosomes:
    • The first $22$ of the $23$ pairs of chromosomes in both males and females.
    • The two members of each autosomal pair are virtually identical in appearance and gene location, making them homologous.
  • Sex Chromosomes:
    • The $23$rd pair of chromosomes determining biological sex.
    • In females, it consists of a homologous pair ($XX$).
    • In males, it consists of a nonhomologous pair ($XY$).
• Karyotype (Karyogram)
  • An ordered systematic display of an individual's full set of chromosomes.
  • Key structural features include centromeres, kinetochores, cohesin proteins, and sister chromatids.

Chromosomal Aberrations and Numerical Abnormalities

• Euploid Cells
  • Cells with an exact multiple of the normal haploid number ($23$) of chromosomes.
  • Both haploid ($23$) and diploid ($46$) cells are standard euploid forms.
  • Polyploidy:
    • Occurs when a euploid cell has more than the diploid number of chromosomes.
    • Triploidy: A zygote possessing three copies of each chromosome, totaling $69$ chromosomes.
    • Tetraploidy: A zygote possessing four copies of each chromosome, totaling $92$ chromosomes.
    • Neither triploid nor tetraploid fetuses survive to viability.
• Aneuploidy
  • A somatic cell that does not contain an exact multiple of $23$ chromosomes.
  • Trisomy: A cell containing three copies of a single chromosome. Infants can survive with trisomy of certain autosomes.
  • Monosomy: The presence of only one copy of a given chromosome in a cell. Monosomy of any autosome is lethal.
  • General genetic rule: Loss of chromosome material is more severe than gain of extra material ("It is better to have extra than less").
• Nondisjunction
  • The primary underlying mechanism causing aneuploidy.
  • Defined as the failure of homologous chromosomes or sister chromatids to separate normally during meiotic (Meiosis I or Meiosis II) or mitotic division.

Autosomal Aneuploidies

• Partial Trisomy
  • Condition where only an extra portion of a specific chromosome is present in each cell.
• Chromosomal Mosaics
  • Trisomies or genetic aberrations that occur in only a subset of cells throughout the body, resulting from mitotic nondisjunction during embryonic development.
• Down Syndrome (Trisomy 21)
  • The best-known and most common autosomal aneuploidy.
  • Caused by an extra copy of chromosome $21$.
  • Incidence: Approximately $1:800$ live births.
  • Clinical Presentation and Characteristics:
    • Intellectual disability
    • Low nasal bridge
    • Epicanthal folds
    • Protruding tongue
    • Low-set ears
    • Poor muscle tone (hypotonia)
  • Risk Factor: Risk increases markedly with maternal age, particularly above $35$ years.

Sex Chromosome Aneuploidies

• Trisomy X ($47,XXX$)
  • A female possessing three X chromosomes in each cell.
  • Clinical Features: Highly variable; can include sterility, menstrual irregularity, and/or intellectual disability.
  • Symptom severity increases progressively with each additional X chromosome ($48,XXXX$ or $49,XXXXX$).
• Turner Syndrome ($45,X$)
  • Females possessing only a single X chromosome.
  • Clinical Presentation and Characteristics:
    • Underdeveloped ovaries resulting in sterility
    • Short stature (average height $\sim 4\text{ feet } 7\text{ inches}$)
    • Webbing of the neck
    • Peripheral edema
    • Underdeveloped breasts with widely spaced nipples
    • High rate of spontaneous fetal abortion
  • The single X chromosome is inherited from the mother in the majority of cases.
• Klinefelter Syndrome ($47,XXY$)
  • Individuals possessing at least two X chromosomes and one Y chromosome.
  • Clinical Presentation and Characteristics:
    • Overall male physical appearance
    • Gynecomastia (development of female-like breasts)
    • Small testes (hypogonadism); individual is usually sterile
    • Sparse body hair
    • Long upper and lower limbs
  • Can include complex karyotypes such as $48,XXXY$
  • Physical and cognitive abnormalities increase in severity with each extra X chromosome.

Structural Abnormalities of Chromosomes

• Chromosome Breakage and Clastogens
  • If chromosome breakage occurs, physiologic repair mechanisms usually repair the break, but structural alterations may result.
  • Clastogens: Harmful agents that increase the frequency of chromosome breakage.
    • Examples include ionizing radiation, toxic chemicals, and viruses.
• Deletion
  • Breakage and loss of a portion of chromosomal DNA.
  • Cri du Chat Syndrome ("Cry of the Cat"):
    • Caused by a deletion of the short arm of chromosome $5$
    • Characteristics: High-pitched cat-like cry, low birth weight, severe intellectual disability, and microcephaly.
• Duplication
  • Presence of a repeated gene or gene sequence.
  • Consequences are generally less severe than deletions because gaining genetic material is better tolerated than losing material.
  • Duplication in the same region of chromosome $5$ that causes Cri du Chat leads to intellectual disability but less severe physical abnormalities.
• Inversions
  • Occurs when two breaks take place on a single chromosome followed by the reversal and reattachment of the inverted segment (e.g., sequence $ABCDEFG$ becomes $ABEDCFG$).
  • Position Effect: Gene expression changes caused by an altered chromosomal position without changing the total genetic content.
• Translocations
  • The interchanging of genetic material between nonhomologous chromosomes.
  • Reciprocal Translocation: Occurs when two nonhomologous chromosomes break and their segments are rejoined in an abnormal arrangement.
  • Robertsonian Translocation: Occurs when the long arms of two nonhomologous acrocentric chromosomes fuse at the centromere, forming a single large chromosome.
• Fragile Sites and Fragile X Syndrome
  • Fragile Sites: Chromosomal areas that develop distinctive breaks or gaps when cells are cultured in specific deficient media.
  • Fragile X Syndrome:
    • Located on the long arm of the X chromosome.
    • Associated with substantial intellectual disability; it is the second most common genetic cause of intellectual disability after Down syndrome.
    • Exhibits a significantly higher incidence in males because they possess only a single X chromosome.

Principles of Genetics and Allelic Variation

• Terminology
  • Locus: The specific physical position occupied by a gene along a chromosome.
  • Allele: An alternative form of a specific gene located at a given locus.
  • Polymorphism: A locus that possesses two or more alleles occurring with appreciable frequency in a population.
  • Homozygous: Loci on a pair of homologous chromosomes containing identical alleles.
  • Heterozygous: Loci on a pair of homologous chromosomes containing different alleles.
• Genotype vs. Phenotype
  • Genotype: The specific genetic composition of an organism at a given locus ("what they have").
  • Phenotype: The observable, outward physical or biochemical demonstration of an organism's genetic makeup ("what they demonstrate").
  • Phenylketonuria (PKU) Example:
    • An infant born with the PKU genotype cannot metabolize phenylalanine properly.
    • If left untreated, severe intellectual disability becomes a core feature of the PKU phenotype.
    • If dietary intervention is administered, the child demonstrates an outwardly normal PKU phenotype.
• Dominance and Recessiveness
  • Dominant Allele: An allele whose effect is observable when paired with a different allele in a heterozygous state (represented by a capital letter).
  • Recessive Allele: An allele whose effects are hidden or masked in the presence of a dominant allele (represented by a lowercase letter).
  • Codominant Alleles: Occurs when both alleles in a heterozygote are fully expressed in the phenotype.
  • Carrier: An individual who harbors a disease-causing recessive allele but is phenotypically normal.

Patterns of Single-Gene Inheritance

• Modes of Single-Gene Transmission
  • Autosomal dominant
  • Autosomal recessive
  • X-linked dominant
  • X-linked recessive
• Pedigree Analysis
  • A stylized family tree chart used to track specific genetic traits across generations.
  • Proband: The initial affected individual within a family who brings the genetic disorder to medical attention.

Autosomal Dominant Inheritance

• Transmission Dynamics
  • Affected offspring are most commonly produced by the mating of a normal parent ($dd$) with an affected heterozygous parent ($Dd$).
  • On average, $50\%$ ($1/2$) of the offspring will be normal ($dd$), and $50\%$ ($1/2$) will be heterozygous ($Dd$) and express the disease.
• Recurrence Risk
  • The probability that a given individual will inherit or express a genetic disease.
  • When one parent is affected by an autosomal dominant condition and the other parent is un-affected:
    • The occurrence and recurrence risk for each child is $50\%$ ($1/2$).
    • Each pregnancy is an independent event; previous child outcomes do not alter the $50\%$ risk for future children.
• Penetrance
  • The percentage of individuals with a specific disease genotype who actually demonstrate the expected disease phenotype.
  • Incomplete Penetrance: Occurs when an individual inherits a disease-causing genotype but fails to express the clinical phenotype.
  • Retinoblastoma Example: Retinoblastoma (a malignant eye tumor in children) displays incomplete penetrance at $90\%$.
• Expressivity
  • The extent or severity of variation in phenotype exhibited by individuals with a specific identical genotype.
  • Neurofibromatosis Type 1: Classic autosomal dominant example showing variable expressivity (ranging from mild café-au-lait spots to severe neurofibromas).
  • Factors Causing Variable Expressivity:
    • Modifier genes
    • Environmental factors
    • Different specific mutations occurring at the same gene locus
• Epigenetics and Genomic Imprinting
  • Epigenetics: Chemical modifications of DNA/chromatin (e.g., methylation) that alter gene expression without changing the underlying DNA sequence, resulting in different phenotypes.
  • Genomic Imprinting: Epigenetic process where one parent preferentially inactivates (imprints) a gene during gamete transmission to offspring.

Autosomal Recessive Inheritance

• Transmission Dynamics
  • In the majority of cases, both parents of an affected individual are asymptomatic heterozygous carriers ($Dd$).
  • Offspring Risk Breakdown:
    • $25\%$ ($1/4$) homozygous normal ($DD$)
    • $50\%$ ($1/2$) heterozygous carrier ($Dd$)
    • $25\%$ ($1/4$) homozygous affected ($dd$)
  • The recurrence risk for offspring of two carrier parents is $25\%$.
  • Males and females are affected in equal proportions.
  • Cystic Fibrosis: Classic example of an autosomal recessive single-gene disease.
• Consanguinity
  • The mating of two biologically related individuals.
  • Dramatically increases the recurrence risk for autosomal recessive conditions by increasing the likelihood that both parents share rare recessive alleles inherited from a common ancestor.

X-Linked Inheritance

• Mechanisms of Sex-Linked Inheritance
  • Refers to genetic conditions caused by mutations on sex chromosomes.
  • Most sex-linked traits are located on the X chromosome and are designated as X-linked.
  • X-Linked Recessive: The most common form of sex-linked inheritance.
  • Males are primarily affected because they possess only one X chromosome ($XY$) and lack a second X chromosome to counteract an abnormal gene.
• X Inactivation (Lyon Hypothesis)
  • One X chromosome in female somatic cells is permanently inactivated early during embryonic development.
  • Once inactivation occurs in a cell, all daughter cells derived from it retain the exact same X chromosome inactivation.
  • Inactivation Characteristics: Random, fixed, and incomplete.
• Sex Determination
  • Biological male sex is determined by the SRY gene located on the Y chromosome, which triggers the cascades of downstream sex-determining genes.
• Pedigree Characteristics of X-Linked Recessive Inheritance
  • Affected males cannot pass the mutated X gene to their sons ($Y$ chromosome passed to sons).
  • Affected males pass the mutated X gene to all of their daughters, who become obligate carriers.
  • Sons born to female carrier mothers have a $50\%$ ($1/2$) probability of inheriting the affected X chromosome and expressing the condition.

Sex-Limited and Sex-Influenced Traits

• Sex-Limited Traits
  • Traits that can occur in only one sex, usually due to structural or anatomical differences.
  • Examples: Inherited testicular or ovarian defects.
• Sex-Influenced Traits
  • Traits that occur much more frequently or severely in one sex than the other, despite being autosomal.
  • Examples:
    • Male-pattern baldness
    • Autosomal dominant breast cancer

Linkage Analysis and Gene Mapping

• Crossing Over and Recombination
  • During Meiosis I, homologous chromosomes exchange segments in a process known as crossover.
  • Crossover leads to the recombination of alleles located on the same chromosome.
  • The frequency of genetic recombination is proportional to the physical distance separating two loci on a chromosome.
  • Linkage: Loci that are located very close together on a chromosome are unlikely to undergo recombination during crossover and tend to be inherited together.
• Human Genome Project
  • Successfully generated a comprehensive physical and gene map of all human chromosomes.
  • Identified specific genes responsible for over $5000$ genetic conditions.

Polygenic and Multifactorial Inheritance

• Polygenic Traits
  • Traits in which phenotypic variation is caused by the combined additive effects of multiple gene loci.
• Multifactorial Traits
  • Traits in which phenotypic variation is driven by a combination of complex genetic factors and environmental factors.
  • Characterized by a threshold of liability, where an individual must surpass a cumulative genetic/environmental threshold to express the phenotypic disease.
• Recurrence Risks in Multifactorial Diseases
  • Recurrence risks can change substantially between different geographic populations or demographic groups.
  • Recurrence risk increases significantly if more than one family member is clinically affected.
  • Empirical risks for multifactorial diseases are estimated from direct clinical observation and epidemiological data rather than simple Mendelian ratios.