Comprehensive Genetics and Nursing Study Guide

Importance of Genetics in Nursing Practice

• Genetics influences various aspects of healthcare including disease risk, medication response, screening, prevention, and patient education.

• Nurses play a critical role in:

  • Identifying genetic risk factors through assessment.

  • Collecting and interpreting family histories (pedigrees).

  • Providing patient education and emotional support.

  • Recognizing ethical, legal, and psychosocial implications of genetic information.

Basic Genetic Terminology

• Gene: A segment of DNA that codes for a specific protein.

• Genome: The entire genetic material of an individual.

• Chromosome: A structure containing DNA. Humans typically have $46$ chromosomes arranged in $23$ pairs.

• Allele: Different forms of a single gene. One member of a pair or series of genes that occupy a specific position on a specific chromosome.

• Genotype: The complete genetic constitution of an organism; the specific combination and location of genes/alleles on chromosomes (e.g., $AA$ or $Aa$).

• Phenotype: The physical appearance or observable biologic, physiologic, and molecular traits (e.g., stature, blood type) resulting from genetic and environmental influences.

• Mutation: A permanent change in the DNA sequence.

• Carrier: A person who possesses one recessive allele for a trait but does not exhibit symptoms of the disease.

• Penetrance: The likelihood that a gene will be expressed.

• Expressivity: The degree to which a trait is expressed in an individual.

DNA, Genes, and Chromosomes

• DNA contains the instructions for chromosomes.

• Each chromosome includes many different genes.

• Chromosomes come in pairs; upon conception, an individual receives one of each pair from each parent.

• Autosomes: The $22$ pairs of numbered chromosomes.

• Sex Chromosomes: The $23^{rd}$ pair.

  • $XX$: Female.

  • $XY$: Male.

Inheritance Patterns and Single Gene Disorders

• Single Gene Disorders: Occur due to a mutation in one gene.

  • Autosomal Disorders: Affect genes on one of the $22$ autosomal pairs.

  • X-Linked Disorders: Affect the sex chromosomes.

• Allele Dominance:

  • Dominant: A single copy of a gene is sufficient to express the character (usually denoted by a capital letter).

  • Recessive: Two copies of the gene are required to express the character.

Autosomal Dominant Inheritance

• Characteristics:

  • Affects one parent (usually).

  • Appears in every generation.

  • Males and females are affected equally.

  • There is a $50\%$ chance of inheritance for offspring if one parent is a heterozygous carrier ($Aa$) and the other is unaffected ($aa$).

• Examples:

  • Huntington Disease: A progressive, fatal neurodegenerative disorder causing movement, cognitive, and psychiatric disturbances. Symptoms usually begin in adulthood.

  • Marfan Syndrome: Connective tissue disorder.

    • Skeletal: Tall, thin habitus, long limbs and fingers (arachnodactyly), joint hypermobility, scoliosis.

    • Cardiovascular (Life-threatening): Aortic aneurysm/dissection, mitral valve prolapse, heart murmurs.

    • Ocular: Lens subluxation, myopia, risk for retinal detachment.

  • Neurofibromatosis Type 2: Characterized by tumor growth along nerves (e.g., bilateral acoustic neuromas/vestibular schwannomas). Symptoms include hearing loss, tinnitus, and balance problems, often presenting in adolescence.

  • Osteogenesis Imperfecta (Brittle Bone Disease): Connective tissue disorder due to a defect in type I collagen. Characterized by fragile bones that fracture easily with minimal trauma.

• Nursing Implications: Early screening, education on disease progression, and referrals for family planning and counseling.

Autosomal Recessive Inheritance

• Characteristics:

  • Both parents are usually asymptomatic carriers ($Aa$).

  • Often skips generations.

  • Probabilities for offspring (Carrier x Carrier):

    • $25\%$ Affected ($aa$).

    • $50\%$ Carriers ($Aa$).

    • $25\%$ Unaffected ($AA$).

• Examples:

  • Cystic Fibrosis (CF): Multisystem disorder involving thick, sticky secretions. Defective chloride transport leads to decreased water in secretions, causing airway obstruction, pancreatic duct obstruction (malabsorption), and GI/reproductive issues.

  • Sickle Cell Disease: Genetic blood disorder.

  • Tay-Sachs Disease: Neurodegenerative condition.

  • Phenylketonuria (PKU): Requires strict dietary management.

  • Albinism: Defect in melanin production. Symptoms include very light skin/hair, inability to tan, high risk of skin cancer, and ocular issues (reduced acuity, nystagmus, strabismus, photophobia).

• Nursing Implications: Carrier screening education, newborn screening follow-up, and teaching adherence to medications and nutrition.

X-Linked Recessive Inheritance

• Characteristics:

  • Usually affects males ($X^{R}Y$) because they have only one X chromosome.

  • Females are typically carriers ($XX^{R}$).

  • There is no father-to-son transmission (fathers provide the Y chromosome to sons).

• Examples:

  • Hemophilia A and B: Bleeding disorders caused by clotting factor deficiency (Factor VIII in Hemophilia A). Results in unstable fibrin clots and prolonged bleeding. Common sites include joints (hemarthrosis), muscles, and internal organs.

  • Duchenne Muscular Dystrophy (DMD): Caused by a mutation/deletion of the dystrophin gene. Leads to progressive muscle degeneration starting in childhood.

  • Color Blindness: missing or damaged photo pigment genes on the X chromosome. Red-Green is the most common sex-linked type.

• Color Blindness Types:

  • Complete (Achromatopsia): Total inability to see color.

  • Red-Green: Protanopia, deuteranopia, protanomaly, deuteranomaly.

  • Blue-Yellow: Tritanopia, tritanomaly.

Chromosomal Disorders

• Definition: Result from extra or missing chromosomes.

• Down Syndrome (Trisomy 21):

  • Extra copy of chromosome $21$.

  • Features: Upward slanting eyes, epicanthal folds, single palmar crease, small low-set ears, broad flat nose, protruding tongue, hypotonia.

  • Comorbidities: Congenital heart defects ($40\%$), cataracts, intellectual disability, hypothyroidism, immune dysfunction.

  • Nursing: Monitor for respiratory infections, routine thyroid screening, and maintain neutral neck positioning (due to atlantoaxial instability).

• Turner Syndrome (XO):

  • Affects females; one X chromosome is missing or altered ($45,X$).

  • Features: Short stature, webbed neck, lymphedema of hands/feet, skeletal abnormalities.

  • Reproductive: Early loss of ovarian function, primary amenorrhea, infertility.

• Klinefelter Syndrome (XXY):

  • Affects males; extra X chromosome.

  • Features: Gynecomastia, infertility.

Genetic Testing and Nursing Responsibilities

• Types of Testing:

  • Diagnostic: Confirms a disease (e.g., Karyotype for Down syndrome).

  • Carrier Screening: Identifies recessive condition carriers.

  • Prenatal: Amniocentesis ($\ge 15$ weeks), Chorionic Villus Sampling (CVS, $10-13$ weeks).

  • Newborn Screening: Early intervention (e.g., PKU).

  • Predictive: Identifies future risk (e.g., BRCA1/BRCA2).

  • Pharmacogenomic: Determines drug response.

• Nursing Roles:

  • Before Testing: Provide informed consent, explain purpose and risks (screening vs. testing), assess emotional readiness.

  • During Testing: Monitor maternal/fetal status and maintain aseptic technique.

  • After Testing: Monitor for complications (bleeding, infection, fluid leakage), provide emotional support, and refer to counseling.

Pharmacogenomics

• Definition: The study of how genetic differences affect drug metabolism and response, allowing for personalized treatment.

• Examples:

  • Warfarin: Variants in $CYP2C9$ and $VKORC1$ genes affect sensitivity. Slower metabolism necessitates lower doses to avoid bleeding risk. Monitor INR closely.

  • Clopidogrel (Plavix): $CYP2C19$ poor metabolizers cannot effectively activate the drug, increasing clotting risk.

  • Statins: $SLCO1B1$ variants increase risk of statin-induced myopathy or rhabdomyolysis.

Epigenetics and Multifactorial Disorders

• Epigenetics: Changes in gene expression without altering the DNA sequence (e.g., DNA methylation, histone modification). Often influenced by environment (smoking, diet, stress) and can be reversible.

• Multifactorial Genetic Disorders: Caused by a combination of multiple genes (polygenic) and environmental factors.

  • Examples: Heart disease, Type 2 diabetes, hypertension, certain cancers, neural tube defects (spina bifida), cleft lip/palate.

  • Characteristics: Variable expression and difficult to predict recurrence risk.

Assessment and Pedigrees

• Three-Generation Pedigree: The gold standard for risk assessment.

  • First Gen: Parents.

  • Second Gen: Patient and siblings.

  • Third Gen: Patient's children.

• Purpose: Identify inheritance patterns, assess risk to relatives, and guide preventive care.

Ethical, Legal, and Social Issues

• Informed Consent: Patients must understand risks/benefits.

• Autonomy: Respecting the patient's choice to test or share results.

• Confidentiality: Genetic info is sensitive; protect privacy.

• Discrimination: Concerns regarding insurance or employment.

• GINA (Genetic Information Nondiscrimination Act): Protects against health insurance and employment discrimination.

Questions & Discussion

• Q: If an unaffected parent and an affected heterozygous parent ($Aa$) for an autosomal dominant trait are expecting, what is the chance of an unaffected child?

• A: $50\%$. (Rationale: The cross is $Aa \times aa$, resulting in $50\%$ $Aa$ and $50\%$ $aa$).

• Q: Two phenotypically normal individuals have an affected child. What is the conclusion?

• A: They both carried the disease allele (autosomal recessive carrier status).

• Q: A father has an X-linked recessive disorder and the mother is unaffected/not a carrier. What is the outcome?

• A: All daughters will be carriers ($100\%$). Sons will be unaffected ($0\%$) because they get the Y from the father.

• Q: A woman is a carrier for Hemophilia A and the father is unaffected. What is the chance a son will have hemophilia?

• A: $50\%$. (Rationale: The mother has one affected X to pass to her sons).

• Q: Nursing intervention for a child with Hemophilia A?

• A: Apply firm pressure to venipuncture sites for at least $10$ minutes. (Rationale: Prolonged pressure is needed due to factor deficiency).