Genetic Disorders

Core Concepts

DNA, Genes, and Chromosomes: The Basics

DNA is the molecule that carries genetic instructions, organized into units called genes. Genes are packaged into chromosomes — humans have 46 (23 pairs), one pair being the sex chromosomes (XX in people assigned female, XY in people assigned male). Each person inherits one copy of each pair from each parent, so every gene exists in two copies, called alleles. A mutation is a change in the DNA sequence that may have no effect, a small effect, or a devastating one. Most genetic variation is normal — disorders arise when a change disrupts a gene's function or when chromosomes are abnormal in number or structure.

Chromosomal Disorders

Chromosomal disorders involve whole chromosomes rather than single genes — usually an abnormal number (aneuploidy) or a structural change — and the effects are widespread because each chromosome carries many genes. Classic examples:

• Trisomy 21 (Down syndrome): an extra copy of chromosome 21; the most common viable chromosomal disorder, associated with characteristic physical features, intellectual disability of variable degree, and increased risk of heart defects and other health conditions.
• Turner syndrome (monosomy X): a person assigned female at birth with only one X chromosome; features include short stature and ovarian dysfunction.
• Klinefelter syndrome (XXY): a person assigned male at birth with an extra X chromosome; features include tall stature and reduced fertility.

Chromosomal disorders are usually not inherited — they arise as errors in egg, sperm, or early cell division; risk for some (like trisomy 21) increases with parental age.

Single-Gene (Mendelian) Disorders

Single-gene disorders are caused by mutations in one gene and follow recognizable inheritance patterns (detailed in the next topic):

• Autosomal dominant: one altered copy of the gene causes the disorder. An affected person typically has an affected parent, and each child has a 50% chance of inheriting it. Example: Huntington disease, which appears in adulthood.
• Autosomal recessive: both copies must be altered. Affected people usually have unaffected carrier parents; each child of two carriers has a 25% chance of being affected. Examples: cystic fibrosis, sickle cell disease, Tay-Sachs disease.
• X-linked recessive: the altered gene is on the X chromosome; males (one X) are affected much more often, while females (two Xs) are usually carriers. Examples: hemophilia A, Duchenne muscular dystrophy.

Recognizing the pattern — who is affected across generations, and which sexes — is the skill nurses practice when building a family history.

Multifactorial Disorders

Most common diseases — type 2 diabetes, coronary artery disease, hypertension, many cancers — are multifactorial: multiple genes interacting with environment and lifestyle (diet, activity, smoking, stress). They cluster in families but do not follow simple Mendelian patterns, and risk is modified by behavior — which is why the nursing message is never "your genes decide your fate."

Types of Mutations (Concept Level)

Mutations vary by mechanism: point mutations (a single DNA letter changed), deletions or insertions (letters removed or added — which can shift the reading frame), duplications, and structural changes such as translocations (a piece of one chromosome attaching to another). The effect depends on whether the change alters a protein's function; some mutations are inherited, others arise spontaneously.

The Nurse's Role: Assessment, Education, Referral

• Assessment: a three-generation family history is the core nursing tool — ask about affected relatives, age at diagnosis, and ethnicity (some disorders are more common in certain populations, e.g., sickle cell disease in people of African ancestry). Document it clearly; patterns often emerge only when the whole picture is on paper.
• Education: explain in plain language what a genetic disorder is, what a positive family history does and does not mean, and what testing might offer. Correct misconceptions — "it skipped a generation" usually means a carrier: the gene did not skip; the expression of the condition did.
• Referral and support: genetic testing and risk calculations are provided by genetics professionals (genetic counselors, medical geneticists). The nurse facilitates referral, prepares the patient for what testing involves, and supports the family through results; confidentiality is critical — genetic information is sensitive, and the patient decides who knows it.
• Advocacy: protections against genetic discrimination exist in some jurisdictions; nurses can point patients to reputable sources and counseling. Scope note: nurses assess, educate, and refer — they do not diagnose genetic conditions or order genetic tests without provider or counselor involvement and institutional policy.

Common Confusions