Genetics

Core Concepts

DNA, genes, and chromosomes

A human cell typically contains 46 chromosomes in 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Each chromosome is DNA; genes are the segments that carry instructions for making proteins. Because a person inherits one chromosome of each pair from each parent, they have two copies of every gene — one from each. Alternative forms of a gene are called alleles; their combination is the genotype, which contributes to observable traits (phenotype). A karyotype — an organized display of a person's chromosomes — detects chromosome number and structure problems.

Patterns of inheritance

Four patterns organize most single-gene inheritance:

• Autosomal dominant: one altered allele is enough to produce the condition. An affected person has a 50% chance of passing the altered allele to each child, and affected individuals usually have an affected parent.
• Autosomal recessive: two altered alleles are required, one from each parent. People with one altered allele are carriers — typically symptom-free but able to pass the allele on. Two carriers have, per child, a 25% chance of an affected child, a 50% chance of a carrier, and a 25% chance of inheriting neither.
• X-linked: the gene sits on the X chromosome. In X-linked recessive conditions, a person with one X chromosome (typically XY males) is affected by a single altered allele, so these conditions are far more common in males; people with two X chromosomes usually need two altered alleles.
• Multifactorial: most common conditions involve multiple genes interacting with the environment — "genetic risk" is a probability, not a destiny.

Chromosomal abnormalities

Problems can involve chromosome number (aneuploidy — e.g., trisomy, three copies of a chromosome instead of two) or structure (deletions, duplications, translocations). Aneuploidy usually arises from errors in egg or sperm formation (nondisjunction). The fertility-relevant fact: most aneuploid conceptions do not survive — they end in early loss, often before the pregnancy is recognized. Risk for some trisomies increases with parental age; precise figures are guideline- and population-dependent and should come from current sources.

Carrier screening, prenatal testing, and counseling

• Carrier screening tests a person or couple for recessive conditions (e.g., cystic fibrosis, sickle cell disease) they could pass on without having themselves. Panels and recommendations vary by organization, region, and ancestry; nurses apply current institutional guidance.
• Prenatal screening (cell-free DNA screening, ultrasound markers) estimates the risk that a fetus has a chromosomal condition; diagnostic testing (chorionic villus sampling, amniocentesis) confirms or excludes it. Screening is not diagnosis.
• Genetic counseling is provided by specialized professionals and is non-directive: options are explained and supported without steering the person's decision. The nurse gathers family history, educates in plain language, supports the person, and facilitates referral — never interpreting results authoritatively or making reproductive decisions for anyone.

The nurse's role: pedigree, education, support, referral

A three-generation family history (parents, siblings, children, and more distant relatives, with conditions and ages) is the first genetic "test" and is squarely within nursing practice: from it the nurse identifies who might benefit from carrier screening or genetic counseling and documents findings accurately. The nurse also protects confidentiality and knows that scope of practice, available testing, and legal protections vary by country, state, and institution.

Common Confusions