Genes, inheritance and genetic conditions

DNA stores biological information using a sequence of chemical bases. A gene is a region of DNA associated with a functional product; genes are not the whole genome. DNA is organized into chromosomes. The human genome refers to the full complement of genetic material, including nuclear and mitochondrial DNA. Typical human body cells have 23 pairs of nuclear chromosomes, but there are biological exceptions and variation. Avoid treating a simplified diagram as a description of every cell or person.

Different versions of a DNA sequence are called variants; alternative versions of a gene are often described as alleles. Many variants do not cause disease. Genetic information can contribute to health, but environment and other factors may also matter. A result requires appropriate interpretation rather than an assumption that every difference is harmful.

Fictional exercise: a brochure uses gene, chromosome and genome as if they are synonyms. Rewrite it using a nested description: DNA is organized into chromosomes, genes are regions of DNA, and the genome includes the full genetic material. Protect personal genetic information and refer individual questions to an appropriate genetics professional.

Practice: Draw a labelled diagram connecting cell nucleus, chromosome, DNA and gene, then add mitochondrial DNA separately. Explain why a variant is not automatically a diagnosis.

In a simplified autosomal dominant model, one disease-associated copy can be sufficient for the condition. In an autosomal recessive model, disease-associated variants in both copies of the relevant gene are generally needed. A person with one such recessive copy may be a carrier. X-linked inheritance concerns genes on the X chromosome; the pattern depends on chromosome copies and the particular condition. Mitochondrial DNA is usually inherited through the egg. Many conditions involve multiple genes and environmental factors rather than a simple single-gene pattern.

For a simplified autosomal recessive example with two carriers, each carrying one relevant variant in the same gene, a pregnancy has a 25% chance of inheriting both variants, a 50% chance of inheriting one and a 25% chance of inheriting neither. Each pregnancy is a new event; these percentages do not promise one affected child in every four. Real risk assessment requires confirmation of the variants and family circumstances.

New variants can also arise without a previously known family history. Penetrance and expression can vary: inheriting a variant may not predict exactly whether, when or how severely a condition appears. Use this lesson to understand vocabulary, not to calculate an individual family’s risk or advise reproductive decisions.

Practice: Use a fictional two-by-two inheritance grid for the simplified carrier example. Explain why a previous child’s result does not guarantee the next result, and list questions for a genetics professional.

Cystic fibrosis is associated with disease-causing variants in the CFTR gene and follows an autosomal recessive pattern. It can affect several body systems, including the lungs and digestive system. Huntington disease is associated with an expanded repeat in the HTT gene and follows an autosomal dominant pattern; its effects include neurological and other changes. These examples illustrate that different kinds of DNA changes and inheritance patterns can have different consequences.

A genetic condition is not always inherited from a parent: some variants arise newly. Not every variant in a gene is disease-causing, and different conditions can share symptoms. A family history, a screening result and a confirmed diagnosis are different forms of information. Clinical assessment and appropriate testing are needed to interpret an individual situation.

Fictional scenario: a worker hears that a person has a relative with a genetic condition and assumes the person must have it too. Correct the assumption, respect privacy and direct questions to the relevant clinician or genetic counsellor. Do not disclose family information to unrelated colleagues or use an online ancestry result as a substitute for clinical interpretation.

Practice: Compare the two examples by gene, simplified inheritance pattern and broad effects. Then explain why none of those facts diagnoses a fictional person based only on family history.