Unit
- Genome organization: The human genome contains approximately 3 billion base pairs organized into 23 pairs of chromosomes, including 22 autosomes and one pair of sex chromosomes. Each chromosome consists of DNA wrapped around histone proteins, forming chromatin that condenses during cell division.
- Mendelian inheritance: Autosomal dominant, autosomal recessive, and X-linked patterns follow predictable segregation ratios. In autosomal dominant conditions, a single mutated allele is sufficient to cause disease; examples include Huntington disease and Marfan syndrome. Autosomal recessive disorders require two mutated alleles, such as cystic fibrosis and sickle cell disease.
- Non-Mendelian patterns: Mitochondrial inheritance is passed exclusively through the maternal line, affecting energy-production tissues. Genomic imprinting results in differential expression depending on parental origin, as seen in Prader-Willi and Angelman syndromes. Multifactorial inheritance involves multiple genes and environmental contributions, common in diabetes and heart disease.
- Chromosomal abnormalities: Numerical changes like trisomy 21 (Down syndrome) arise from nondisjunction during meiosis. Structural abnormalities include deletions, duplications, translocations, and inversions, each with distinct clinical consequences. Karyotyping and chromosomal microarray are standard diagnostic tools.
- DNA replication and mutation: DNA polymerase proofreading mechanisms maintain fidelity, but errors can lead to single nucleotide variants, insertions, deletions, or repeat expansions. Mutations in coding regions may alter protein function; those in regulatory regions affect gene expression.
- Genetic variation in populations: Allele frequencies differ across populations due to genetic drift, founder effects, and natural selection. Polymorphisms, such as SNPs, occur in more than 1% of the population and often underlie common disease susceptibility.
- Penetrance and expressivity: Reduced penetrance means not all individuals with a mutation develop symptoms, complicating risk assessment. Variable expressivity describes the range of severity among affected individuals, even within the same family.
- Genotype vs phenotype correlation: The relationship between genetic makeup and observable traits is influenced by modifier genes, epigenetic changes, and environmental factors. Understanding these interactions is critical for interpreting test results and counseling patients.
- Genetic testing technologies: Next-generation sequencing and polymerase chain reaction techniques allow comprehensive analysis of genes and chromosomes. These methods detect point mutations, copy number variations, and structural rearrangements, enabling precise diagnosis, carrier screening, and prenatal testing for a wide range of genetic disorders.
- Ethical and legal considerations: Genetic counseling involves informed consent, privacy protection under laws like GINA, and management of incidental findings. Clinicians must balance patient autonomy with the potential for genetic discrimination and the psychological impact of test results on patients and families.
- Pharmacogenomics: Genetic variations influence drug metabolism, efficacy, and adverse reactions. For example, CYP2D6 polymorphisms affect response to antidepressants, and HLA-B*5701 screening prevents hypersensitivity to abacavir. Incorporating pharmacogenomic data into prescribing decisions improves treatment outcomes and minimizes adverse drug reactions and hospitalizations.
Scope of Practice
Role: Clinical Geneticist vs Genetic Counselor
Within scope:
- Clinical geneticists diagnose genetic conditions, order and interpret tests, and manage treatment plans.
- Genetic counselors assess family history, calculate recurrence risks, and support patient decision-making.
- Both provide education about inheritance, testing options, and available resources.
Outside scope:
- Genetic counselors do not diagnose medical conditions or prescribe treatments.
- Clinical geneticists typically do not provide long-term psychosocial counseling.
- Neither performs direct patient care tasks reserved for primary care physicians without collaboration.
Terminology
- Allele
- One of two or more versions of a gene at a given locus.
- Genotype
- The genetic constitution of an individual at a specific locus.
- Phenotype
- The observable characteristics resulting from genotype and environment.
- Penetrance
- The proportion of individuals with a mutation who exhibit symptoms.
Quick Revision
- The human genome comprises 23 chromosome pairs with organized DNA structure.
- Mendelian patterns include autosomal dominant, recessive, and X-linked inheritance.
- Non-Mendelian patterns include mitochondrial, imprinting, and multifactorial inheritance.
- Chromosomal abnormalities can be numerical or structural and are detected via karyotype or microarray.
- Penetrance and expressivity affect the manifestation of genetic conditions.