genomics

Genetics and Genomics in NursingIntroduction to Genetics and Genomics

Genetics is the study of the general mechanisms of heredity and the variation of inherited traits. Genomics is a broader field that studies the function of all nucleotide sequences within the entire genome of a species, including coding and non-coding regions of DNA.

Key Terms

  • Gene: The basic unit of heredity, a segment of DNA that contains instructions for producing a specific protein.

  • Genome: The complete set of genes for a specific species. The human genome contains approximately 20,000 to 25,000 genes.

  • Proteome: The complete set of proteins a person makes at a given time under certain conditions. The study of the proteome is called proteomics.

  • DNA (Deoxyribonucleic Acid): The genetic material in a cell that contains genes. It is a double-stranded molecule composed of nucleic acids.

  • Chromosome: A temporary structure, visible during cell division, that is a large chunk of DNA containing many genes.

  • Nucleus: The part of the cell containing the chromosomes and most of the cell's DNA.

  • Mitochondrial DNA (mtDNA): A small amount of DNA found in the mitochondria.

DNA Structure

  • DNA is composed of four nucleic acids: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T).

  • Adenine (A) and Guanine (G) are purines (double-ring structures).

  • Cytosine (C) and Thymine (T) are pyrimidines (single-ring structures).

  • In DNA, Adenine pairs with Thymine (A-T), and Guanine pairs with Cytosine (G-C). These are called complementary base pairs.

  • DNA exists as a double helix, with a sugar-phosphate backbone.

Cell Division

  • Mitosis: A process of cell division that results in two identical daughter cells, each with the same number of chromosomes and DNA as the parent cell. This is crucial for growth and repair.

    • Phases of Mitosis:

      • G1G1​: Cell growth and nutrient uptake.

      • SS: DNA replication.

      • G2G2​: Production of proteins for cell division.

      • MM: Actual cell division (mitosis and cytokinesis).

  • Apoptosis: Programmed cell death, a natural process for removing old or damaged cells.

Chromosomes and PloidyChromosome Structure

  • Chromosomes are composed of DNA and are visible under a microscope during the MM phase of cell division.

  • Centromere: The region that constricts a chromosome, connecting two sister chromatids.

  • Telomeres: Specialized structures at the ends of chromosomes that protect the DNA.

  • Gene Locus: The specific location of a gene on a chromosome (e.g., the insulin gene locus is at 11q13).

Ploidy

  • Ploidy refers to the number of complete sets of chromosomes in a cell.

  • Haploid (1N): A complete set of one of each chromosome (23 chromosomes in humans). Gametes (sperm and egg) are haploid.

  • Diploid (2N): Contains two complete sets of chromosomes, one from each parent (46 chromosomes in humans). Somatic cells are diploid.

  • Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes (XXXX for females, XYXY for males).

Gene Expression and Inheritance PatternsAlleles and Genotypes

  • Allele: An alternative form of a gene at a given locus.

  • Genotype: An individual's genetic makeup for a specific gene pair or their total genetic makeup.

  • Phenotype: The observable expression of a specific trait or characteristic (e.g., eye color, blood type).

  • Homozygous: Having two identical alleles for a particular gene (e.g., AAAA or aaaa).

  • Heterozygous: Having two different alleles for a particular gene (e.g., AaAa).

Dominant and Recessive Traits

  • Dominant Trait: A trait that is apparent or expressed when only one copy or dose of the gene is present (e.g., brown eyes, widow's peak). If an individual is heterozygous (AaAa), the dominant allele (AA) is expressed.

  • Recessive Trait: A trait that is apparent or expressed only when two copies or doses of the gene are present or if one copy is missing (e.g., blue eyes, attached earlobes). If an individual is heterozygous (AaAa), the recessive allele (aa) is not expressed.

Other Inheritance Patterns

  • Codominant Expression: When each of two alleles present is expressed equally when both are present (e.g., blood type ABAB).

  • Incomplete Dominance: A blending of traits when heterozygous (e.g., pink flowers from red and white parents).

  • X-linked Inheritance: Traits carried on the XXchromosome.

    • Hemizygous: Males have only one XXchromosome, so any allele on the XXchromosome is expressed, whether dominant or recessive.

    • X-linked recessive traits occur more frequently in males because they only need one copy of the recessive allele to express the trait.

  • Autosomal Dominant (AD): Traits controlled by genes on autosomal chromosomes that are expressed when at least one dominant allele is present. They appear in every generation and affect males and females equally.

  • Autosomal Recessive (AR): Traits controlled by genes on autosomal chromosomes that are expressed only when both alleles are recessive. They can skip generations and affect males and females equally.

  • Pleiotropy: When a single gene has effects in more than one tissue or organ.

  • Modifier Genes: Genes that contribute to the phenotype but are not the primary cause of its expression.

Pedigrees

A pedigree is a chart or "family tree" that maps genetic disorders within a family.

  • Circles represent females.

  • Squares represent males.

  • Shaded symbols indicate individuals affected by the trait.

  • Half-shaded symbols indicate carriers.

Aneuploidy

Aneuploidy is a change in the number of individual chromosomes or chromosomal segments, resulting in a cell having an abnormal number of chromosomes (e.g., 45 or 47 instead of the typical 46). This occurs due to improper separation of chromosomes during cell division (nondisjunction).

Types of Aneuploidy

  • Monosomy: The loss of a single chromosome (e.g., Turner Syndrome, 45,X45,X).

  • Nullisomy: The loss of a pair of chromosomes.

  • Trisomy: The addition of a single chromosome (e.g., Down Syndrome - Trisomy 21, Edwards Syndrome - Trisomy 18, Patau Syndrome - Trisomy 13).

  • Tetrasomy: The addition of a pair of chromosomes.

Incidence and Survival

  • Autosomal aneuploidy is common, with an incidence of about 1 in 160 live births.

  • Trisomy 13, 21, and 18 are the most common autosomal aneuploidies.

  • Aneuploidies that involve the loss or addition of entire chromosomes are often not compatible with fetal life.

Mosaicism

Mosaicism is the presence of two or more cell populations with different genotypes within a single individual that developed from a single fertilized egg. This occurs when a mutation or chromosomal abnormality arises after conception.

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leading to gametes with an abnormal number of chromosomes.

Examples of Aneuploidies

  • Down Syndrome (Trisomy 21): An extra copy of chromosome 21.

  • Edwards Syndrome (Trisomy 18): An extra copy of chromosome 18.

  • Patau Syndrome (Trisomy 13): An extra copy of chromosome 13.

  • Klinefelter Syndrome: XXYXXY genotype in males, resulting in an extra XX chromosome.

  • Turner Syndrome: XOXO genotype in females, resulting in a missing XX chromosome.

Factors Influencing Aneuploidy

  • Maternal Age: The risk of aneuploidy, particularly Down Syndrome, increases significantly with maternal age over 35.

  • Aging: Increased age is associated with a higher risk of certain aneuploidies, which can contribute to conditions like cancer due to defects in the immune system and less effective cell function.

Somatic Mutations

Somatic mutations occur after conception and affect only a select group of cells within an individual, such as a blood cell. These mutations are not passed on to offspring.

Genomics in Health and Illness

  • Genomics: The study of the role of genes and their interaction in health and illness. It considers genetic predispositions and environmental factors.

  • Genetic Variation: Differences in DNA sequences among individuals.

  • Genetic Susceptibility: Having one or more gene variations that increase an individual's risk for a specific disease.

  • Genetic Resistance: Having gene variations that protect against the development of specific diseases.

  • Complex Traits: Diseases resulting from the actions of multiple genes and/or the combined influence of genes and the environment (e.g., diabetes, obesity, cancer).

  • Genetic Ancestry: Using genetic analysis to determine an individual's family origins and migration patterns.

Genetic Analysis in Specific Conditions

  • Sudden Unexplained Death: Genetic factors, particularly mutations in genes like the ryanodine receptor, can be a cause of sudden unexplained deaths in young individuals.

  • Sudden Infant Death Syndrome (SIDS): While the exact cause is unknown, genetic predispositions may play a role.

  • Gene Doping: The use of genetic modification to enhance athletic performance, which is detected through forensic scientific methods.

Forensic Applications of Genetics

  • DNA Profiling/Fingerprinting: A technique used to identify individuals based on unique patterns (polymorphisms) in their DNA. It is used in crime scene investigations, paternity testing, and identifying skeletal remains.

  • DNA Phenotyping: A method that uses genetic evidence to predict physical traits, such as facial features, ancestry, and hair/eye color, to aid in identifying suspects.

  • DNA Databases: Collections of DNA profiles used for comparison in criminal investigations (e.g., CODIS).

DNA Testing Process

  1. Quantitation: Determining the amount of DNA retrieved from a crime scene specimen.

  2. Amplification: Copying specific regions of the DNA using Polymerase Chain Reaction (PCR).

  3. Detection: Separating and identifying DNA fragments, often using electrophoresis and fluorescence detection, to create a DNA profile.

Clinical Applications of Genetics

  • Bone Marrow Transplantation: Matching Human Leukocytic Antigen (HLA) alleles between donor and recipient is crucial to minimize immune rejection.

  • Genetic Counseling: Providing information and support to individuals and families regarding genetic conditions, risks, and inheritance patterns.

  • Pharmacogenomics: Studying how genes affect a person's response to drugs, allowing for personalized medicine.

Nursing Role in Genetics and Genomics

  • Assessment: Gathering family history and identifying genetic risks.

  • Planning: Developing care plans that incorporate genetic information.

  • Implementation: Providing education and support to patients and families regarding genetic testing, conditions, and resources.

  • Evaluation: Assessing the effectiveness of genetic interventions and patient understanding.

  • Client and Family Support: Offering emotional and informational support throughout the genetic testing and diagnosis process.

  • Resource Linkages: Connecting patients with genetic specialists and relevant educational resources.