Genetics Study Guide: Complex Traits, Cytogenetics, DNA, and Gene Expression

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Chapter 5 – Complex Traits

Polygenic and Multifactorial Traits

Complex traits are influenced by multiple genetic and environmental factors. Understanding the distinction between polygenic and multifactorial traits is essential for interpreting inheritance patterns.

Polygenic traits: Controlled by more than one gene. Each gene contributes additively to the phenotype.
Multifactorial traits: Controlled by multiple genes and environmental factors. Examples include height, skin color, and risk for common diseases.
Phenotypic classes: Polygenic traits often show a range of phenotypes due to the number of dominant alleles present. The number of phenotypic classes is calculated as: For example, 3 genes yield 7 classes.

Twin Studies and Heritability

Twin studies are used to estimate the genetic and environmental contributions to traits. Heritability measures the degree to which offspring phenotypes correlate with parental phenotypes.

Heritability: The proportion of phenotypic variation in a population attributable to genetic variation. Higher correlation between parent and offspring phenotypes indicates higher heritability.
Twin studies: Compare monozygotic (identical) and dizygotic (fraternal) twins to assess genetic influence.

Human Traits: Obesity and Skin Color

Several human traits illustrate the interplay between genetics and environment.

Leptin: Hormone that signals the brain to decrease hunger and increase fat burning. Mutations in leptin or leptin resistance can lead to obesity.
Thrifty gene hypothesis: Suggests that genes favoring fat storage were advantageous for survival during long ocean voyages, possibly explaining high obesity rates in Pacific island populations.
Melanin: Pigment in skin that protects against UV-induced skin cancer and folate degradation. However, insufficient UV exposure can reduce vitamin D production.

Genome-Wide Association Studies (GWAS)

GWAS are used to identify genetic variants associated with traits.

Manhattan plot: Visual representation of GWAS results, showing the significance of associations across the genome. Peaks indicate potential genes involved in trait inheritance.

De Novo Mutation

De novo mutations are new genetic changes not inherited from either parent.

De novo mutation: Mutation arising spontaneously in a gamete or early embryo. Risk increases with paternal age due to continuous division of sperm precursor cells.

Chapter 6 – Cytogenetics

Recognizing Cytogenetic Disorders

Cytogenetic disorders are identified by analyzing karyotypes, which display the number and structure of chromosomes.

Karyotype: Visual representation of chromosomes, used to detect abnormalities.

Key Cytogenetic Terms

Understanding chromosomal terminology is crucial for diagnosing and discussing genetic disorders.

Polyploid: Having more than two complete sets of chromosomes.
Aneuploid: Having an abnormal chromosome number that is less than a complete set (e.g., monosomy, trisomy).
Monosomy: Presence of only one copy of a particular chromosome.
Trisomy: Presence of three copies of a particular chromosome.
Uniparental disomy: Both copies of a chromosome are inherited from the same parent. Example: Prader-Willi and Angelman syndromes.
Fragile chromosome: Chromosome with multiple repeats that can disrupt gene function.
Robertsonian translocation: Fusion of parts of different chromosomes, often involving acrocentric chromosomes.

Screening for Chromosomal Abnormalities

Several methods are used to detect chromosomal abnormalities in fetuses.

Amniocentesis: Withdrawal of amniotic fluid for analysis; most common method.
Chorionic villus sampling: Sampling placental cells; higher risk of miscarriage but can be performed earlier.
Fetal DNA sampling from maternal blood: Low risk, but technically challenging.

Parental Age and Genetic Risk

Advanced parental age increases the risk of genetic abnormalities.

Maternal age: Older mothers have increased risk of non-disjunction due to prolonged arrest of oocytes in meiosis I, leading to aneuploidy.
Paternal age: Older fathers have increased risk of de novo mutations due to continuous division of sperm precursor cells.

Chapter 8 – DNA

Discovery of DNA as Genetic Material

Key experiments established DNA as the carrier of genetic information.

Bacterial transformation: Dead virulent cells can transfer genetic material to live non-virulent cells, making them virulent. DNA was identified as the transforming substance.
Bacteriophage experiments: Viruses labeled with 32P (DNA) and 35S (protein) showed that only DNA entered bacteria, confirming DNA as genetic material.

Structure of DNA

The structure of DNA was elucidated by several scientists and is fundamental to genetics.

Watson, Crick, Wilkins, and Franklin: Discovered the double helix structure. Watson, Crick, and Wilkins received the Nobel Prize.
Double helix: Two antiparallel strands of nucleotides.
Nucleotide: Composed of phosphate, deoxyribose sugar, and a base (A, C, T, G).
Phosphodiester bond: Connects 5' phosphate to 3' hydroxyl group of next nucleotide.
Base pairing: Hydrogen bonds between A-T and C-G.

DNA Replication

DNA replication ensures genetic continuity during cell division.

Replication fork: Region where DNA strands separate for replication.
Replication bubble: Area of separated DNA with forks at both ends.
Direction: DNA synthesis proceeds 5' to 3'.
Leading strand: Synthesized continuously toward the replication fork.
Lagging strand: Synthesized in fragments (Okazaki fragments) away from the fork.

Genome Organization

DNA is highly condensed to fit within the cell nucleus.

Histones: Proteins around which DNA is wrapped, forming nucleosomes ("beads on a string").
Supercoiling: Further compacts DNA into chromosomes.

Chromosome Structure

Chromosomes have specialized regions important for cell division and stability.

Centromere: Constricted region where sister chromatids attach.
Kinetochore: Protein complex at the centromere where microtubules attach during mitosis.
Telomere: Repetitive DNA sequences at chromosome ends that protect against damage.

Chapter 9 – Gene Expression and Proteins

Proteins and Amino Acids

Proteins are polymers of amino acids, each with unique properties.

Amino acids: 20 types, each with a distinct R group.
N-terminus: The first amino acid in a protein chain.
C-terminus: The last amino acid in a protein chain.
Protein folding: Determines function; misfolding can cause disease.

RNA Structure and Function

RNA is similar to DNA but has distinct differences.

Ribose: Sugar in RNA (instead of deoxyribose).
Uracil: Replaces thymine in RNA.
Single-stranded: RNA is typically single-stranded.

Gene Expression: Transcription and Translation

Gene expression involves transcription (making RNA) and translation (making protein).

Transcription: DNA is copied into RNA by RNA polymerase. The template strand is used to synthesize pre-mRNA.
RNA processing: Introns (non-coding regions) are removed, exons (coding regions) are joined, a 5' cap is added, and a poly-A tail is attached.
Regulation: Transcription factors bind to promoters to regulate RNA polymerase activity.
Translation: mRNA is decoded by ribosomes to synthesize proteins. Each codon (three bases) specifies an amino acid. The start codon is AUG.
tRNA: Adaptor molecule that pairs with codons and brings the correct amino acid.
Ribosome: Moves along mRNA, adding amino acids until a stop codon is reached.

Genetic Code Table (Purpose: Codon-to-Amino Acid Translation)

Codon	Amino Acid
AUG	Methionine (Start)
UUU	Phenylalanine
UAA, UAG, UGA	Stop
Additional info: Use a full code table for practice; only key codons shown here.

Prions

Prions are infectious proteins that cause disease by inducing misfolding in normal proteins.

Prion diseases: Include Creutzfeldt-Jakob disease and mad cow disease.
Mechanism: Misfolded prion proteins aggregate and damage tissue.