🧬AP Biology Unit 5 – Heredity

Key Concepts and Terminology

• Alleles refer to different versions of a gene that can result in variations in the expressed trait
• Genotype represents the genetic makeup of an organism, while phenotype is the observable physical characteristics
• Homozygous individuals possess two identical alleles for a specific gene, while heterozygous individuals have two different alleles
• Dominant alleles mask the expression of recessive alleles when present together in a heterozygous individual
• Punnett squares are used to predict the probability of offspring inheriting specific traits based on the genotypes of the parents
• Pedigree charts illustrate the inheritance patterns of traits within a family across multiple generations
• Incomplete dominance occurs when neither allele is completely dominant, resulting in a blended phenotype (red and white flowers producing pink offspring)

Mendelian Genetics

• Gregor Mendel, known as the "father of modern genetics," conducted experiments on pea plants to study inheritance patterns
• Mendel's Law of Segregation states that allele pairs separate during gamete formation, with each gamete receiving one allele from each pair
• Mendel's Law of Independent Assortment proposes that the inheritance of one trait does not influence the inheritance of another trait
• Monohybrid crosses involve the examination of a single trait, while dihybrid crosses consider two traits simultaneously
• Test crosses are performed to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual
• Mendel's principles of inheritance form the foundation for understanding the transmission of traits from parents to offspring
  • These principles were derived from his meticulous experiments and statistical analysis of pea plant traits

Beyond Mendelian Inheritance

• Codominance is a phenomenon where both alleles in a heterozygous individual are expressed equally (ABO blood types)
• Polygenic traits are influenced by multiple genes, each contributing a small effect to the overall phenotype (height, skin color)
• Epistasis occurs when the expression of one gene is influenced by the presence or absence of another gene
• Pleiotropy refers to a single gene influencing multiple seemingly unrelated phenotypic traits (sickle cell anemia affecting red blood cells and resistance to malaria)
• Genomic imprinting is an epigenetic phenomenon where gene expression depends on the parent of origin (Prader-Willi and Angelman syndromes)
• Environmental factors can interact with genetic factors to modify the expression of traits (diet influencing weight)

Chromosomes and DNA

• Chromosomes are thread-like structures composed of DNA and proteins, found in the nucleus of eukaryotic cells
• DNA (deoxyribonucleic acid) is the genetic material that carries the instructions for an organism's development and function
• The double helix structure of DNA consists of two complementary strands held together by hydrogen bonds between nitrogenous bases
• The four nitrogenous bases in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C), with A pairing with T and G pairing with C
• Histones are proteins that help package and condense DNA into chromatin, which further coils to form chromosomes
• The number of chromosomes varies among species, with humans having 23 pairs (46 total) of chromosomes
  • One pair consists of the sex chromosomes (X and Y), which determine an individual's biological sex

Gene Expression and Regulation

• Gene expression is the process by which the information encoded in a gene is used to synthesize functional gene products (proteins or RNA)
• Transcription is the first step of gene expression, where the DNA sequence of a gene is copied into a complementary RNA molecule
• Translation is the process in which the genetic code carried by mRNA is decoded to produce a specific amino acid sequence, forming a protein
• The genetic code is the set of rules that defines the relationship between the sequence of nucleotides in mRNA and the amino acid sequence of proteins
• Regulatory sequences, such as promoters and enhancers, control the initiation and rate of transcription
• Transcription factors are proteins that bind to regulatory sequences and influence gene expression by promoting or repressing transcription
• Alternative splicing allows a single gene to produce multiple protein isoforms by selectively including or excluding specific exons during mRNA processing

Genetic Mutations and Variation

• Mutations are changes in the DNA sequence that can alter gene function and lead to genetic variation
• Point mutations involve the substitution, insertion, or deletion of a single nucleotide and can be classified as silent, missense, or nonsense mutations
• Frameshift mutations occur when the number of nucleotides inserted or deleted is not a multiple of three, shifting the reading frame and altering the amino acid sequence
• Chromosomal mutations include large-scale changes such as deletions, duplications, inversions, and translocations of chromosomal segments
• Mutations can be spontaneous or induced by environmental factors such as radiation, chemicals, or viruses
• Genetic variation is essential for evolution, as it provides the raw material for natural selection to act upon
• Single nucleotide polymorphisms (SNPs) are common variations in DNA sequence that occur at specific positions in the genome and contribute to genetic diversity

Inheritance Patterns in Humans

• Autosomal dominant disorders (Huntington's disease) manifest when an individual inherits one dominant allele, even if the other allele is recessive
• Autosomal recessive disorders (cystic fibrosis) require an individual to inherit two recessive alleles, one from each parent, to express the disorder
• X-linked disorders are caused by mutations in genes located on the X chromosome and typically affect males more frequently than females
  • X-linked recessive disorders (hemophilia) are more common in males because they only need one recessive allele to express the disorder
  • X-linked dominant disorders (Rett syndrome) affect females more severely as they can inherit the dominant allele from either parent
• Mitochondrial disorders are caused by mutations in the mitochondrial DNA and are inherited exclusively from the mother through the egg cell
• Genetic counseling helps individuals and families understand and adapt to the medical, psychological, and familial implications of genetic disorders

Applications and Modern Techniques

• Polymerase chain reaction (PCR) is a technique used to amplify specific DNA sequences, enabling their detection and analysis
• DNA sequencing determines the precise order of nucleotides in a DNA molecule, allowing for the identification of mutations and variations
• Genome editing technologies, such as CRISPR-Cas9, enable the precise modification of DNA sequences by introducing targeted mutations or corrections
• Gene therapy involves the introduction of functional genes into cells to replace or correct defective genes, potentially treating genetic disorders
• Personalized medicine utilizes an individual's genetic information to tailor medical treatments and interventions based on their specific genetic profile
• Genetic engineering allows the transfer of genes from one organism to another, creating genetically modified organisms (GMOs) with desired traits
• Forensic genetics applies genetic principles and techniques to legal investigations, such as DNA fingerprinting for identification purposes