🐾General Biology II Unit 5 – Extensions of Mendelian Genetics

Key Concepts

• Mendelian inheritance patterns describe simple genetic traits controlled by a single gene with dominant and recessive alleles
• Non-Mendelian inheritance patterns deviate from simple Mendelian ratios and involve multiple genes, environmental factors, or complex mechanisms
• Gene interactions occur when multiple genes influence a single trait, leading to modified phenotypic ratios and outcomes
• Sex-linked inheritance involves genes located on the sex chromosomes (X and Y) and results in different inheritance patterns between males and females
• Pedigree analysis is a tool used to study the inheritance of genetic traits within families across multiple generations
• Real-world applications of genetics include genetic testing, personalized medicine, and agricultural biotechnology
• Understanding the extensions of Mendelian genetics is crucial for comprehending complex genetic phenomena and their implications in various fields

Beyond Simple Inheritance

• Incomplete dominance occurs when the heterozygous phenotype is intermediate between the two homozygous phenotypes (snapdragon flower color)
• Codominance results in the expression of both alleles in the heterozygous state, leading to a distinct phenotype (human ABO blood types)
• Multiple alleles refer to the existence of more than two alleles for a given gene, increasing the number of possible genotypes and phenotypes
• Polygenic inheritance involves the cumulative effect of multiple genes on a single trait, resulting in a continuous range of phenotypes (human skin color, height)
• Pleiotropy occurs when a single gene influences multiple seemingly unrelated traits (phenylketonuria affecting mental development and pigmentation)
• Epistasis involves the interaction between different genes, where one gene can mask or modify the expression of another gene (fruit fly eye color)
• Environmental factors can influence the expression of genes, leading to variations in phenotype even among individuals with the same genotype (plant height affected by nutrient availability)

Non-Mendelian Inheritance Patterns

• Mitochondrial inheritance involves the transmission of genetic material from the mitochondria, which is inherited exclusively from the mother
  • Mitochondrial DNA is separate from the nuclear genome and follows a unique inheritance pattern
  • Mitochondrial disorders are caused by mutations in the mitochondrial DNA and can affect energy production and cellular function
• Genomic imprinting is an epigenetic phenomenon where the expression of a gene depends on its parental origin
  • Imprinted genes are differentially expressed based on whether they are inherited from the mother or father
  • Imprinting disorders, such as Prader-Willi syndrome and Angelman syndrome, result from disruptions in imprinted gene expression
• Uniparental disomy (UPD) occurs when an individual inherits both copies of a chromosome or chromosomal region from one parent
  • UPD can lead to imprinting disorders or uncover recessive mutations
• Trinucleotide repeat expansions are unstable DNA sequences that can expand across generations, leading to genetic disorders (Huntington's disease, fragile X syndrome)
• Mosaicism refers to the presence of two or more genetically distinct cell populations within an individual, arising from postzygotic mutations (McCune-Albright syndrome)
• Epigenetic modifications, such as DNA methylation and histone modifications, can alter gene expression without changing the underlying DNA sequence

Gene Interactions

• Complementary gene action occurs when two or more genes contribute to the same phenotype, and the presence of dominant alleles at all loci is required for the trait to be expressed
• Duplicate gene action involves two or more genes that independently produce the same phenotype, and the presence of a dominant allele at any one locus is sufficient for trait expression
• Additive gene action refers to the cumulative effect of multiple genes on a quantitative trait, where each gene contributes a small and equal effect to the phenotype
• Epistatic interactions can be dominant or recessive, depending on whether the masking effect is caused by a dominant or recessive allele
  • Dominant epistasis: The presence of a dominant allele at one locus masks the expression of alleles at another locus
  • Recessive epistasis: The presence of recessive alleles at one locus masks the expression of alleles at another locus
• Modifier genes can enhance, suppress, or alter the expression of other genes, influencing the phenotypic outcome
• Gene interactions can lead to modified phenotypic ratios that deviate from the expected Mendelian ratios, requiring more complex genetic analysis

Sex-Linked Inheritance

• Sex-linked genes are located on the sex chromosomes (X and Y) and exhibit different inheritance patterns in males and females
• X-linked inheritance involves genes located on the X chromosome
  • X-linked recessive traits are more common in males because they only have one X chromosome
  • Carrier females can pass X-linked recessive alleles to their offspring without expressing the trait themselves
• Y-linked inheritance involves genes located on the Y chromosome and is passed exclusively from father to son
  • Y-linked traits are rare and affect only males
• X-inactivation is the process by which one of the two X chromosomes in female cells is randomly inactivated, resulting in dosage compensation
  • Barr bodies are the inactive X chromosomes visible in the nucleus of female cells
• Sex-limited traits are autosomal traits that are expressed differently in males and females due to hormonal or physiological differences (male pattern baldness)
• Sex-influenced traits are autosomal traits that have different penetrance or expressivity in males and females (familial hypercholesterolemia)

Pedigree Analysis

• Pedigrees are diagrams that represent the inheritance of genetic traits within a family across multiple generations
• Pedigree symbols include squares for males, circles for females, and various shading and line patterns to indicate affected individuals and relationships
• Autosomal dominant inheritance is characterized by the presence of the trait in every generation, equal transmission to both sexes, and affected individuals having an affected parent
• Autosomal recessive inheritance is characterized by the trait skipping generations, equal occurrence in both sexes, and unaffected parents having affected children
• X-linked recessive inheritance is characterized by the trait being more common in males, no male-to-male transmission, and affected males having carrier mothers
• Pedigree analysis can help determine the mode of inheritance, estimate recurrence risks, and identify potential carriers of genetic conditions

Real-World Applications

• Genetic testing can be used for diagnostic purposes, carrier screening, and predictive testing for inherited disorders (BRCA1/2 testing for breast and ovarian cancer risk)
• Personalized medicine utilizes genetic information to tailor medical treatments and interventions based on an individual's genetic profile (pharmacogenomics)
• Preimplantation genetic diagnosis (PGD) allows for the screening of embryos for genetic disorders before implantation during in vitro fertilization
• Agricultural biotechnology employs genetic techniques to develop crops with improved traits, such as increased yield, pest resistance, and nutritional content (genetically modified organisms)
• Forensic genetics uses DNA analysis for crime scene investigation, paternity testing, and identification of human remains
• Gene therapy aims to treat or prevent genetic disorders by introducing functional copies of genes into cells or modifying the expression of existing genes (sickle cell anemia, spinal muscular atrophy)
• Population genetics studies the distribution and changes in allele frequencies within populations, providing insights into evolution and genetic diversity

Study Tips and Tricks

• Create a glossary of key terms and definitions to help you remember important concepts and terminology
• Use mnemonic devices or acronyms to memorize complex processes or lists of information (PMAT for the stages of mitosis: Prophase, Metaphase, Anaphase, Telophase)
• Practice solving genetic problems and pedigrees to reinforce your understanding of inheritance patterns and probability calculations
• Utilize visual aids, such as diagrams, flowcharts, and concept maps, to organize and connect different ideas and concepts
• Engage in active recall by testing yourself with flashcards, practice questions, or self-quizzing techniques
• Collaborate with classmates to discuss concepts, share study strategies, and work through challenging problems together
• Seek clarification from your instructor or teaching assistants for any topics or concepts that are unclear or confusing
• Relate genetic concepts to real-world examples and applications to make the information more meaningful and memorable