General Genetics Unit 3 Review: Extensions of Mendelian Genetics

Key Concepts and Terminology

• Alleles are different versions of a gene that can result in varying traits
• Genotype refers to the genetic makeup of an organism, while phenotype is the observable characteristics
• Dominance describes the relationship between alleles, with dominant alleles masking the effects of recessive alleles
• Codominance occurs when both alleles are expressed equally in the phenotype
• Incomplete dominance results in a blending of traits, creating an intermediate phenotype
• Pleiotropy is when a single gene influences multiple seemingly unrelated phenotypic traits
• Epistasis involves the interaction between genes, where one gene influences the expression of another
• Sex-linked genes are located on the sex chromosomes (X and Y) and are inherited differently between males and females

Beyond Simple Dominance

• Incomplete dominance deviates from simple dominant-recessive patterns, resulting in a phenotype that is a blend of both alleles (red and white flowers producing pink offspring)
• Codominance allows both alleles to be expressed simultaneously, such as in human blood types (A and B alleles producing AB blood type)
• Multiple alleles exist when there are more than two possible alleles for a given gene, increasing the number of possible genotypes and phenotypes
• Polygenic traits are influenced by multiple genes, each contributing a small effect to the overall phenotype (height, skin color)
  • The additive effect of these genes results in a continuous range of phenotypes rather than distinct categories
• Pleiotropy occurs when a single gene affects multiple traits, such as the gene responsible for sickle cell anemia also providing resistance to malaria

Multiple Alleles and Blood Types

• The ABO blood group system in humans is determined by three alleles: A, B, and O
  • The A and B alleles are codominant, while the O allele is recessive
• Genotypes AA and AO result in blood type A, BB and BO in blood type B, AB in blood type AB, and OO in blood type O
• The Rh blood group system is determined by the presence (+) or absence (-) of the Rh factor on red blood cells
  • The Rh+ allele is dominant, while the Rh- allele is recessive
• Compatible blood types are crucial for safe blood transfusions to avoid adverse immune reactions
• Multiple alleles increase genetic diversity within a population and can have evolutionary significance in adapting to environmental pressures

Gene Interactions and Epistasis

• Epistasis occurs when the expression of one gene is influenced by the presence of one or more "modifier genes"
• Dominant epistasis involves a dominant allele at one gene masking the expression of alleles at another gene
  • In fruit flies, the presence of a dominant allele for gray body color masks the effects of the alleles for wing shape
• Recessive epistasis occurs when a recessive allele at one gene masks the expression of alleles at another gene
• Duplicate genes have similar functions, and the presence of a dominant allele in either gene results in the same phenotype (redundancy)
• Complementary genes require the presence of dominant alleles at both genes to produce a specific phenotype
• Epistatic interactions can alter expected Mendelian ratios and complicate the interpretation of genetic crosses

Sex-Linked Inheritance

• Sex-linked genes are located on the sex chromosomes (X and Y) and exhibit unique inheritance patterns
• In humans, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY)
• X-linked recessive traits are more common in males because they only need one recessive allele to express the trait
  • Examples include color blindness and hemophilia
• X-linked dominant traits affect both males and females, but are more severe in males due to the lack of a second X chromosome
• Y-linked traits are inherited from father to son and are less common than X-linked traits
• Sex-limited traits are autosomal but are expressed differently in males and females due to hormonal differences (male pattern baldness)

Pedigree Analysis and Inheritance Patterns

• Pedigrees are diagrams that show the inheritance of traits within a family across multiple generations
• Autosomal dominant inheritance is characterized by affected individuals in every generation and both males and females equally affected
  • Huntington's disease follows an autosomal dominant pattern
• Autosomal recessive inheritance is characterized by the "skip generation" pattern, where affected individuals have unaffected parents
  • Cystic fibrosis is an example of an autosomal recessive disorder
• X-linked recessive inheritance is characterized by affected males and unaffected carrier females, with no male-to-male transmission
• Mitochondrial inheritance is maternal, with all offspring of an affected mother displaying the trait
• Pedigree analysis helps determine the mode of inheritance, estimate risks for future offspring, and provide genetic counseling

Environmental Influences on Gene Expression

• Gene expression can be influenced by environmental factors, resulting in phenotypic plasticity
• Epigenetic modifications, such as DNA methylation and histone modifications, can alter gene expression without changing the DNA sequence
  • These modifications can be influenced by diet, stress, and exposure to toxins
• Temperature-dependent sex determination in some reptiles is an example of environmental influence on gene expression
  • In many turtles, the incubation temperature of the eggs determines the sex of the offspring
• Siamese cats have temperature-sensitive pigment production, resulting in darker coloration on cooler body parts (ears, face, tail)
• The study of gene-environment interactions is crucial for understanding complex traits and diseases, such as obesity and cancer

Real-World Applications and Case Studies

• Genetic testing can identify individuals at risk for inherited disorders and guide preventive measures or treatment
  • BRCA1 and BRCA2 gene testing can assess the risk of developing breast and ovarian cancer
• Pharmacogenomics studies how genetic variations influence drug response, allowing for personalized medicine
  • Genetic testing can help determine the optimal dose of blood thinners like warfarin
• Agricultural applications include the development of genetically modified crops with enhanced traits (pest resistance, drought tolerance)
• Forensic genetics uses DNA evidence to identify suspects, determine familial relationships, and solve crimes
  • The Golden State Killer case was solved using genetic genealogy and DNA from crime scenes
• Genetic counseling helps individuals and families understand and adapt to the implications of genetic disorders
• Research on gene therapy aims to treat genetic diseases by introducing functional copies of genes into cells
  • Luxturna is a gene therapy approved for the treatment of a rare form of inherited blindness