🐾General Biology II Unit 4 – Mendelian Genetics

Key Concepts and Terminology

• Allele: Alternative forms of a gene that can occur at a specific locus on a chromosome
• Genotype: Genetic makeup of an organism, typically represented by letters (e.g., AA, Aa, aa)
  • Homozygous: Having two identical alleles for a particular gene (AA or aa)
  • Heterozygous: Having two different alleles for a particular gene (Aa)
• Phenotype: Observable characteristics or traits of an organism determined by its genotype and environmental factors
• Dominant allele: Allele that masks the effect of a recessive allele and is expressed in the phenotype when present (typically represented by a capital letter)
• Recessive allele: Allele that is masked by a dominant allele and is only expressed in the phenotype when two copies are present (typically represented by a lowercase letter)
• Punnett square: Diagram used to predict the probability of genotypes and phenotypes in the offspring of a genetic cross
• Monohybrid cross: Genetic cross involving a single gene with two alleles
• Dihybrid cross: Genetic cross involving two genes, each with two alleles

Mendel's Experiments and Discoveries

• Gregor Mendel, an Austrian monk, is considered the father of modern genetics
• Conducted experiments on pea plants to study inheritance patterns
• Focused on seven distinct traits in pea plants (seed shape, seed color, flower color, pod shape, pod color, flower position, and plant height)
• Performed controlled crosses between plants with contrasting traits and analyzed the offspring
• Discovered that traits are inherited as discrete units (genes) rather than blending together
• Observed that certain traits consistently appeared more frequently than others, leading to the concept of dominant and recessive alleles
• Laid the foundation for the principles of inheritance and the field of genetics

Laws of Inheritance

• Law of Segregation (Mendel's First Law): During gamete formation, the two alleles for a gene separate (segregate) from each other, and each gamete receives only one allele
  • Explains the observation that traits do not blend but are passed on as distinct units
• Law of Independent Assortment (Mendel's Second Law): During gamete formation, the segregation of alleles for one gene occurs independently of the segregation of alleles for other genes
  • Applies to genes located on different chromosomes or far apart on the same chromosome
  • Results in the random distribution of alleles in the gametes
• These laws form the basis for predicting the inheritance of traits and the probability of offspring genotypes and phenotypes

Punnett Squares and Probability

• Punnett squares are used to predict the probability of genotypes and phenotypes in the offspring of a genetic cross
• Involve drawing a grid with the possible gametes from each parent along the top and left side
• Each cell in the grid represents a potential offspring genotype and is filled in by combining the alleles from the corresponding gametes
• Probability of each genotype is calculated by counting the number of times it appears in the Punnett square and dividing by the total number of cells
• In a monohybrid cross between two heterozygous individuals (Aa x Aa), the expected genotypic ratio is 1:2:1 (1 AA, 2 Aa, 1 aa)
• The phenotypic ratio depends on the dominance relationship between the alleles (3:1 dominant to recessive for complete dominance)

Patterns of Inheritance

• Complete dominance: One allele is completely dominant over the other, and the heterozygous phenotype is indistinguishable from the homozygous dominant phenotype (e.g., yellow vs. green pea seeds)
• Incomplete dominance: The heterozygous phenotype is an intermediate between the two homozygous phenotypes (e.g., pink flowers in snapdragons)
• Codominance: Both alleles are expressed equally in the heterozygous phenotype (e.g., AB blood type in humans)
• Multiple alleles: More than two alleles exist for a particular gene (e.g., ABO blood type system in humans)
• Polygenic inheritance: A trait is influenced by multiple genes, each with a small effect (e.g., human skin color, height)
• Sex-linked inheritance: Genes located on the sex chromosomes (X and Y) that exhibit different inheritance patterns in males and females (e.g., color blindness, hemophilia)

Genetic Variations and Exceptions

• Mutations: Changes in the DNA sequence that can alter gene function and lead to new alleles
  • Point mutations: Single nucleotide changes (substitutions, insertions, or deletions)
  • Chromosomal mutations: Large-scale changes in chromosome structure or number (e.g., translocations, duplications, deletions)
• Non-Mendelian inheritance: Patterns of inheritance that do not follow Mendel's laws
  • Epistasis: Interaction between genes where one gene influences the expression of another gene
  • Pleiotropy: A single gene affects multiple seemingly unrelated traits
  • Genomic imprinting: Differential expression of genes depending on whether they are inherited from the mother or father
• Environmental influences: Factors such as temperature, nutrition, and stress can affect the expression of genes and modify the phenotype

Applications in Modern Genetics

• Genetic testing: Analysis of an individual's DNA to determine the presence or absence of specific alleles associated with genetic disorders or traits
  • Carrier screening: Identifies individuals who carry a recessive allele for a genetic disorder
  • Prenatal testing: Detects genetic abnormalities in a developing fetus (e.g., amniocentesis, chorionic villus sampling)
• Gene therapy: Introduction of functional genes into cells to replace or correct defective genes causing genetic disorders
• Genetic engineering: Manipulation of an organism's DNA to modify its characteristics or produce desired substances (e.g., genetically modified crops, recombinant proteins)
• Personalized medicine: Tailoring medical treatments to an individual's genetic profile to optimize effectiveness and minimize side effects
• Forensic genetics: Application of genetic principles to legal matters, such as DNA fingerprinting for identification purposes in criminal investigations or paternity testing

Review and Practice Problems

• Monohybrid cross: In pea plants, tall (T) is dominant to short (t). Cross a heterozygous tall plant with a short plant. What are the expected genotypic and phenotypic ratios in the offspring?
• Dihybrid cross: In pea plants, yellow (Y) is dominant to green (y), and round (R) is dominant to wrinkled (r). Cross two plants heterozygous for both traits (YyRr x YyRr). What are the expected genotypic and phenotypic ratios in the offspring?
• Incomplete dominance: In snapdragons, red (R) and white (W) flowers are homozygous, while pink (RW) is heterozygous. Cross a red flower with a white flower. What are the expected genotypic and phenotypic ratios in the offspring?
• Codominance: In humans, the ABO blood type system is determined by the alleles IA, IB, and i. IA and IB are codominant, while i is recessive. What are the possible genotypes and phenotypes for blood types A, B, AB, and O?
• Sex-linked inheritance: In humans, red-green color blindness is caused by a recessive allele (c) on the X chromosome. A color-blind man marries a woman who is a carrier for color blindness. What is the probability of their children being color-blind?