👨‍👩‍👦‍👦General Genetics Unit 2 – Mendelian Genetics: Core Concepts

What's This Unit All About?

• Explores the fundamental principles of inheritance discovered by Gregor Mendel in the 19th century
• Lays the foundation for understanding how traits are passed from parents to offspring
• Introduces key concepts such as genes, alleles, genotypes, and phenotypes
• Covers the basic patterns of inheritance, including dominant and recessive traits
• Examines the role of probability in predicting the outcomes of genetic crosses
• Extends Mendelian principles to more complex patterns of inheritance
• Highlights the relevance of Mendelian genetics in fields like agriculture, medicine, and genetics research

Key Terms You Need to Know

• Gene: A segment of DNA that encodes a specific trait or characteristic
• Allele: One of the alternative forms of a gene that can occur at a particular locus
• Genotype: The genetic makeup of an individual, typically represented by letters (e.g., AA, Aa, aa)
• Phenotype: The observable characteristics or traits of an individual, determined by the genotype and environmental factors
• Dominant allele: An allele that masks the effect of a recessive allele and is expressed in the phenotype when present
• Recessive allele: An allele that is only expressed in the phenotype when no dominant allele is present
• Homozygous: Having two identical alleles for a particular gene (e.g., AA or aa)
• Heterozygous: Having two different alleles for a particular gene (e.g., Aa)
• Punnett square: A diagram used to predict the probability of offspring genotypes and phenotypes in a genetic cross

The Basics: Mendel's Laws

• Mendel's First Law (Law of Segregation): Each individual possesses two alleles for each gene, which segregate during gamete formation
  • Each parent contributes one allele to the offspring
  • Alleles segregate independently during meiosis
• Mendel's Second Law (Law of Independent Assortment): The inheritance of one trait is independent of the inheritance of other traits
  • Genes for different traits are inherited independently of each other
  • Allows for the prediction of offspring ratios for multiple traits
• Mendel's experiments involved true-breeding pea plants with contrasting traits (e.g., purple vs. white flowers)
• Monohybrid cross: A genetic cross involving a single trait controlled by one gene
• Dihybrid cross: A genetic cross involving two traits controlled by two different genes

Genetic Crosses and Probability

• Probability is a key tool in predicting the outcomes of genetic crosses
• The probability of an event is calculated by dividing the number of favorable outcomes by the total number of possible outcomes
• Multiplication rule: The probability of two independent events occurring together is the product of their individual probabilities
  • Used to calculate the probability of specific genotypes or phenotypes in a genetic cross
• Addition rule: The probability of an event occurring is the sum of the probabilities of all the ways it can occur
  • Used to calculate the probability of an offspring having a particular genotype or phenotype
• Testcross: A genetic cross between an individual with an unknown genotype and an individual with a known homozygous recessive genotype
  • Helps determine the unknown genotype based on the offspring phenotypes

Punnett Squares: Your New Best Friend

• A visual tool for predicting the probability of offspring genotypes and phenotypes in a genetic cross
• Involves 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
• The genotypic and phenotypic ratios can be determined by counting the occurrences of each genotype or phenotype in the Punnett square
• Monohybrid Punnett square: Involves a single gene with two alleles (e.g., Aa x Aa)
• Dihybrid Punnett square: Involves two genes with two alleles each (e.g., AaBb x AaBb)
  • Requires considering the independent assortment of alleles for each gene

Beyond Simple Inheritance

• Incomplete dominance: A pattern of inheritance where the heterozygous phenotype is intermediate between the two homozygous phenotypes
  • Example: Red and white flower cross-produces pink flowers in the F1 generation
• Codominance: A pattern of inheritance where both alleles in a heterozygous individual are expressed in the phenotype
  • Example: ABO blood types in humans (IA and IB alleles are codominant)
• Multiple alleles: Some genes have more than two alleles, resulting in a wider range of phenotypes
  • Example: Coat color in rabbits (C, cch, ch, and c alleles)
• Polygenic inheritance: Traits that are influenced by multiple genes, each with a small effect on the phenotype
  • Example: Human height and skin color
• Environmental influences: Some traits are affected by both genetic and environmental factors
  • Example: Plant height influenced by both genotype and nutrient availability

Real-World Applications

• Agriculture: Mendelian principles are used in selective breeding to develop crops and livestock with desirable traits
  • Example: Breeding disease-resistant or high-yielding crop varieties
• Medicine: Understanding Mendelian inheritance helps predict the risk of genetic disorders and develop targeted therapies
  • Example: Sickle cell anemia is caused by a recessive allele of the hemoglobin gene
• Genetic counseling: Professionals use Mendelian genetics to assess the risk of inherited disorders and provide guidance to families
  • Example: Predicting the probability of a child inheriting a genetic disorder based on family history
• Forensic science: Genetic markers following Mendelian inheritance patterns are used in DNA profiling and paternity testing
  • Example: Using short tandem repeats (STRs) to match DNA samples in criminal investigations

Common Pitfalls and How to Avoid Them

• Confusing genotype and phenotype: Remember that genotype refers to the genetic makeup, while phenotype refers to the observable characteristics
  • Pay attention to the context and the specific terms used in questions
• Misinterpreting probability: Probability predicts the likelihood of outcomes in a large number of events, not the exact outcome of a single event
  • Avoid assuming that high probability guarantees a specific result in a small sample size
• Overlooking the role of the environment: Phenotypes can be influenced by both genetic and environmental factors
  • Consider the potential impact of environmental conditions on the expression of a trait
• Forgetting to consider all possible genotypes: In complex crosses, it's easy to miss some of the possible genotype combinations
  • Systematically list all the possible genotypes before calculating probabilities or drawing conclusions
• Misapplying Mendel's laws: Mendel's laws apply to traits controlled by single genes with complete dominance
  • Be cautious when applying Mendelian principles to more complex patterns of inheritance, such as incomplete dominance or polygenic traits