Introduction to Mendelian Genetics
Mendelian genetics refers to the principles of inheritance first discovered by Gregor Mendel in the 1860s through his experiments with pea plants. These principles explain how traits are passed from parents to offspring through genes and form the foundation of modern genetics. Understanding Mendelian genetics is crucial for predicting inheritance patterns in organisms, diagnosing genetic disorders, and developing applications in agriculture, medicine, and biotechnology.
Core Concepts and Terminology
| Term | Definition |
|---|---|
| Gene | A segment of DNA that codes for a specific trait |
| Allele | Different versions of the same gene |
| Dominant allele | An allele that expresses its trait even when only one copy is present (denoted with capital letters, e.g., “A”) |
| Recessive allele | An allele that only expresses its trait when two copies are present (denoted with lowercase letters, e.g., “a”) |
| Genotype | The genetic makeup of an organism (e.g., AA, Aa, aa) |
| Phenotype | The observable traits resulting from the genotype |
| Homozygous | Having two identical alleles for a gene (AA or aa) |
| Heterozygous | Having two different alleles for a gene (Aa) |
| Punnett square | A diagram used to predict the outcome of genetic crosses |
Mendel’s Laws of Inheritance
First Law: Law of Segregation
- Each individual has two alleles for each gene
- Alleles separate during gamete formation (meiosis)
- Each gamete receives only one allele from each gene pair
- Fertilization restores the paired condition in offspring
Second Law: Law of Independent Assortment
- Alleles of different genes segregate independently during gamete formation
- The inheritance of one trait does not affect the inheritance of another trait
- Only applies to genes located on different chromosomes
Third Law: Law of Dominance
- When two different alleles are present, the dominant allele will mask the recessive allele
- Recessive traits only appear when two recessive alleles are present
Types of Inheritance Patterns
Complete Dominance
- One allele completely masks the other (e.g., AA and Aa show same phenotype)
- Example: Purple flower color (A) is dominant over white (a)
- AA = purple flowers
- Aa = purple flowers
- aa = white flowers
Incomplete Dominance
- Neither allele is completely dominant; heterozygotes show intermediate phenotype
- Example: Red (R) and white (r) flowers
- RR = red flowers
- Rr = pink flowers
- rr = white flowers
Codominance
- Both alleles are expressed simultaneously in the heterozygote
- Example: Red (C^R) and white (C^W) flower color
- C^R C^R = red flowers
- C^R C^W = red and white spotted flowers
- C^W C^W = white flowers
Multiple Alleles
- More than two alleles exist for a gene in a population
- Example: Human blood types (A, B, O alleles)
Sex-Linked Inheritance
- Genes located on sex chromosomes (X or Y)
- Example: Red-green color blindness (located on X chromosome)
Monohybrid and Dihybrid Crosses
Monohybrid Cross (One Trait)
- Cross between parents differing in one trait
- Example: Crossing tall (TT) with short (tt) pea plants
- Punnett square for F₁ generation (Tt × Tt):
| T | t
-----------
T | TT| Tt
-----------
t | Tt| tt- Phenotypic ratio: 3:1 (dominant)
- Genotypic ratio: 1:2:1 (homozygous dominant:heterozygousrecessive)
Dihybrid Cross (Two Traits)
- Cross between parents differing in two traits
- Example: Crossing round yellow (RRYY) with wrinkled green (rryy) pea seeds
- F₁ generation: All RrYy (round yellow)
- F₂ generation ratio: 9:3:3:1
- 9/16 round yellow (R_Y_)
- 3/16 round green (R_yy)
- 3/16 wrinkled yellow (rrY_)
- 1/16 wrinkled green (rryy)
Calculating Probabilities in Genetic Crosses
Multiplication Rule
- Used to calculate the probability of independent events occurring together
- Probability (A and B) = Probability (A) × Probability (B)
- Example: Probability of having a child with two recessive traits from heterozygous parents (Aa Bb × Aa Bb)
- Probability of aa = 1/4
- Probability of bb = 1/4
- Probability of aa and bb = 1/4 × 1/4 = 1/16
Addition Rule
- Used to calculate the probability of either of two mutually exclusive events occurring
- Probability (A or B) = Probability (A) + Probability (B)
- Example: Probability of having either AA or Aa genotype = Probability (AA) + Probability (Aa)
Common Genetic Disorders Following Mendelian Inheritance
| Disorder | Inheritance Pattern | Description |
|---|---|---|
| Cystic fibrosis | Autosomal recessive | Affects respiratory and digestive systems |
| Huntington’s disease | Autosomal dominant | Progressive neurological disorder |
| Sickle cell anemia | Autosomal recessive | Abnormal hemoglobin causing misshapen red blood cells |
| Hemophilia | X-linked recessive | Blood clotting disorder, more common in males |
| Tay-Sachs disease | Autosomal recessive | Fatal disorder affecting nerve cells in brain |
| Phenylketonuria (PKU) | Autosomal recessive | Inability to break down phenylalanine |
Pedigree Analysis
Pedigree Symbols
- Square: Male
- Circle: Female
- Filled symbol: Affected individual
- Unfilled symbol: Unaffected individual
- Half-filled symbol: Carrier of recessive trait
- Horizontal line connecting symbols: Marriage/mating
- Vertical line descending: Offspring
Inheritance Pattern Clues
- Autosomal dominant:
- Appears in every generation
- Affected individuals have at least one affected parent
- Males and females equally affected
- Autosomal recessive:
- Often skips generations
- Affected individuals typically have unaffected parents who are carriers
- More common in consanguineous marriages
- Males and females equally affected
- X-linked recessive:
- More males affected than females
- No male-to-male transmission
- All daughters of affected males are carriers
- Sons of carrier females have 50% chance of being affected
Common Challenges and Solutions
| Challenge | Solution |
|---|---|
| Determining genotype from phenotype | Perform test crosses with recessive homozygotes |
| Distinguishing inheritance patterns | Create and analyze pedigrees over multiple generations |
| Identifying carriers of recessive traits | Use molecular genetic testing or observe offspring patterns |
| Analyzing complex traits | Consider environmental factors and potential polygenic inheritance |
| Predicting outcomes with incomplete penetrance | Account for reduced expression probability in calculations |
Best Practices for Solving Genetics Problems
- Identify the inheritance pattern based on the description of the trait
- Assign appropriate symbols (capital for dominant, lowercase for recessive)
- Determine the genotypes of parents based on their phenotypes and other information
- Set up a Punnett square to visualize possible gametes and offspring
- Calculate the probability of specific genotypes or phenotypes
- Consider special cases like sex-linked traits, incomplete dominance, etc.
- Check your work by ensuring probabilities sum to 1 or 100%
Applications of Mendelian Genetics
- Agricultural breeding: Developing crops with desired traits
- Genetic counseling: Predicting inheritance of genetic disorders
- Forensic science: DNA profiling for identification
- Medicine: Diagnosing and treating genetic disorders
- Conservation: Managing genetic diversity in endangered species
Resources for Further Learning
- Textbooks:
- “Genetics: A Conceptual Approach” by Benjamin A. Pierce
- “Introduction to Genetic Analysis” by Griffiths et al.
- Online Resources:
- Khan Academy’s Genetics section
- National Human Genome Research Institute (NHGRI) resources
- Learn.Genetics by the University of Utah
- Interactive Tools:
- Virtual Genetics Lab (VGLII)
- PhET Interactive Simulations: Gene Expression
- Online Punnett Square Calculator
- Research Journals:
- Nature Genetics
- Genetics
- Journal of Heredity
This cheatsheet provides a comprehensive overview of Mendelian genetics principles, focusing on practical applications and problem-solving approaches. For specific genetic disorders or more complex inheritance patterns, consult specialized resources or genetic counselors.