🌱 Mendelian Genetics

Terminology

Gene: Unit of heredity, codes for trait Allele: Alternative version of gene Dominant: Allele that masks other (uppercase, e.g., A) Recessive: Allele that is masked (lowercase, e.g., a) Homozygous: Same alleles (AA or aa) Heterozygous: Different alleles (Aa) Genotype: Genetic makeup (e.g., Aa) Phenotype: Physical appearance (e.g., tall)

Mendel's Laws

Law of Segregation

• Each parent has two alleles for each gene
• Alleles separate during gamete formation
• Each gamete gets one allele
• Fertilization restores pairs

Law of Independent Assortment

• Genes for different traits assort independently
• Applies to genes on different chromosomes
• Exception: linked genes on same chromosome

Monohybrid Cross

One trait considered

Example: Tall (T) × Short (t) pea plants

• P generation: TT × tt
• F₁ generation: All Tt (100% tall)
• F₁ × F₁: Tt × Tt
• F₂ generation: 1 TT : 2 Tt : 1 tt
  • Genotypic ratio: 1:2:1
  • Phenotypic ratio: 3:1 (3 tall : 1 short)

Dihybrid Cross

Two traits considered

Example: Round Yellow (RRYY) × Wrinkled Green (rryy)

• F₁: All RrYy (round, yellow)
• F₂ (RrYy × RrYy): 9:3:3:1 ratio
  • 9 Round Yellow
  • 3 Round Green
  • 3 Wrinkled Yellow
  • 1 Wrinkled Green

Testcross

Purpose: Determine unknown genotype

Method: Cross with homozygous recessive (tt)

• If offspring all dominant phenotype → unknown is TT
• If offspring 1:1 ratio → unknown is Tt

Probability Rules

Product Rule (AND)

• Probability of independent events together
• Multiply probabilities
• Example: Probability of Tt AND Yy?
  • P(Tt) = 1/2, P(Yy) = 1/2
  • P(Tt AND Yy) = 1/2 × 1/2 = 1/4

Sum Rule (OR)

• Probability of alternative events
• Add probabilities
• Example: Probability of TT OR Tt?
  • P(TT) = 1/4, P(Tt) = 1/2
  • P(TT OR Tt) = 1/4 + 1/2 = 3/4

Pedigree Analysis

Pedigree: Family tree showing trait inheritance

Symbols:

• Circle = female
• Square = male
• Filled = affected
• Half-filled = carrier
• Horizontal line = mating
• Vertical line = offspring

Determining inheritance pattern:

• Dominant: appears in every generation
• Recessive: skips generations, affected children from unaffected parents

Key Concepts

1. Dominant alleles mask recessive alleles
2. Segregation: alleles separate during gamete formation
3. Independent assortment: different genes assort independently
4. Monohybrid F₂: 3:1 phenotypic ratio
5. Dihybrid F₂: 9:3:3:1 phenotypic ratio
6. Testcross: reveals unknown genotype
7. Product rule: multiply probabilities (AND)
8. Sum rule: add probabilities (OR)