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)

Punnett Square:

         |  ty
    -----|--------
    TY   | TtYy (tall, yellow)
    -----|--------
    Ty   | Ttyy (tall, green)
    -----|--------
    tY   | ttYy (short, yellow)
    -----|--------
    ty   | ttyy (short, green)

Offspring genotypes:

• TtYy: tall, yellow
• Ttyy: tall, green
• ttYy: short, yellow
• ttyy: short, green

(b) Phenotypic Ratio:

Count each phenotype:

• Tall, yellow: 1 (TtYy)
• Tall, green: 1 (Ttyy)
• Short, yellow: 1 (ttYy)
• Short, green: 1 (ttyy)

$\boxed{\text{Phenotypic ratio: } 1:1:1:1}$

Or grouped:

• Tall: 2 (50%)
• Short: 2 (50%)
• Yellow: 2 (50%)
• Green: 2 (50%)

(c) Probability of tall with green seeds:

Phenotype: Tall, green Genotype: Ttyy

From Punnett square: 1 out of 4 offspring

$P(\text{tall, green}) = \frac{1}{4}$

$\boxed{\text{Probability} = 25\% \text{ or } 0.25}$

Alternative calculation (using multiplication rule):

$P(\text{tall}) = P(T-) = \frac{1}{2}$ $P(\text{green}) = P(yy) = \frac{1}{2}$

Since traits assort independently: $P(\text{tall AND green}) = \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

Key Concept - Testcross:

• Crossing with homozygous recessive reveals genotype of unknown
• Used to determine if organism is homozygous or heterozygous
• Mendel used this to confirm his laws

Mendel's Laws Applied:

1. Law of Segregation: Each gamete gets one allele per gene
2. Law of Independent Assortment: T/t segregates independently of Y/y