genetics-sg

biology

🧬 GENETICS EXPLAINED

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🔁 Genetic Recombination

Type	What It Means	Example
Crossing Over	Recombination between linked genes on the same chromosome	AaBb x aabb → more parentals than recombinants
Independent Assortment	Recombination between unlinked genes (on different chromosomes)	AaBb x aabb → 1:1:1:1 expected
More recombinants = genes are farther apart on a chromosome
Recombination Frequency Formula = (recombinants ÷ total offspring) × 100 = % or map units

• Max recombination frequency = 50% = unlinked
• If genes are linked, recombination frequency is <50%
• Parental types = most frequent
• Recombinants = less frequent

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🔢 Genotypic vs Phenotypic Ratios

Cross Type	Genotypic Ratio	Phenotypic Ratio
Monohybrid (Aa x Aa)	1:2:1 (AA:Aa:aa)	3:1 (dom : rec)
Dihybrid (AaBb x AaBb)	1:2:1:2:4:2:1:2:1	9:3:3:1
Incomplete/Codominance (Rr x Rr)	1:2:1	1 red : 2 pink : 1 white
AaBb x aabb (unlinked)	—	1:1:1:1 (A_B_, A_bb, aaB_, aabb)

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🧬 Key Terms

• Wild Type = most common natural phenotype
• Barr Body = inactive X chromosome in females
• Hemizygous = only one allele present (e.g., males for X genes)
• Genomic Imprinting = some genes are silenced based on parent origin (epigenetics!)

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🎲 Epistasis & Polygenic Traits

• Epistasis: one gene masks another
  • Example ratio = 9:3:4
• Polygenic Inheritance: many genes affect one trait
  • Use formula: 2n + 1 = phenotypes
  • Example: skin color, height

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⚖️ Dominance Types

Type	Result
Complete	Dominant masks recessive (Aa = dominant)
Incomplete	Blend (Rr = pink from red x white)
Codominance	Both show (e.g. AB blood type)

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👪 PEDIGREE ANALYSIS — HOW TO DETERMINE MODE OF INHERITANCE

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Step-by-Step Gameplan:

1. Does the trait skip generations?

   • Yes → likely recessive
   • No → likely dominant

2. Are both sexes affected equally?

   • Yes → probably autosomal
   • No (mostly one gender) → likely sex-linked

3. Are there any carriers?

   • Yes = autosomal recessive
   • ONLY females can be carriers = sex-linked recessive

4. Is every affected daughter’s father also affected?

   • Yes → think X-linked recessive

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🎯 Quick Cheat Sheet:

Inheritance	Key Clues
Autosomal Dominant	Trait appears in every generation; both sexes equally; affected parents → affected kids
Autosomal Recessive	Skips generations; affected kids from unaffected parents; both sexes equally
Sex-Linked Recessive	Mostly males; skips generations; daughters only affected if dad has it too; mom can be carrier
Sex-Linked Dominant	No male-to-male transmission; all daughters of affected dads are affected

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🧬 Barr Bodies

• One Barr body = 2 X chromosomes
• Females = XX (1 Barr body)
• Males = XY (0 Barr bodies)
• So: Yes → 1 Barr body = Female

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 Generation I
  I1([♂ Unaffected]):::unaffected -->|married| I2([♀ Unaffected]):::unaffected
  
   Generation III (from affected II1)
  II1 --> III1([♂ Unaffected]):::unaffected
  II1 --> III2([♀ Affected]):::affected

   Styles
  classDef unaffected fill:#ffffff,stroke:#000000,color:#000000;
  classDef affected fill:#000000,stroke:#000000,color:#ffffff;

flowchart TD
     Styling
    classDef default fill:#ffffff,stroke:#000000,color:#000000,font-size:12px;
    class A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P default;