Genetics and Mendelian Inheritance

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Genetics — Key Facts for Sri Lanka A/L Examination

Mendel’s Laws:

Law	Statement
Law of Dominance	One allele masks another (dominant over recessive)
Law of Segregation	Alleles separate during gamete formation
Law of Independent Assortment	Alleles of different genes assort independently

Key Terms:

Term	Definition
Gene	Unit of heredity on a chromosome
Allele	Alternative form of a gene
Dominant	Allele that expresses in heterozygote (AA or Aa)
Recessive	Allele only expresses in homozygote (aa)
Homozygous	Both alleles same (AA or aa)
Heterozygous	Different alleles (Aa)
Genotype	Genetic makeup (e.g., Aa, aa)
Phenotype	Physical appearance (e.g., tall, short)

Punnett Square (for monohybrid cross):

Parent: Tt × Tt
         T    t
    T   TT   Tt
    t   Tt   tt

Offspring: 3 tall : 1 short
Genotypic ratio: 1 TT : 2 Tt : 1 tt

⚡ A/L Exam Tip: Capital letter = dominant allele, lowercase = recessive allele!

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Genetics — Detailed Study Guide

Monohybrid Cross

Example: In pea plants, tall (T) is dominant over short (t). Cross: Tall (TT) × Short (tt)

F₁ Generation: All Tall (Tt)
F₁ × F₁: Tt × Tt
F₂ Generation:
         T    t
    T   TT   Tt   → Tall
    t   Tt   tt   → Short
    
Ratio: 3 Tall : 1 Short

Test Cross:

• Crossing unknown genotype with homozygous recessive
• If all offspring show dominant phenotype → unknown was homozygous dominant
• If offspring show 1:1 ratio → unknown was heterozygous

Dihybrid Cross

Example: Round (R) dominant over wrinkled (r), Yellow (Y) dominant over green (y). Cross: RRYY × rryy

F₁: All Round Yellow (RrYy)
F₁ × F₁: RrYy × RrYy

Gametes: RY, Ry, rY, ry

F₂ phenotypic ratio:
9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green

Dihybrid Ratio = 9:3:3:1 (when F₁ self-crossed)

⚡ A/L PYQ: “In a dihybrid cross, what is the genotypic ratio?” Answer: 1:2:1:2:4:2:1:2:1 (9 genotypes, 4 phenotypes in 9:3:3:1 ratio)

Extensions of Mendel

Incomplete Dominance:

• Neither allele is fully dominant
• F₁ shows intermediate phenotype
• Example: Snapdragon flowers (red × white → pink F₁)

Codominance:

• Both alleles expressed equally
• Example: MN blood group (M, N, MN antigens)

Multiple Alleles:

• More than two allele options for a gene
• Example: ABO blood groups
  • I^A, I^B, i (three alleles)
  • I^A and I^B are codominant
  • Both I^A and I^B are dominant over i

ABO Blood Group Genetics:

Genotype	Blood Group
I^A I^A or I^A i	A
I^B I^B or I^B i	B
I^A I^B	AB
ii	O

⚡ A/L Important: Both I^A I^B and I^A i produce different blood groups (AB vs A), showing complete vs incomplete relationships!

Chromosome Theory of Inheritance

Key Points:

• Genes are located on chromosomes
• One chromosome = many genes
• Homologous chromosomes carry allele pairs
• Sex chromosomes (X and Y) determine sex in mammals

Sex Determination:

Organism	Sex Determination
Humans	XY (male heterogametic)
Birds	ZW (female heterogametic)
Drosophila	XY (male heterogametic)
Plants	Various mechanisms

Human Sex Chromosomes:

• Female: XX
• Male: XY
• 50% chance of male or female offspring

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Genetics — Complete Notes for A/L Sri Lanka

Linkage and Crossing Over

Linkage:

• Genes on same chromosome tend to inherit together
• Complete linkage: No crossing over (rare)
• Incomplete linkage: Crossing over can occur

Crossing Over:

• Exchange of genetic material between homologous chromosomes
• Occurs during Prophase I of meiosis
• Creates genetic recombination
• Recombinant frequency measures distance between genes

Morgan’s Experiments (Drosophila):

• White eye mutation (recessive, X-linked)
• Male white × Female red → All red F₁
• F₂: All females red, males 50% red : 50% white
• Showed X-linkage of eye color gene

Autosomal Linkage Example:

• In peas: Flower color (P) and pollen shape (L) are linked
• If genes are close on chromosome, they’re “linked”
• They don’t assort independently

Mutations

Types of Mutations:

Type	Definition	Example
Gene mutation	Change in single gene	Sickle cell anemia
Chromosomal mutation	Change in chromosome structure	Deletion, duplication
Genomic mutation	Change in chromosome number	Down syndrome (trisomy 21)

Gene Mutations (Point mutations):

• Substitution: One base replaced (e.g., A instead of G)
• Insertion: Extra base added
• Deletion: Base removed
• All can cause frame-shift mutations (reading frame changes)

Chromosomal Mutations:

• Deletion: Loss of chromosome segment
• Duplication: Extra copy of segment
• Inversion: Segment reversed
• Translocation: Segment moved to another chromosome

Gene Mutations in Humans:

Condition	Mutation Type	Effect
Sickle cell anemia	Substitution (point)	Abnormal hemoglobin
Phenylketonuria	Enzyme deficiency	Mental retardation
Cystic fibrosis	Deletion	Mucus buildup

Human Genetics

Pedigree Analysis:

• Circles = females, Squares = males
• Shaded = affected individual
• Horizontal line = parents
• Vertical line = offspring

Autosomal Recessive Traits:

• Appears in both sexes equally
• Often skips generations
• Both parents usually carriers (heterozygous)

X-linked Recessive Traits:

• More common in males
• Never passed father to son
• Affected mother → all sons affected

Examples:

• Autosomal recessive: Albinism, Cystic fibrosis
• X-linked recessive: Hemophilia, Red-green color blindness, Duchenne muscular dystrophy

⚡ A/L Important: Hemophilia is X-linked recessive - affected males inherit from carrier mothers. Queen Victoria was a carrier and passed it to several European royal families!

GCE A/L Sri Lanka Past Paper Tips

Common Questions:

1. “State Mendel’s three laws of inheritance” (6 marks)
2. “Explain the terms: genotype, phenotype, homozygous, heterozygous” (4 marks)
3. “A heterozygous tall pea plant is crossed with a homozygous short plant. Using a Punnet square, show the offspring genotypes and phenotypes” (6 marks)
4. “Describe how sex is determined in humans” (5 marks)
5. “Explain the mechanism of crossing over and its importance” (8 marks)

Calculation Questions:

• Probability problems (product rule, sum rule)
• Expected vs observed ratios (chi-square test)
• Pedigree analysis problems

⚡ A/L Strategy: For genetics problems, always define your symbols first (e.g., T = tall, t = short). This shows examiners you understand the notation!

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