Genetics, Heredity and Molecular Biology

🟢 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	Principle
Law of Dominance	One allele masks another	Dominant traits expressed
Law of Segregation	Alleles separate during gamete formation	Each gamete receives one allele
Law of Independent Assortment	Genes on different chromosomes assort independently	Creates genetic variation

Key Terminology:

• Allele: Different forms of a gene
• Genotype: Genetic makeup (e.g., Tt, tt)
• Phenotype: Physical appearance
• Homozygous: Same alleles (TT or tt)
• Heterozygous: Different alleles (Tt)
• Carrier: Heterozygous individual with recessive allele

⚡ A/L Exam Tip: Remember Punnett Square ratios — for monohybrid cross: 3:1 (phenotype), 1:2:1 (genotype)

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Genetics — Detailed Study Guide

Mendelian Genetics

Monohybrid Cross (Single gene inheritance):

P generation:      TT (tall) × tt (dwarf)
Gametes:           T         t
F₁ generation:     Tt (all tall)
F₁ gametes:        T, t
F₂ generation:     TT : Tt : tt
                    1  :  2  :  1
Phenotypic ratio:  3 tall : 1 dwarf

Test Cross: Crossing unknown genotype with homozygous recessive

• If all offspring show dominant trait → unknown is homozygous dominant
• If offspring show 1:1 ratio → unknown is heterozygous

Dihybrid Cross (Two genes):

F₂ Phenotypic Ratio: 9:3:3:1
- 9 dominant for both traits
- 3 dominant for first, recessive for second
- 3 recessive for first, dominant for second
- 1 recessive for both traits

Formula for Dihybrid Cross: $$(3:1)^2 = 9:3:3:1$$

Non-Mendelian Inheritance:

Type	Pattern	Example
Incomplete dominance	Intermediate phenotype	Snapdragon flower color (red × white = pink)
Co-dominance	Both alleles expressed	ABO blood groups (I^A, I^B)
Multiple alleles	More than 2 allele options	ABO blood groups, flower color
Pleiotropy	One gene affects multiple traits	Marfan syndrome
Epistasis	One gene masks another	Squash color

Chromosomal Theory

DNA Structure (Watson-Crick Model): $$5’ \rightarrow 3’ \text{ antiparallel strands}$$

Feature	Description
Double helix	Two coiled strands
Sugar-phosphate backbone	Exterior of helix
Nitrogenous bases	Interior (A-T, G-C)
Base pairing	A=T (2 H-bonds), G≡C (3 H-bonds)

Chargaff’s Rule: [A] = [T] and [G] = [C]

DNA Replication (Semi-conservative):

Parental DNA →
1. Unwinding (helicase)
2. Strand separation
3. New strand synthesis (DNA polymerase, 5'→3')
4. Semi-conservative daughter molecules

Key Enzymes:

Enzyme	Function
Helicase	Unwinds double helix
DNA polymerase	Synthesizes new strand
Ligase	Joins Okazaki fragments
Topoisomerase	Relieves supercoiling

Gene Expression

Central Dogma: $$DNA \xrightarrow{\text{Transcription}} mRNA \xrightarrow{\text{Translation}} Protein$$

Transcription (Eukaryotes):

• RNA polymerase reads 3’→5’ strand
• mRNA synthesized 5’→3’
• Post-processing: 5’ cap, poly-A tail, intron removal

Translation:

Component	Role
mRNA	Carries genetic code
tRNA	Delivers amino acids
rRNA	Ribosome structure
Ribosome	Protein synthesis site

Genetic Code:

• Codon: 3 nucleotides = 1 amino acid
• Start codon: AUG (methionine)
• Stop codons: UAA, UAG, UGA
• 64 possible codons, 20 amino acids (degeneracy)

⚡ A/L PYQ: “Describe the process of DNA replication” Answer: DNA replication is semi-conservative — each daughter DNA molecule contains one original strand and one newly synthesized strand. Enzymes involved include helicase (unwinding), DNA polymerase (new strand synthesis), and ligase (joining fragments).

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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

Human Genetics

Karyotype:

• 46 chromosomes (23 pairs)
• 22 pairs autosomes + 1 pair sex chromosomes
• XX = female, XY = male

Sex-Linked Inheritance:

Disease	Gene Location	Inheritance	Symptoms
Hemophilia A	X chromosome	XᵇXᵇ (carrier) XᵇY (affected)	Bleeding disorder
Red-green color blindness	X chromosome	XᵇXᵇ (carrier) XᵇY (affected)	Cannot distinguish red/green
Duchenne muscular dystrophy	X chromosome	XᵇXᵇ (carrier) XᵇY (affected)	Progressive muscle weakness

Punnett Square for Hemophilia (carrier mother × normal father):

        Xᵇ    X
   X    XᵇX   XX (normal daughter)
   Y    XᵇY   XY (affected son)
   
Result: 25% affected sons, 25% carrier daughters

Pedigree Analysis:

Symbol	Meaning
■ □	Affected male/female
○ ◯	Affected female/male
◻ ▢	Unaffected male/female
⊡ ⊗	Carrier female/male
—	Married couple

Mutations

Gene Mutations (Point Mutations):

Type	Example	Effect
Substitution	CAT → CGT	May change amino acid (missense)
Insertion	CAT → CGAT	Frameshift mutation
Deletion	CAT → CT	Frameshift mutation

Chromosomal Mutations:

Type	Description	Example
Deletion	Loss of chromosome segment	Cri-du-chat (deletion 5p)
Duplication	Repeated segment	Drosophila Bar eye
Inversion	Segment reversed
Translocation	Segment moves to another chromosome	Chronic myeloid leukemia
Aneuploidy	Abnormal chromosome number	Down syndrome (trisomy 21)

Mutation Rate: ~1 in 10⁶ base pairs per replication

Genetic Disorders (Sri Lankan Relevance)

Common Genetic Disorders:

Disorder	Cause	Features
Thalassemia	Autosomal recessive	Abnormal hemoglobin, anemia
Diabetes Type 2	Polygenic + environment	Blood sugar regulation failure
G6PD deficiency	X-linked recessive	Hemolysis with oxidant drugs
Phenylketonuria	Autosomal recessive	Cannot metabolize phenylalanine

Thalassemia in Sri Lanka:

• High prevalence due to carrier status (1 in 20 Sri Lankans)
• Major public health concern
• Prenatal screening recommended
• Treatment: Blood transfusions, iron chelation

Genetic Counseling:

• Assessing risk for inherited conditions
• Pedigree analysis
• Carrier detection
• Prenatal diagnosis options

DNA Technology Applications

DNA Fingerprinting:

Process:
1. DNA extraction
2. PCR amplification of VNTR regions
3. Gel electrophoresis
4. Southern blotting/probe hybridization
5. Band pattern comparison

Applications:

• Forensic identification (99.99% accuracy)
• Paternity testing
• Population genetics studies
• Wildlife conservation (identifying individual animals)

Gene Therapy:

• Functional gene insertion to treat genetic diseases
• Current approaches: Ex vivo (cells modified outside body), In vivo (direct delivery)
• Successfully used for: SCID, inherited blindness, blood disorders
• Ethical considerations ongoing

Human Genome Project:

• Completed 2003
• Identified ~20,500 genes
• Enabled personalized medicine advances

⚡ A/L Strategy: Genetics problems require practice. Master Punnett squares for monohybrid and dihybrid crosses, understand pedigree analysis, and memorize the steps of DNA replication and central dogma.

⚡ Common A/L Questions:

1. “Explain Mendel’s laws of inheritance” (10 marks)
2. “Describe the structure of DNA and its replication” (12 marks)
3. “What is the difference between gene and chromosomal mutations?” (8 marks)
4. “Calculate expected ratios from given genetic crosses” (10 marks)

⚡ Exam Tip: Always state phenotypic and genotypic ratios clearly. When answering inheritance questions, show your Punnett square workings — partial credit is awarded!

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