Genetics (IB)

• A gene is a heritable factor controlling a specific characteristic; a sequence of DNA bases coding for a polypeptide
• Alleles are different forms of a gene, occupying the same locus on homologous chromosomes
• Humans have 46 chromosomes (23 pairs): 22 pairs of autosomes + 1 pair of sex chromosomes (XX or XY)
• Homologous chromosomes: same genes at the same loci, but may have different alleles
• Karyotype: the number and appearance of chromosomes; used to detect abnormalities (e.g., trisomy 21 — Down syndrome)
• Genome: the entire genetic information of an organism

Meiosis produces four genetically different haploid cells from one diploid cell.

Meiosis I (Reduction Division)

• Prophase I: Homologous chromosomes pair (bivalents); crossing over occurs at chiasmata
• Metaphase I: Bivalents align randomly at the equator (independent assortment)
• Anaphase I: Homologous chromosomes separate (reduction: $2n → n$)
• Telophase I: Two haploid cells formed

Meiosis II (Similar to Mitosis)

• Sister chromatids are separated
• Result: four haploid cells, each genetically unique

Sources of Genetic Variation

1. Crossing over (prophase I): exchange of alleles between homologous chromosomes
2. Independent assortment (metaphase I): random orientation of bivalents ($2^{23}$ combinations in humans)
3. Random fertilisation: any sperm can fuse with any egg
4. Mutation: random changes in DNA

Non-disjunction

• Failure of chromosomes to separate properly
• Results in gametes with abnormal chromosome numbers
• Down syndrome: trisomy 21 (three copies of chromosome 21)

Key Terms

• Dominant: expressed when one or two copies present
• Recessive: only expressed when homozygous
• Codominant: both alleles expressed in heterozygote
• Genotype: allele combination (e.g., Bb)
• Phenotype: observable characteristic
• Homozygous: two identical alleles (BB or bb)
• Heterozygous: two different alleles (Bb)
• Carrier: heterozygote for a recessive condition

Monohybrid Crosses

• Bb × Bb → 3 dominant : 1 recessive
• Bb × bb → 1 dominant : 1 recessive (test cross)

ABO Blood Groups (Codominance + Multiple Alleles)

• Three alleles: $I^A$, $I^B$, $I^O$
• $I^A$ and $I^B$ are codominant; $I^O$ is recessive
• Genotypes: $I^AI^A$ or $I^AI^O$ → Type A; $I^BI^B$ or $I^BI^O$ → Type B; $I^AI^B$ → Type AB; $I^OI^O$ → Type O

Sex-Linked Inheritance

• Genes on the X chromosome (e.g., colour blindness, haemophilia)
• Males ($XY$) are more likely to be affected (only one X, no second copy to mask recessive allele)

Pedigree Charts

• IB requires you to deduce genotypes from pedigree diagrams
• Filled symbols = affected; half-filled = carrier; circles = female; squares = male
• Key clues: if two unaffected parents have an affected child → both are carriers → condition is autosomal recessive

PCR (Polymerase Chain Reaction)

• Amplifies DNA: Denaturation (95°C) → Annealing (55°C) → Extension (72°C)
• Uses Taq polymerase (heat-stable); $2^n$ copies after $n$ cycles

Gel Electrophoresis

• Separates DNA fragments by size (smaller move further)
• DNA is negatively charged → moves towards positive electrode

Genetic Engineering

1. Gene cut using restriction enzymes (produce sticky ends)
2. Gene inserted into vector (plasmid) using DNA ligase
3. Vector introduced into host cell
4. Host expresses the gene

Applications

• Insulin production by GM bacteria
• GM crops: pest resistance (Bt crops), herbicide tolerance, Golden Rice (vitamin A)

Gene Therapy

• Somatic: affects only the individual (not heritable)
• Germ line: affects gametes (heritable; currently banned for humans)

Cloning

• Somatic cell nuclear transfer: nucleus from body cell → enucleated egg → embryo → clone
• Example: Dolly the sheep
• Plants: tissue culture from meristems

• Completed 2003; mapped all ~20,000–25,000 human genes
• Benefits: identifying disease genes, pharmacogenomics, understanding evolution
• Concerns: genetic discrimination, privacy, patent issues

Question: A man with blood type A ($I^AI^O$) and a woman with blood type B ($I^BI^O$) have children. What are the possible blood types of their offspring? (3 marks)

Solution:

Possible blood types: A, B, AB, and O — each with 25% probability.

• Genes are DNA sequences coding for polypeptides; alleles are different versions at the same locus.
• Meiosis produces 4 haploid cells with genetic variation from crossing over, independent assortment, and random fertilisation.
• Inheritance patterns: dominant/recessive, codominance (ABO blood groups), sex-linkage (X-linked).
• Biotechnology: PCR amplifies DNA; gel electrophoresis separates fragments; genetic engineering uses restriction enzymes + ligase + vectors.
• Gene therapy and cloning have applications but raise ethical concerns.