Genetics, Variation and Reproduction
DNA and Chromosomes
DNA (deoxyribonucleic acid) is a double-stranded helix made of nucleotides. Each nucleotide contains:
- A phosphate group
- A deoxyribose sugar
- A nitrogenous base (A, T, C, or G)
Base pairing is complementary and specific: A–T and C–G (held by hydrogen bonds).
Genes: segments of DNA that code for a specific protein (via mRNA → polypeptide). Each gene occupies a specific locus on a chromosome.
Alleles: different versions of the same gene. Dominant alleles are expressed even when one copy is present (heterozygous); recessive alleles are only expressed when two copies are present (homozygous recessive).
Human body cells have 46 chromosomes (23 homologous pairs). Chromosome 23 = sex chromosomes: XX (female), XY (male).
Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Sexual reproduction (gametes) |
| Daughter cells produced | 2 | 4 |
| Chromosome number | 46 (diploid) | 23 (haploid) |
| Genetically identical? | Yes | No (genetic variation) |
| Where? | Body cells | Gonads (testes/ovaries) |
Meiosis produces variation through: crossing over (exchange of alleles between homologous chromosomes) and independent assortment (random orientation of chromosome pairs).
Punnett Squares and Monohybrid Inheritance
Use Punnett squares to predict offspring ratios. Convention: dominant allele = capital letter; recessive = lower case.
Example — cystic fibrosis (autosomal recessive; alleles F/f): Both parents are carriers (Ff × Ff):
| F | f | |
|---|---|---|
| F | FF | Ff |
| f | Ff | ff |
Ratio: 1 FF : 2 Ff : 1 ff → probability of affected child (ff) = 1/4 (25%).
Codominance: both alleles are expressed in the heterozygote. Example: ABO blood group (Iᴬ and Iᴮ are codominant; i is recessive).
Sex-linked inheritance: genes on the X chromosome. Colour blindness and haemophilia are X-linked recessive — more common in males (only one X chromosome).
Genetic Disease
- Cystic fibrosis — autosomal recessive; affects mucus production → breathing and digestion.
- Sickle cell disease — autosomal recessive (codominant); abnormal haemoglobin.
- Huntington's disease — autosomal dominant (only one allele needed); progressive neurodegenerative.
- Down syndrome — chromosomal; trisomy 21 (three copies of chromosome 21); caused by non-disjunction during meiosis.
Genetic testing can identify carriers and affected individuals. Ethical considerations: privacy, discrimination, psychological impact.
Variation
Genetic variation — differences in DNA (alleles); inherited; source: mutations, sexual reproduction. Environmental variation — caused by environment (e.g. language spoken, scars, diet-induced weight). Many characteristics show continuous variation (controlled by many genes + environment; e.g. height, weight) or discontinuous variation (controlled by one or few genes; e.g. blood group, tongue rolling).
⚠Common mistakes
- Confusing mitosis and meiosis — mitosis produces 2 identical diploid cells; meiosis produces 4 haploid cells with genetic variation.
- A carrier is heterozygous (Ff) — they do not show the disease but can pass it on.
- Dominant diseases (e.g. Huntington's) require only ONE dominant allele — even heterozygotes are affected.
- XY = male; XX = female. The Y chromosome from the father determines the sex of the offspring.
AI-generated · claude-opus-4-7 · v3-wjec-biology