Genetics
DNA structure and the genetic code
DNA (deoxyribonucleic acid) is a double helix made of two strands of nucleotides. Each nucleotide has:
- A deoxyribose sugar
- A phosphate group
- One of four nitrogenous bases: Adenine, Thymine, Guanine, Cytosine
Base pairing rules (complementary base pairs):
- A pairs with T (and T with A)
- G pairs with C (and C with G)
A gene is a section of DNA that codes for a specific protein (sequence of amino acids). Humans have about 20,000–25,000 genes located on 46 chromosomes (23 pairs) in the nucleus.
Chromosomes and alleles
Humans have 23 pairs of homologous chromosomes (46 total). Each pair carries genes for the same traits but may have different versions.
An allele is an alternative form of a gene. If two alleles are the same, the organism is homozygous; if different, it is heterozygous.
- Dominant allele: expressed even when only one copy is present (represented with capital letter, e.g. B).
- Recessive allele: only expressed when two copies are present (lower case, e.g. b).
- Genotype: the alleles an organism carries (e.g. Bb).
- Phenotype: the observable characteristic (e.g. brown eyes).
Monohybrid crosses
A Punnett square shows the possible offspring genotypes from a cross.
Example: Brown eyes B dominant over blue (b). Cross Bb × Bb:
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Ratio: 3 brown (BB + Bb + Bb) : 1 blue (bb) — 75% brown, 25% blue.
Sex determination
Sex chromosomes: females are XX, males are XY.
- All eggs carry X.
- Sperm carry either X or Y.
- 50% chance of XX (female) or XY (male) at fertilisation.
Genetic diseases
Cystic fibrosis (CF): autosomal recessive — caused by two copies of a faulty recessive allele (f). Affects cell membrane chloride channels → thick mucus in lungs and digestive system. Carriers (Ff) are unaffected.
Huntington's disease: autosomal dominant — caused by one copy of the dominant allele (H). Progressive neurological disorder with late onset. Even Hh individuals will develop the disease.
Variation
Continuous variation: a range of values with no distinct categories; influenced by many genes and the environment. Example: height, skin colour, mass.
Discontinuous variation: distinct categories with no intermediates; usually controlled by one or few genes. Example: blood group (A, B, AB, O), tongue rolling, attached vs free earlobes.
Variation arises from:
- Genetic differences — mutations, sexual reproduction (crossing over, independent assortment).
- Environmental factors — diet, exercise, disease, climate.
⚠Common mistakes
- Confusing genotype and phenotype — genotype is the alleles (Bb); phenotype is the trait (brown eyes).
- Misapplying dominant/recessive in CF — carriers have genotype Ff but are NOT affected (recessive disease).
- Forgetting that Punnett square ratios are probabilities, not guaranteed outcomes in a family.
- Sex determination — writing that the mother determines sex; it is the father's sperm (X or Y) that determines sex.
AI-generated · claude-opus-4-7 · v3-ccea-biology