CB3 — Genetics
📖Definition— Key terminology
| Term | Definition |
|---|---|
| Gene | Section of DNA coding for a specific protein (trait) |
| Allele | A version of a gene (e.g., tall T or short t) |
| Genotype | The allele combination an organism carries (e.g., Tt) |
| Phenotype | The observable characteristic (e.g., tall) |
| Homozygous | Both alleles the same (TT or tt) |
| Heterozygous | Two different alleles (Tt) |
| Dominant | Allele expressed when only one copy present (capital letter) |
| Recessive | Allele expressed only when two copies present (lowercase) |
| Carrier | Heterozygous individual who carries a recessive allele without showing the condition |
Monohybrid inheritance and Punnett squares
Monohybrid inheritance involves a single gene with two alleles. Use a Punnett square to predict offspring genotype ratios.
Example — cystic fibrosis (autosomal recessive, alleles F = unaffected, f = cystic fibrosis):
Two carrier parents: Ff × Ff
| F | f | |
|---|---|---|
| F | FF | Ff |
| f | Ff | ff |
Ratio: 1 FF : 2 Ff : 1 ff → 1:2:1 genotype ratio; 3 unaffected : 1 affected phenotype ratio. Probability of affected child = 1/4 (25%).
Genetic diseases
Cystic fibrosis (CF): autosomal recessive. Gene on chromosome 7 codes for CFTR protein (chloride ion channel). Faulty CFTR → thick mucus accumulates in lungs, pancreas, digestive tract. Symptoms: breathing difficulties, frequent infections, poor nutrient absorption.
Sickle cell anaemia: autosomal recessive. Mutation in haemoglobin gene → abnormal HbS (sickle-shaped red blood cells). Symptoms: anaemia, pain crises, organ damage. Heterozygous carriers (HbA HbS) have sickle cell trait — some protection against malaria (explains persistence of allele in malaria-endemic regions).
Huntington's disease: autosomal dominant. Only one copy of the allele (H) causes disease. Late onset (30–50 years), progressive neurodegeneration, no cure. Even children of affected individuals have 50% risk (Hh × hh cross).
Sex determination
Humans have 23 pairs of chromosomes. The 23rd pair are the sex chromosomes:
- XX = female
- XY = male
All eggs carry X; sperm carry either X or Y. Whether sperm carrying X or Y fertilises the egg determines biological sex. Each birth: 50% probability of XX (female), 50% XY (male).
Sex-linked inheritance
Some genes are located on the X chromosome (X-linked). Because males have only one X (hemizygous), a single recessive allele causes the condition in males — females need two copies.
Colour blindness (X-linked recessive, alleles X^B = normal, X^b = colour blind):
- Carrier female: X^B X^b (normal vision but can pass on allele)
- Affected male: X^b Y (colour blind)
- Affected female: X^b X^b (needs two copies — rarer)
Cross: carrier female × unaffected male (X^B X^b × X^B Y):
| X^B | Y | |
|---|---|---|
| X^B | X^B X^B | X^B Y |
| X^b | X^B X^b | X^b Y |
→ 1/2 of sons colour blind; daughters all unaffected (1/2 carriers).
Variation: continuous and discontinuous
Discontinuous variation: distinct categories, no intermediates. Controlled by one or a few genes. Examples: blood group (A, B, AB, O), ability to roll tongue, sex. Shown as bar chart.
Continuous variation: a range of values with no distinct categories. Controlled by many genes (polygenic) + environment. Examples: height, mass, intelligence. Shown as histogram/normal distribution curve.
Sources of variation:
- Genetic: mutations (random changes in DNA sequence), meiosis (crossing over + independent assortment), sexual reproduction (random fertilisation).
- Environmental: diet, exercise, disease, temperature, UV exposure.
Mutations
A mutation is a change in the base sequence of DNA. Mutations can be:
- Substitution: one base replaced by another (may change one amino acid or be silent).
- Deletion/insertion: base added or removed → frameshift mutation (affects all subsequent codons) → usually non-functional protein.
Most mutations are neutral (non-coding DNA or synonymous change). Some are harmful (disease-causing), rarely beneficial. Mutations are the ultimate source of all genetic variation and the raw material for natural selection.
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