CB3.3 — Genetic inheritance (Edexcel 1SC0)
📖Definition— Key terms
- Gene: a section of DNA coding for a characteristic.
- Allele: a version of a gene.
- Dominant allele: expressed even if only one copy is present (capital letter).
- Recessive allele: only expressed when two copies are present (lower case letter).
- Genotype: the alleles an organism has (e.g. Bb).
- Phenotype: the physical characteristic shown (e.g. brown eyes).
- Homozygous: two identical alleles (BB or bb).
- Heterozygous: two different alleles (Bb).
Monohybrid cross and Punnett squares
Example: cystic fibrosis (autosomal recessive). C = normal allele; c = cystic fibrosis allele.
Two carrier parents (Cc × Cc):
| C | c | |
|---|---|---|
| C | CC | Cc |
| c | Cc | cc |
Ratio: 3 normal : 1 affected (25% chance of cystic fibrosis).
Sex determination
Humans have 23 pairs of chromosomes. The 23rd pair determines sex:
- XX = female
- XY = male
Cross: XX × XY → 50% XX (female) : 50% XY (male).
Codominance
Both alleles are expressed equally. Example: blood groups — I^A and I^B are codominant → blood group AB.
Sex-linked traits
Some genes are carried on the X chromosome (sex-linked). Males (XY) only have one X — so a recessive allele on that X is expressed (no second X to mask it). Example: red-green colour blindness; haemophilia.
AI-generated · claude-opus-4-7 · v3-edexcel-combined-science