Genetics, inheritance and Punnett squares
DNA, genes and chromosomes
- DNA — a double helix made of two strands of nucleotides; carries the genetic code.
- Gene — a section of DNA that codes for a single protein, controlling a specific characteristic.
- Chromosome — a long molecule of DNA wound around proteins. Humans have 46 chromosomes (23 pairs).
- Allele — different version of a gene (e.g. brown vs blue eye allele).
- Genotype — the alleles an organism has (e.g. Bb).
- Phenotype — the physical characteristic shown (e.g. brown eyes).
Dominant and recessive
- A dominant allele is expressed when present in either one or two copies. Conventionally a capital letter B.
- A recessive allele is expressed only when there are two copies (bb). Conventionally a lowercase letter.
- Homozygous = two identical alleles (BB or bb). Heterozygous = two different alleles (Bb).
Punnett squares
Used to predict offspring genotypes from a parental cross.
Example: Bb x Bb (both parents heterozygous brown-eyed).
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
- Genotype ratio 1 BB : 2 Bb : 1 bb.
- Phenotype ratio 3 brown : 1 blue.
Inherited disorders
Most GCSE examples are caused by recessive alleles, requiring two copies for the disorder to show.
- Cystic fibrosis — recessive, affects mucus production (lungs and digestion).
- Sickle cell anaemia — recessive, abnormal red blood cells.
- Huntington's disease — dominant, affects the nervous system in mid-adulthood.
If both parents are carriers (Aa) of a recessive disorder, each child has a 1/4 chance of being affected.
WJEC exam tip
Always state phenotype OR genotype clearly when answering inheritance questions. "3 brown : 1 blue" is a phenotype ratio. "1 BB : 2 Bb : 1 bb" is a genotype ratio. Examiners deduct marks if you blur the two.
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