Biology · Both
Inheritance & Genetics
DNA, genes and chromosomes, dominant and recessive alleles, genetic diagrams and variation.
Genetic information is carried in DNA. You must understand genes, alleles, and how characteristics are inherited using simple genetic diagrams.
DNA, genes and chromosomes
• **DNA** – molecule that carries genetic instructions (double helix)
• **Gene** – section of DNA that codes for a particular protein / characteristic
• **Chromosome** – structure made of DNA; humans have 23 pairs (46 total)
• **Allele** – different version of the same gene (e.g. brown eyes / blue eyes)
One allele comes from each parent.
Dominant and recessive
• **Dominant allele** – always shows in the phenotype if present (capital letter, e.g. B)
• **Recessive allele** – only shows if two copies are present (lowercase, e.g. b)
**Genotype** – the alleles an organism has (e.g. Bb)
**Phenotype** – the characteristic expressed (e.g. brown eyes)
Homozygous = two same alleles (BB or bb)
Heterozygous = two different alleles (Bb)
Worked example
B = brown eyes (dominant), b = blue eyes. What eye colour is genotype Bb?
Solution
**Brown eyes** (dominant allele B is expressed).
Genetic diagrams (Punnett square)
Used to predict offspring genotypes and phenotypes.
Example: Bb × Bb
Offspring genotypes: BB, Bb, Bb, bb
Ratio: 3 brown : 1 blue (phenotype)
Practice questions
Try these without looking at the answers first.
Q1.How many chromosomes in a normal human body cell?
Show answer
46 (23 pairs)
Q2.What is an allele?
Show answer
A different version of a gene
Q3.Genotype bb – homozygous or heterozygous?
Show answer
Homozygous (recessive)