Explore how traits are inherited from parents through genes, learn to read Punnett squares, and discover how dominant and recessive alleles determine our characteristics.
DNA (deoxyribonucleic acid) is the molecule that carries all genetic information in living organisms. It is stored in the nucleus of every cell and is shaped as a double helix โ two strands twisted around each other like a spiral staircase. DNA is made of smaller units called nucleotides, each containing one of four bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). The sequence of these bases forms the genetic code.
DNA is coiled and packaged into structures called chromosomes, found in the nucleus. Humans have 46 chromosomes arranged in 23 pairs โ one chromosome in each pair inherited from each parent.
A gene is a specific section of DNA that codes for a particular trait โ for example, the gene that determines eye colour. Each gene occupies a fixed position (called a locus) on a chromosome.
An allele is an alternative form of a gene. Because chromosomes come in pairs, every individual carries two alleles for each gene โ one on each chromosome in the pair.
| Term | Meaning | Example |
|---|---|---|
| Dominant allele | Expressed even when only one copy is present. Written as an uppercase letter. | B = brown eyes (dominant) |
| Recessive allele | Only expressed when two copies are present. Written as a lowercase letter. | b = blue eyes (recessive) |
| Genotype | The genetic make-up โ which alleles an organism carries. | BB, Bb, or bb |
| Phenotype | The physical characteristic that is actually expressed. | Brown eyes or blue eyes |
A Punnett square is a grid used to predict the possible genotypes (and phenotypes) of offspring from a genetic cross. The alleles of one parent are placed along the top; the alleles of the other parent are placed along the left side.
Monohybrid cross example โ Bb ร Bb (heterozygous brown-eyed parents):
This means there is a 75% chance of brown eyes and a 25% chance of blue eyes in each offspring.
Gregor Mendel (1822โ1884) was an Austrian monk who discovered the basic principles of inheritance by carefully cross-breeding pea plants and recording the results over many years. He tracked traits such as seed colour, seed shape, plant height, and pod colour. His work was largely ignored during his lifetime but is now recognised as the foundation of genetics. His findings led to two laws: the Law of Segregation (alleles separate when gametes form) and the Law of Independent Assortment (genes for different traits are inherited independently).
In codominance, neither allele is dominant or recessive โ both are fully expressed in the phenotype. The best example is the ABO blood type system: a person with genotype IAIB has blood type AB, showing both A and B antigens on red blood cells simultaneously.
The sex of a child is determined by the father's sperm, not the mother's egg. All eggs carry an X chromosome. Sperm carry either an X or a Y chromosome. If a Y-carrying sperm fertilises the egg, the offspring is male (XY). If an X-carrying sperm fertilises the egg, the offspring is female (XX). The Y chromosome carries the SRY gene, which triggers male development.
Not all characteristics are purely genetic. Some traits are influenced by the environment:
Select parent genotypes and a trait. The Punnett square updates instantly, showing all possible offspring combinations with genotype and phenotype ratios.