Why do children resemble their parents?
Heredity is the passing of traits from parents to offspring. The instructions for traits are carried in genes, which are sections of DNA.
Where genes live
- DNA is coiled tightly into chromosomes, found in the nucleus of each cell.
- Humans have 46 chromosomes, in 23 pairs. One chromosome of each pair comes from each parent.
- Each chromosome carries hundreds to thousands of genes.
- Sex cells (sperm and egg) have only 23 chromosomes, so when they join, the offspring has the full 46.
Alleles
A gene can come in different versions called alleles. For example, a pea plant gene for flower colour has a purple allele and a white allele.
Because you get one chromosome from each parent, you have two alleles for each gene.
- Dominant allele: shows its trait even with one copy. Written as a capital letter (P).
- Recessive allele: shows its trait only with two copies. Written as a lowercase letter (p).
Genotype and phenotype
The genotype is the pair of alleles. The phenotype is the trait you can see.
| Genotype | Name | Phenotype (pea flower) |
|---|---|---|
| PP | Homozygous dominant | Purple |
| Pp | Heterozygous | Purple |
| pp | Homozygous recessive | White |
Notice that PP and Pp look the same. You can’t always tell the genotype just by looking.
Gregor Mendel
In the 1860s, an Austrian monk named Gregor Mendel bred thousands of pea plants. He found that traits are passed in predictable patterns and that some traits can "hide" for a generation and reappear later. He’s called the father of genetics.
Punnett squares
A Punnett square predicts the chances of offspring genotypes.
Cross two heterozygous purple plants (Pp × Pp):
| P | p | |
|---|---|---|
| P | PP | Pp |
| p | Pp | pp |
Results:
- Genotypes: 1 PP : 2 Pp : 1 pp
- Phenotypes: 3 purple : 1 white (75% purple, 25% white)
These are probabilities, not guarantees, like flipping a coin.
Beyond simple dominance
Not every trait follows the simple pattern.
| Pattern | What happens | Example |
|---|---|---|
| Incomplete dominance | A blend appears | Red × white snapdragons give pink |
| Codominance | Both traits show | Red and white hairs on a roan cow |
| Multiple alleles | More than two alleles exist | Human blood types A, B, AB, O |
| Polygenic traits | Many genes affect one trait | Human height and skin colour |
Genes and environment
Many traits are affected by both genes and the environment. Your genes influence your height, but so do nutrition and health. Hydrangea flowers have genes for colour, but soil acidity decides whether they’re blue or pink.
Mutations
A mutation is a change in DNA. Some mutations cause genetic disorders; others create new variations that can be helpful.
Common mistakes
- Thinking dominant means more common. Some dominant alleles are rare.
- Thinking a 3:1 ratio means exactly 3 out of every 4 offspring. It’s a probability.
- Confusing genotype and phenotype.
Try it yourself
In guinea pigs, black fur (B) is dominant over white fur (b).
- Cross a homozygous black guinea pig (BB) with a white one (bb). What are the offspring?
- Cross two of those offspring (Bb × Bb). Draw the Punnett square.
- What percentage of offspring will be white?
(Answers: all Bb, black; 1 BB : 2 Bb : 1 bb; 25% white)
Quick recap
- Heredity passes traits through genes on chromosomes.
- Each person has two alleles for each gene, one from each parent.
- Dominant alleles show with one copy; recessive alleles need two.
- Genotype is the allele pair; phenotype is the visible trait.
- Punnett squares predict probabilities of offspring traits.