Chromosomes, genome and genes
Inside the nucleus of a cell, DNA is wound around proteins and packed into thread-like chromosomes. A human body cell has 46 chromosomes, which is 23 pairs.
The complete set of DNA of an organism is its genome. A gene is a stretch of DNA that carries the instruction for one product, often a protein. Genes sit in a fixed place on a chromosome, called its locus.
Only a small part of human DNA makes genes; the rest has other jobs or is not yet understood.
Alleles: forms of a gene
The two chromosomes in a pair are called a homologous pair. They carry the same genes in the same order, but the gene on one can be a different version from the gene on the other. These versions are alleles.
Example: an eye-colour gene may have a B allele (brown) and a b allele (blue). A person with BB or Bb has brown eyes because B is dominant (shows up even when only one copy is there); bb gives blue eyes. A person with two different alleles is called heterozygous; with two same alleles, homozygous.
Meiosis and fertilisation: shuffling alleles
Meiosis is a special cell division that makes gametes (egg and sperm). In it the number of chromosomes is halved: each gamete gets just one chromosome of each pair. Which one it gets is random, so a person with 23 pairs can make 223 (about 8 million) different gametes.
At fertilisation egg and sperm join, and the chromosome number returns to 46. Each child gets one set from each parent. This is why children are like parents but not exact copies, and why brothers and sisters differ.
Sometimes a pair does not separate (called nondisjunction), and a gamete gets too many or too few chromosomes. This can cause conditions such as Down syndrome (an extra chromosome 21).
Polygenic inheritance and continuous traits
Mendel's pea traits were controlled by one gene each (two clear groups: tall or short). But many human traits are controlled by many genes at once. This is polygenic inheritance. Each tall allele adds a little height. Because many alleles can be added in many combinations, the result is a smooth range, called continuous variation.
Plot how many people have each height and you get a bell-shaped curve: most people in the middle and a few at the extremes. Environment (food, health) also matters.
One code for all life
All living things, from bacteria to humans, write genes in the same four-letter DNA code (A, T, G, C) and read it in the same way. This shared genetic code is evidence that all life is related.
Try it
In the 3D, set each of the three height genes to 1, then press "Make 100 children". Which total height is most common? Now set all three to 2.
Key formulas and definitions
- Human body cell: 46 chromosomes = 23 pairs; gamete: 23
- Genome = all the DNA of an organism
- Gamete types from n pairs = 2^n (23 pairs → 2^23 ≈ 8.4 million)
- Polygenic trait = many genes add up → bell-shaped curve
- Genotype BB, Bb = brown (B dominant); bb = blue
Worked examples
1. A fruit-fly cell has 4 pairs of chromosomes. How many chromosomes in a body cell and in a gamete?
Body cell = 4 × 2 = 8 chromosomes. A gamete has one of each pair = 4 chromosomes.
2. How many different gametes can an organism with 3 pairs of chromosomes make (ignoring crossing over)?
2^3 = 8 different gametes.
3. Both parents are Bb for eye colour (B = brown, b = blue). What are the possible genotypes of children?
Each parent gives B or b. Children: BB, Bb, Bb, bb. Three out of four have brown eyes (BB, Bb, Bb) and one in four has blue eyes (bb).
4. Three height genes each have a tall allele (T) and a short allele (t). A person is Tt Tt Tt. How many tall alleles? What happens if two such people have children?
They have 3 tall alleles. Children can have 0 to 6 tall alleles, but most will have about 3, with fewer at the two ends. This makes a bell-shaped curve.
Common mistakes
- Mixing up gene and allele. A gene is the instruction; an allele is one version of that gene.
- Thinking a gamete gets 46 chromosomes. A gamete gets 23 (one of each pair).
- Thinking one gene controls height. Height is polygenic and also depends on food and health.
- Thinking dominant means "most common". Dominant only means the allele shows up when it is present.