Chromosomes, DNA and genes
Every living cell has a control room called the nucleus. Inside it are thread-like packets called chromosomes. They are easiest to see when a cell is about to divide.
A chromosome is made of one very long molecule of DNA, wrapped around proteins and coiled many times so it fits in a tiny space. If you stretched out the DNA from one human cell, it would be about 2 metres long.
A gene is a short stretch of DNA that holds one instruction, for example how to make one protein. Genes control traits such as blood group or hair type.
So the order of size is: gene < DNA molecule < chromosome < nucleus < cell. A chromosome has hundreds to thousands of genes.
Chromosome numbers and pairs
Each kind of living thing has its own fixed number of chromosomes in a body cell.
- Human: 46 (23 pairs)
- Fruit fly: 8 (4 pairs)
- Pea plant: 14 (7 pairs)
- Rice: 24 (12 pairs)
- Dog: 78 (39 pairs)
A bigger number does not mean a cleverer or more complex animal. It is only a count.
In humans, 22 pairs are the same in boys and girls. They are called autosomes. The 23rd pair is the sex chromosomes: girls have XX, boys have XY.
The two chromosomes in a pair are called homologous chromosomes. They have the same size, the same shape and the same genes in the same places. One came from the mother and one from the father.
How genes pass on: egg and sperm
Body cells have 46 chromosomes. If egg and sperm also had 46, the baby would have 92, and the next baby 184. That would never work.
So the sex cells (gametes) are made by a special cell division called meiosis. It cuts the number in half. Each egg or sperm gets just one chromosome from every pair: 23 chromosomes.
At fertilisation, egg (23) + sperm (23) join to make the first cell of the baby, the zygote, with 46 chromosomes in 23 pairs. Half the genes are from the mother, half from the father.
Which chromosome of a pair goes into a particular egg is a matter of chance. With 23 pairs there are more than 8 million possible eggs, and the same for sperm. That is why brothers and sisters are different.
The father's sperm decides the baby's sex. An egg always carries X. A sperm carries X or Y. X + X gives a girl, X + Y gives a boy. The chance is about half and half.
Genes sit on chromosomes: Sutton and Morgan
Mendel (1860s) talked about 'factors' but did not know where they were. In 1902 Walter Sutton (and Theodor Boveri) noticed that chromosomes behave exactly like Mendel's factors: they come in pairs, they separate into the gametes, and they join again at fertilisation. So, they said, the factors must sit on the chromosomes.
Around 1910, Thomas Hunt Morgan proved it with fruit flies. He found one white-eyed male among red-eyed flies. When he bred it, white eyes passed on together with the sex of the fly. This showed that the gene for eye colour lies on the X chromosome. It was the first clear proof that a gene is a part of a particular chromosome.
Genes are in a line
Genes are lined up one after another on a chromosome, like beads on a string. Each gene has its own fixed place called its locus. In a pair, the same locus on both chromosomes holds the same gene, though maybe in different versions (alleles).
Genes close together on the same chromosome tend to travel together. Morgan's student Alfred Sturtevant used this idea: the more often two genes get separated, the farther apart they are. From such numbers he drew the first gene map, a straight line with genes placed along it.
Today scientists read the whole line of letters of DNA in a person, which is called sequencing the genome.
Try it: build a baby cell
Use the 3D above. Go to step 6, press Fertilise, then press Mix again five times. Write down boy or girl each time. About half should be boys. Then, with a friend, toss a coin for each of 10 pairs: heads means 'take the pink one', tails means 'take the blue one'. Count how different your two gametes are. At home: cut 23 pink and 23 blue paper strips, pair them up and pick one from each pair to make an 'egg'.
Key formulas and definitions
- Human body cell: 2n = 46 (23 pairs)
- Gamete (egg or sperm): n = 23
- n + n = 2n (fertilisation: 23 + 23 = 46)
- Autosomes: 22 pairs; sex chromosomes: 1 pair (XX girl, XY boy)
- Possible gametes from one person: 2^23 = 8 388 608 (without crossing over)
Worked examples
1. A fruit fly body cell has 8 chromosomes. How many are in its egg? How many pairs?
The egg has half: 8 ÷ 2 = 4 chromosomes. A body cell has 4 pairs.
2. A mother gives 23 chromosomes. How many does the child get from the father, and how many in total?
The father also gives 23. Total = 23 + 23 = 46.
3. Order from smallest to largest: chromosome, gene, cell, nucleus, DNA molecule.
Gene, DNA molecule, chromosome, nucleus, cell.
4. A pea plant has 14 chromosomes in a leaf cell. How many chromosomes in a pollen cell (gamete)? How many different chromosome sets can one plant make by chance (2^n)?
Gamete: 14 ÷ 2 = 7. Different sets: 2^7 = 128.
5. A child has 47 chromosomes in a cell instead of 46. Which stage might have gone wrong?
Most likely meiosis, when an egg or sperm formed: that gamete got 24 instead of 23. After fertilisation (24 + 23) the child's cells have 47.
6. Why can a couple have a girl and later a boy?
The mother's egg always carries X. The father's sperm carries X or Y by chance. An X sperm gives a girl, a Y sperm gives a boy, so each baby has about a 50% chance of either.
Common mistakes
- Mixing up the words: a gene is a small part of DNA; a chromosome is a long packet of DNA with many genes.
- Saying the egg has 46 chromosomes. Gametes have 23 (half); only the zygote has 46.
- Thinking more chromosomes means a more advanced animal. Dogs have 78, humans 46.
- Believing the mother decides the baby's sex. The sperm carries X or Y, so the father's sperm decides.