Genes, chromosomes and inheritance

Where the genes are

Genes are not loose in the nucleus. They are carried on thread-like structures called chromosomes. Each chromosome is a long piece of DNA, with many genes along it.

• Body cells have chromosomes in pairs.
• The two chromosomes of a pair carry the same genes, in the same order.
• One chromosome of each pair came from the mother, and one from the father.

A human body cell has $46$ chromosomes, which make $23$ pairs. A pea plant has $14$, which make $7$ pairs.

Gametes carry one set

If the egg and the sperm each carried a full $46$, the zygote would have $92$. The next generation would have $184$, and so on. The DNA would keep doubling.

This does not happen. When gametes form, each one receives only one chromosome of each pair. So a human egg or sperm has $23$ chromosomes.

At fertilisation the two sets come together:

So the zygote has the full number again, in pairs. Every pair has one chromosome from each parent. This is how the number of chromosomes stays constant in a species, generation after generation.

With one copy in each gamete, the number stays at $46$ in every generation. Switch to both copies, and the number doubles each time. By the fifth generation, a body cell would hold $1472$ chromosomes.

Why traits mix in new ways

Recall Mendel's cross with two traits: tall or short, and round or wrinkled seeds. In the second generation, he found new mixtures, such as tall plants with wrinkled seeds.

Chromosomes explain this. The gene for height and the gene for seed shape lie on different chromosomes. When gametes form, each pair is shared out on its own. So the chromosome carrying the height gene does not decide which seed-shape chromosome goes with it.

• Each gamete gets one chromosome of each pair.
• The pairs are shared out independently of each other.
• So the versions of different genes are mixed in new ways in every child.

With $23$ pairs, a single person can make millions of different kinds of gametes. This is a big reason why brothers and sisters differ.

Try it in Python

The program finds the chromosomes in a gamete and in the zygote for four organisms.

You should see that each zygote ends up with the same number as the body cell. The fruit fly's gamete carries $4$.

Practise with new numbers

Common mistakes

• Thinking a gamete carries a full set of pairs. It carries one chromosome of each pair, half the number.
• Thinking the chromosome number grows each generation. Halving in gametes keeps it constant.
• Thinking both chromosomes of a pair come from one parent. One comes from each parent.
• Thinking a gene lies anywhere in the cell. Genes lie on chromosomes in the nucleus.

Remember these

• Genes lie on chromosomes. Body cells have chromosomes in pairs.
• Human body cells: $46$ chromosomes, or $23$ pairs. Human gametes: $23$.
• A gamete carries one chromosome of each pair; fertilisation restores the pairs.
• So the chromosome number stays constant from one generation to the next.
• Genes on different chromosomes are passed on independently, giving new mixtures of traits.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Where are genes found in a cell?

2. How many chromosomes are in a human sperm?

3. What would go wrong if gametes carried both chromosomes of every pair?

4. A body cell of a plant has $16$ chromosomes. How many pairs is that?

5. Why can a tall plant with wrinkled seeds appear when Mendel crosses plants for two traits?