One set in a germ-cell

The idea

Each organism has two sets of all genes, one inherited from each parent, so each germ-cell carries only one gene set; two germ-cells combining restore the normal number, keeping the species' DNA stable.

Counting gene sets

Every cell of a pea plant has two sets of genes, one from each parent. Two germ-cells, one from each parent, join to start a new plant.

How many gene sets must each germ-cell carry?

• One
• Two
• Four

Follow the gene sets

Press Next step.

Halve, then restore

A germ-cell has one set. Two germ-cells together have two sets, which is the normal number. So the amount of DNA stays steady from generation to generation.

Each organism has two sets of all genes, one inherited from each parent, so each germ-cell carries only one gene set. When two germ-cells combine, they restore the normal number, keeping the DNA of the species stable.

Notes

Each germ-cell carries one gene set; two germ-cells combining restore the normal number and keep the species' DNA stable.

Check yourself

A body cell has two gene sets. How many gene sets does a germ-cell carry?

Answer: 1

Each germ-cell has only one gene set. Two germ-cells together make two sets again.

What happens when two germ-cells combine?

Where do the two gene sets of an organism come from?

• They make a cell with a single gene set. Each germ-cell has one set, so two germ-cells together give two sets, the normal number.
• They give more than the normal number of sets. They restore the normal number, not more than it.
• They restore the normal number, keeping the DNA of the species stable — correct. Yes!
• One from each parent — correct. Yes!
• Both from the mother. Each organism has two sets of all genes, one inherited from each parent.
• Both from the father. Each organism has two sets of all genes, one inherited from each parent.