Polyploidy and reverse genetics

Ploidy is the number of complete sets of chromosomes in a cell: haploid (one set), diploid (two), and polyploid (more than two: triploid 3, tetraploid 4, hexaploid 6, octoploid 8). Male bees and ants are haploid.

Polyploidy is rare in animals but common in plants (more than 30% of them). Polyploid plants often have larger leaves and cells and cope better with harsh conditions, so many crops are polyploid: banana is triploid (33 chromosomes), potato and cotton are tetraploid, bread wheat is hexaploid (42 chromosomes, from a basic set of x = 7, so 6 × 7 = 42), and strawberry and sugarcane are octoploid.

An aneuploid has one or a few chromosomes extra or missing, not whole sets. It usually results from chromosomes failing to separate properly in meiosis.

There are two ways to find out what a gene does:

• Forward genetics starts from a visible difference (a trait, like white eyes) and searches for the gene behind it.
• Reverse genetics starts from a known DNA or protein sequence whose job is unknown, changes it on purpose, and watches what happens.

Sequencing whole genomes has found many genes whose jobs are still unknown, so reverse genetics is increasingly important. A gene can be silenced with RNA interference (RNAi), in which small double-stranded RNAs block its expression, or knocked out by targeted disruption through homologous recombination.