Kinds

• 1.
• Gene (point) mutations. Chemical changes within the gene, mainly in the arrangement of nitrogenous bases in DNA.
• The changed gene gives a different protein, and so a new trait.
• The change is often from the dominant to the recessive state (rarely the reverse).
• A gene mutation can happen by substitution (replacing), deletion, or addition (insertion) of nucleotides.

2. Chromosomal mutations. (a) Change in chromosome number:

• Aneuploidy: gain or loss of single chromosomes during meiosis, e.g. Klinefelter syndrome (44 + XXY), one extra chromosome; Turner syndrome (44 + XO), one chromosome fewer.
• Polyploidy: whole sets of chromosomes are multiplied, due to non-separation of chromatids after centromere division and failure of a membrane to form between daughter cells. Types: 3N, 4N, 6N, 8N up to 16N. Polyploidy is common in plants: each gene is present in many copies, so its effect is stronger – the plant is taller and organs (flowers, fruits) larger. Many crops are tetraploid (4N) – cotton, wheat, apple, grapes, pears, strawberries. It is rare in animals because sex determination needs a delicate balance between autosomes and sex chromosomes; it occurs in some hermaphrodite snails and worms. Triploidy in humans is lethal (miscarriage), though polyploid cells occur normally in some liver and pancreas cells. (b) Change in chromosome structure: a piece breaks off, turns through 180° and rejoins (inversion); two non-homologous chromosomes exchange segments (translocation); a small segment is lost (deletion) or gained (duplication). The gene order is altered.