Mutations in the number of chromosomes can arise spontaneously by…

The idea

Chromosome mutations change the number of chromosomes rather than the base sequence. They arise when homologous chromosomes fail to separate at anaphase I, or when sister chromatids fail to separate at anaphase II. This is non-disjunction: both structures are pulled to the same pole, so one daughter cell gains an extra chromosome and another is left one short. A gamete with n plus one chromosomes fertilised by a normal gamete gives a zygote with forty-seven chromosomes in humans, a trisomy, as in Down's syndrome where chromosome 21 is present three times. Non-disjunction of a whole set produces polyploidy, which is common and often viable in plants but almost always lethal in animals.

What earns the marks

• Defining non-disjunction as the failure of homologous chromosomes, or of sister chromatids, to separate.
• Stating that both chromosomes or chromatids move to the same pole.
• Gametes being produced with one more, or one fewer, chromosome than normal.
• The zygote having an abnormal chromosome number after fusion with a normal gamete.
• Correctly identifying how many of the four daughter cells are affected for the division named.

Common mistakes and how to put them right

• Saying non-disjunction changes the base sequence, when it changes chromosome number.

• Confusing non-disjunction with crossing over or with independent segregation.

• Stating that all four gametes are abnormal when non-disjunction occurs in meiosis II.

• Counting chromatids instead of chromosomes when giving the chromosome content of a cell.

• Describing Down's syndrome as the result of a gene mutation.

Exam tips

• In diagram questions count chromosomes, each of which has two chromatids after replication, not chromatids.
• Say both which division failed and which structures failed to separate; each can earn credit.
• Where a question says the second division proceeds normally except in one cell, track each of the four cells separately.