When the chromosome number changes
The conditions so far all came from a single faulty gene. These come from something much larger: a body cell with one chromosome too many, or one too few.
How the number changes
During meiosis a homologous pair sometimes fails to separate, so one gamete ends up with both chromosomes of the pair and the other with neither. If such a gamete is fertilised, every cell of the resulting individual carries the wrong number — 47 instead of 46, or 45.
Down syndrome
47 chromosomes
Three copies of chromosome 21 instead of two. It affects physical development and learning, to a degree that varies a great deal from one person to another.
Turner syndrome
45 chromosomes — X only
A single X chromosome instead of a pair. The individual is female; growth is usually reduced and the ovaries commonly do not develop fully.
Klinefelter syndrome
47 chromosomes — XXY
An extra X alongside the usual X and Y. The individual is male, often taller than average, and the testes commonly do not develop fully.
Count the sex chromosomes in the last two
Turner syndrome is a missing sex chromosome and Klinefelter syndrome an extra one, so both trace back to the same event: the twenty-third pair failing to separate during meiosis. One gamete got both, the other got none, and each produced a different outcome.
These are not inherited from a parent who has them
A gene mutation can be passed down a family for generations. These conditions almost always arise afresh, from one faulty cell division in the formation of one gamete. They are genetic — they involve the chromosomes — without being inherited in the sense the first skill defined.
Wrong number, not wrong gene
These conditions come from a whole chromosome gained or lost when a pair fails to separate in meiosis: 47 chromosomes in Down and Klinefelter syndromes, 45 in Turner syndrome.