Trisomy and reading a karyotype: Down syndrome 47,XX,+21
Trisomy means one extra copy of a single chromosome, giving 47 in total; Down syndrome is usually trisomy 21, with an extra chromosome 21.
Remember
In a karyotype (a person's chromosomes, written in a standard shorthand), 47,XX,+21 means 47 in total, female, with an extra chromosome 21: this is trisomy 21, usually the cause of Down syndrome.
Memory aid
alphabetical order matching descending numbers: Down 21, Edwards 18, Patau 13
Down, Edwards and Patau fall in alphabetical order as the chromosome numbers fall.
A normal body cell holds 46 chromosomes. In trisomy, one chromosome is present three times, so the total is 47. It is one kind of aneuploidy: a chromosome number other than 46, because single chromosomes are gained or lost, rather than whole sets.
A karyotype gives the total number of chromosomes first, then the sex chromosomes, XX in a female and XY in a male, then any abnormality. A plus sign marks an extra chromosome and a minus sign a missing one. So 47,XX,+21 reads as 47 in total, female, with an extra chromosome 21, giving three copies of chromosome 21. In a male the karyotype would be 47,XY,+21.
The total gives the first clue: 47 is one chromosome gained, 45 is one lost (monosomy) and 69 is a whole extra set (polyploidy, such as triploidy, which has three complete sets). Edwards' syndrome has an extra chromosome 18 (+18) and Patau's syndrome an extra chromosome 13 (+13).
The extra chromosome 21 usually comes from non-disjunction, a pair of chromosomes failing to separate in meiosis (the division that makes eggs and sperm). One gamete then carries two copies of chromosome 21, and after fertilisation the embryo has three.
Key facts
- Trisomy
- one chromosome present three times, giving 47 in total.
- Reading a karyotype
- the total number comes first, then the sex chromosomes, then what is gained (+) or lost (-).
- Down syndrome
- usually 47,XX,+21 for a female and 47,XY,+21 for a male.
- Other common trisomies
- Edwards' syndrome is +18 and Patau's syndrome is +13.
- Count check
- 47 is one chromosome gained, 45 is one lost and 69 is a whole extra set.
- Usual cause of trisomy 21
- non-disjunction in meiosis, which usually happens by chance.
- Newborn clues
- low muscle tone (hypotonia), upward-slanting eyes and a single palmar crease (one deep fold across the palm) point to Down syndrome.
Exam traps
- 47,XX,+18 has the right count of 47, but the extra chromosome is 18, giving trisomy 18 (Edwards syndrome) and not Down syndrome.
- A whole extra set of chromosomes is polyploidy, such as triploidy with 69 in total. A karyotype with one extra chromosome is aneuploidy rather than polyploidy.
- Monosomy 21 means one chromosome 21 is missing and would be written 45,XX,-21. The plus sign in 47,XX,+21 shows chromosome 21 gained, not lost.
Read the full notes
Tested in 10 SimpleMed+ questions
Practice it in Genetics, with the answer and a full explanation after every question.
Practice these questions