BRAINCAKEDown's Syndrome
Down's syndrome is a genetic condition caused by an extra copy of chromosome 21. It is also called trisomy 21, and affected people usually have 47 chromosomes in their body cells instead of 46.
In most cases (around 95%) it results from non-disjunction of chromosome 21 during meiosis, usually during the formation of the egg. A gamete with two copies of chromosome 21 (24 chromosomes in total) fuses with a normal gamete (23), giving a zygote with three copies. Because the zygote then divides by mitosis, every cell of the body carries the extra chromosome.
A small number of cases are caused by a translocation, in which part of chromosome 21 is attached to another chromosome, so the person has the extra genetic material even though they have 46 chromosomes.
Sign in free to see the rest of this lesson, the R.E.C.I.P.E. recall steps and the quiz
BrainCake is free. Make an account in seconds and pick up where this page stops.
Key terms in this lesson
- translocation
- A chromosome mutation in which a section of one chromosome breaks off and joins a different, non-homologous chromosome.
- non-disjunction
- The failure of chromosomes or chromatids to separate correctly during meiosis, producing gametes with an abnormal chromosome number.
More in Meiosis and Sexual Reproduction
- Meiosis 2
- The Importance of Meiosis
- Chromosome Mutation
- Non-disjunction
- Turner's Syndrome
- Spermatogenesis
- Oogenesis
- The Male Gamete
All 28 lessons in Meiosis and Sexual Reproduction · All Edexcel AS-level Biology topics