why does non disjunction of the 21 chromosome takes place in down syndrome?

It mostly occurs in old mothers, some think its because the chromatids were next to each other for a longer time in prophase 1 than other mothers as they are older so they stick together and so non disjunction occurs and the child has Down syndrome.

Most of the chromosomal disorders (leading to aneuploidy) non-dis-junction of chromosomes during meioses in gamete formation. nondisjunction is the failure of a chromosome to split correctly during meiosis.

This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for trisomy or monosomy.

Nondisjunction can occur in the meiosis I or meiosis II phases of cellular reproduction.

This is a cause of several medical conditions, including:

•Down Syndrome - trisomy of chromosome 21

•Patau Syndrome - trisomy of chromosome 13

•Edward Syndrome - trisomy of chromosome 18

•Turner Syndrome - only one X chromosome present

The individuals with this Syndromne are trisomic for chromosome 21. (or trisomy 21). These individuals suffer from a variety of phenotypic abnormalitias called Down Syndrome named after Langston Down, who first described the phenotypes associated with the syndrome in 1866. The chromosomal cause of Down syndrome was uncovered in 1958 and was the first recognized human condition associated with aneuploidy.

Characteristics :

1. Broad skull

2. Short stature

3. Stubby hand and feet with characteristic skin creases.

4. Low intelligence (or mental retardation) but are capable of learning many skills. Inspite of their handicaps, they are often happy and affectroriate.

5. Most transonic 21 foetuses do not survive until birth.

6. They have epicauthal folds (which caused this syndrome to be called mongolism originally)

Frequency : 1/700 births in women aged 25 years or under. The frequency increases with age to about 1/100 for women of age 40 and 1/10 for women of age 45. [Chromosome Complement]

46 + addition chromosome 21 = 47

Cause : Most cases of trisomy 21 are the result of non-disjunction during meioses. Non disjunction occurs in both males and females although more common associated with females.

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I have never come across a definitive answer for the WHY of nondisjunction of chromosome 21 in Down Syndrome, but it has definitely been linked to the age of the mother (trisomy also occurs in older eggs of other animals). It is simply regarded as an error in meiosis.

Your question is incorrect. You're implying that the nondisjunction happens because of Down's syndrome. It's the opposite - Down's syndrome is the result of the nondisjunction.

Sometimes meiosis doesn't occur as it's supposed to and the chromosomes don't separate during either meiosis 1 or meiosis 2.

This usually results in the death of the embryo(miscarriage) way before it is born (most of the times women don't even notice that they were pregnant). However in rare cases, such as Down's syndrome the embryo survives, is born and lives.

21 numbers of chromosomes is very small, during second meiotic cell division centromire do not divide. mainly this non disjunction occur in female cell. because when a female was embryo meiosis started and before birth helf of the division was completed other half of the division completed after 12 years one by on cell. so when a second meiotic meiotic cel division occur in old cell the centromire do not divide.