1. The process of Meiosis
Each chromosome is composed of two sister chromatids containing identical genetic information. It is an identical duplicate of the other.
The sister chromatids are joined by a centromere (orange). Therefore, if a single chromosome is pictured as the letter X, then one sister chomatid makes up the left-hand side of the X, and the other makes up the right-hand side.
When the chromosomes synapse (join) during prophase, each gene in each chromosome is brought into contact with its counterpart on that chromosome's homolog. During this process of synapsis, the two homologs (sister chromatids) of each pair exchange segments of DNA in a process known as crossing over. As a result, the gene combinations on each chromosome can be changed. (For example, suppose one homolog initially contained genes for brown eyes and brown hair. After crossing over, it could contain genes for blue eyes and brown hair, where the gene for blue eyes was taken from its homolog.)
While prophase I is proceeding, the centriole pairs move to the opposite ends of the cell and the spindle lengthens and extends toward the "metaphase plate."
Prophase I is the first stage of meiosis(meiosis is the duplication and separation of the chromosomes) In this phase, the chromosomes (shown in red in the diagram) become visible as they condense, that is, as they shorten, coil, and thicken. The nuclear membrane (shown in white) breaks up and disappears.
Prophase I

2. During Metaphase (2nd phase) the chromosomes align on the "metaphase plate."
Next, the spindle fibers attach to the centromeres of each chromosome.
It causes the two members of each chromosome pair to be separated from each other during the next stage of meiosis the anaphase I.
Metaphase I

3. In the third stage, anaphase I, the cell lengthens as it begins the process of division. Homologs of each chromosome pair move toward opposite poles
Anaphase I

4. In this, the fourth stage of meiosis, the chromosomes reach the poles.
At each pole, now, there is a complete haploid set of chromosomes (but each chromosome still has two sister chromatids).
During telophase I, a cleavage furrow appears. By the end of this stage the cell has divided in two.
Telophase I
Telophase I

5. Prophase II Prophase II
Prophase II is the first stage of the second division, and the fifth stage of meiosis as a whole.
It begins with the two daughter cells produced by the first division.
As in Prophase I, the chromosomes are condensed and not yet attached to the spindle apparatus.
Prophase II

6. Genetics and Down Syndrome
Down syndrome is a chromosomal adnormality characterized by die presence of an extra copy of genetic material on the 21st chromosome, either in whole (trisomy 21) or part (translocation)
Each human has 46 chromosomes in each cell, however a child with Down syndrome has 47 chromosomes. There are 3 of the 21 chromosome present.
Down Syndrome can occur in 4 ways:
There are 3 Types of Down Syndrome.
Trisomy 21 (Duplication of a portion of chromosome 21 falls under Trisomy 21)
Mosaic
Translocation

7. Trisomy 21
Trisomy 21 is caused by a meotic nondisjunction (Meiotic = cellular division) (Nondisjunction =failure of chromosome pairs to separate properly during cell division) Thus resulting in an imbalance of chromosomes. Nondisjunction occurs prior to conception and all the cells in the body has the extra no 21 chromosome.
With nondisjunction, a gamete (sperm or an egg Cell) is produced with an extra copy of chromosome no 21. This gamete will then have 24 chromosomes (all reproductive cells normally have 23 chromosomes) When this gamete is combined with a normal gamete from the other parent, the embryo will have 47 chromosome (3 no 21 chromosomes)
Trisomy 21 is the cause of approx. 95% of observed Down syndromes with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.

8. Mosaic Down Syndrome
With Mosaic Down Syndrome some of the cells in the body are normal and some of the cells have the extra chromosome no 21.
Ordinarily, when an egg and a sperm are joined at conception, a single cell is created with a total of 46 chromosomes.
If nothing disrupts the chromosome replication and separation process, each cell in the body should have the same number of chromosomes that were present in the fertilized egg. However, errors can occur in this replication and separation process.

9. Two mechanisms have been proposed to explain why a child may be born with mosaic Down syndrome.
The most likely explanation is that an extra copy of chromosome #21 was present in the egg or the sperm at the time of conception. However, shortly after conception, an error occurred in the chromosome replication and separation process, and the extra copy of the chromosome #21 was not passed on to both cells. In this way, a second cell grouping was created with only 46 chromosomes.
If this error in the chromosome replication and separation process occurred at the 4 cell stage, 1/4 of the cells would have 46 chromosomes and 3/4 would have 47 chromosomes. If the error occurred at the 8 cell stage, 1/8 of the resulting cells would have 46 chromosomes and 7/8 would have 47.
It is also possible that a child with mosaic Down syndrome inherited a total of 46 chromosomes at the time of conception.
After conception, as the cells start to replicate and separate, one of the cells duplicate and separate, but the 1 chromosome no 21’s does not separate. Thus resulting in one cell having 3 chromosomes. This cell then duplicates further and thus resulting in some of the cells having 47 chromosome and other cells having 46.

10. Robertsonian Translocation
A Chromosone translocation is a chromosome abnormality caused by rearrangement of parts between chromosomes.
This occurs between chromosomes that are not members of the same pair. The chromosome breaks at the centromere (thin region of the chromosome) and attaches itself to another chromosome.
The most frequent forms of Robertsonian translocations are between chromosomes 14,13 and 21 and between 21 and 22 and occurs when the long arems of 2 acrocentric chromosomes fuse athe centromere and the 2 short arms are lost. In this case the long arms of chromosome 13 and 14 have funsed but no genetic material was lost. This person is completely normal despite the translocation.

11. However, in unbalanced forms, Robertsonian translocations cause chromosomal deletion or addition and result in syndromes such as trisomy 13 and trisomy 21 (Down Syndrome).
Robertsonian Translocation is the cause of 2-3% of observed saces of Down Syndrome.
Robertsonian Translocation may be hereditry.

12. Duplication of a portion of chromosome 21
(Falls under Trisomy 21)
Rarely, a region of chromosome 21 will undergo a duplication event. This will lead to extra copies of some, but not all, of the genes on chromosome 21. If the duplicated region has genes that are responsible for Down syndrome physical and mental characteristics, such individuals will show those characteristics.
This cause is very rare and no rate estimates are available.
A duplication occurs when part of a chromosome is copied (duplicated) abnormally, resulting in extra genetic material from the duplicated segment.