Down syndrome has a karyotype of XX + 21 or XY + 21 in the most common situation. XX + 21 means that the baby is a girl with an extra chromosome number 21. XY + 21 means that the baby is a boy with an extra chromosome 21. The extra chromosome number 21 is called trisomy i.e. there are three haploid chromosomes number 21.
95% of cases of down’s are due to an extra chromosome. This arises due to meiotic non disjunction in either meiosis one or two (the above picture illustrates both in B and C respectively). In meiosis I these are the homologous chromosomes i.e. the two maternal or paternal chromosomes that form the chromosome pair with 4 chromatids. In meiosis II these are the sister chromatids i.e. the 2 chromatids of each maternal or paternal chromosome that are synthesized during the Synthesis phase (S phase of DNA replication).
This whole process can lead to one oocyte or sperm with an extra chromatid (remember oocytes and sperms are haploid) and one with no chromatids for that particular chromosome number. This is quite tricky to get your head around but try to read the text in accordance with the image above.
Another more rare cause that accounts for only 4% of these cases is robertsonian translocation. Robertsonian translocation most commonly involves chromosomes 14 and 21. It occurs commonly when two acrocentric chromosomes i.e. chromosomes which have their centromeres near the ends fuse and the two short arms get lost. Normally balanced translocations do not cause any harm but unbalanced translocations where the two short arms are lost can lead to still birth, miscarriages and down’s.
Finally only 1% of cases are due to post fertilization errors in mitosis.
Learn more about other reproductive issues here.
Also if you are more interested in learning about other genetic diseases that can affect your family and their symptoms you can read genetics for dummies. I have linked it below.