Down Syndrome
Down Syndrome happens when someone has an extra 21st chromosome. Down syndrome is a numeric error, happening within the chromosomes. It is mostly caused by trisomy 21, where there are three copies of the chromosome rather than the two copies. This occurs by abnormal cell division during the development of the sperm or egg cell. Other ways Down Syndrome can happen is translocation. Unlike trisomy 21, there are only two 21 chromosomes, but some parts of chromosome 21 gets translocated (attached) to another chromosome.
Turner Syndrome
Turner Syndrome happens when a female is missing the X chromosome, or if they are partially missing the X chromosome. Turner Syndrome is a numeric error within the chromosomes. It is caused by monosomy, which is when the chromosome lacks the homologous pair. It can also occur by mosaicism, where some cells have 2 complete X chromosomes, whereas some have one copy of the X chromosome, or if there is a complete copy and an altered copy. This all happens during cell division.