The procedure is the following:
First, homologous chromosomes, one inherited from the mother and one from the father, line up as pairs next to each other (The mother's chromosome 1 pairs up with the father's chromosome 1, mother’s chromosome 2 lines up with fathers’s chromosome 2 etc.). Here breakages occur; the chromosome segments cross over and grow back together. Due to this process, the maternal chromosomes get individual pieces of the paternal chromosomes and vice versa. Again, the former maternal and paternal genetic material is mixed in this process. With 46 chromosomes this results in 2116 different combinations.
This is how the genetic material is reshuffled, and this does not always happen without errors. Through uneven crossover growth, chromosomes can lose parts or gain parts. A grave disease is often the result.
After the chromosomes have lined up in pairs, the homologous copies of the chromosome pairs separate. Again, the former maternal and paternal genetic material is mixed in this process. With 46 chromosomes this results in 2116 different combinations.
A fetus in whose genetic material entire chromosomes are missing or are redundant usually does not develop properly and is not carried to term. A well-known eample is the Down-Syndrom. This illness is due to an extra chromosome 21 and is consequently also called trisomy 21.
After the chromosomes have separated from each other, the cell divides into two daughter cells. The daughter cells then only have one set of chromosomes, i.e. 23 chromosomes.
