Which information of the parent cell ends up in the daughter cell is purely decided by chance. Besides this random process, there is a second random process. In this second random process the DNA will be scrambled in a way that pieces are omitted (deletion), added (duplication), moved from one place to another (translocation) and inverted (inversion). This is a common process, which leads to about 0.4% variation in the DNA.A problem of the second random process is that genetic mistakes can occur. Because of the deletion and duplication process, the chromosomes that come together in a new cell may be shorter or longer. The result of this spontaneous change in the structure of DNA is a so-called copy number variation. Due to the copy number variation chromosomes of different sizes can be combined in a new cell. If this occurs around conception, the result will be a first cell of a human with a genetic variation. This can be either positive or negative. In positive cases this new human will be capable of a special skill that is assessed positively, for example, in sports or science. In negative cases, you have to deal with a syndrome or a severe disability, as in this case the 1q21.1 deletion syndrome.Based on the meiotic process, the syndrome may occur in two ways. a spontaneous deviation (a de novo situation): two chromosomes come together of which one has a copy number variation as a result of the meiosis process.
Symptoms and signs
Recognised symptoms are:
Only one set of genes on the two chromosomes function (haploinsufficiency)
Thrombocytopenia-absent radius (TAR syndrome), in case of a class II-deletion
Neurological-psychiatric problems: schizophrenia; epilepsy; learning problems; cognitive disabilities — mild to moderate; developmental delay — mild to moderate (milestones like sitting, standing and walking; come at a later period in childhood); children show an ataxic gait and fall down a lot
Dysmorphism: slightly unusual facial appearance; disturbed growth; skeletal malformations; small head (microcephaly); prominent forehead; bulbous nose; deep-set eyes; broad thumbs; broad toes; squint; very flexible joints; clavicular pseudoarthrosis (the collarbone doesnt develop normally) (class II-deletion); an extra transverse crease of the fifth finger (class II-deletion)); problems with the development of the vagina (Müllerian aplasia)
Eyes: cataracts
Heart abnormalities and cardiovascular anomalies (30% of the cases): anomalous origin of the coronary artery (Class II-deletion)
Kidneys: missing kidney or floating kidneys
Cancer: neuroblastoma
Sleep disturbancesIt is not clear whether the list of symptoms is complete. Very little information is known about the syndrome. The syndrome can have completely different effects on members of the same family.A common deletion is between 1.0–1.9Mb. Mefford states that the standard for a deletion is 1.35Mb. The largest deletion seen on a living human is over 5 Mb. Cause
Meiosis is the process of dividing cells in humans. In meiosis, the chromosome pairs split and a representative of each pair goes to one daughter cell. In this way the number of chromosomes will be halved in each cell, while all the parts on the chromosome (genes) remain, after being randomized.