Chromosomes and genes can alter as a result of structural changes. These changes are called mutations. The original state can usually not be re-established. The loss of hereditaryinformation is irreversible. A change caused by a mutation is kept throughout all following generations, if it does not cause lethality. A number of different mutations were identified with the help of polytene chromosomes. They occur also in normal chromosomes.
4 processes that cause change in the structure of chromosomes: DELETION DUPLICATION INVERSION TRANSLOCATION
DELETION - is the loss of an inner chromosomal fragment. It can be recognized by a loop of the intact partner chromosome at the site where the missing fragment would have been in thedefect chromosome. The loop allows conclusions about the size and the site of the deletion. 2 kinds of Deletion: Terminal Deletion Intercalary Deletion
Terminal Deletion – it is a deletion that occurs towards the tip or end of a chromosome.The break occurs at the TIP The break occurs at the END
Intercalary Deletion – it is a deletion that occurs from the interior of a chromosome.
DUPLICATION - is the doubling of one or several chromosomalfragments. It shows also as a loop but this time, the loop is in the defect partner chromosome. To find out, whether the loop has been caused by a deletion or a duplication, the band pattern ofthe chromosome has to be analyzed. The mutation is caused by a duplication, if the band pattern of the loop occurs twice.Usually, the pattern can be found again in a domain neighbouring the loop. The loop is caused by a deletion, if a fragment is missing. 4 kinds of Duplication: Tandem Duplication Displaced Homobrachial Duplication Reverse Duplication Displaced Heterobrachial Duplication
Tandem Duplication – the segment which has been added has the same gene order as in the original segment and also located adjacent to it.
Reverse Tandem Duplication – the segment which hasbeen added is placed adjacent to the original segment but with genes in a reverse order.
Displaced Homobrachial Duplication – in this type ofduplication, the additional segment will be adjacent to the original segment but in displaced position in the same arm of the chromosome.
Displaced Heterobrachial Duplication – if theduplicated segment is present on different arm of thesame chromosome, such duplications are regarded as displaced heterobrachial duplication.
INVERSION - is the change of direction of a chromosomal segment. It results from a segment that has broken out of thechromosome and fused again at the same site but with inverted direction. A pairing of homologous chromosomes is possible, though rather complex loops may be formed. 2 kinds of Inversion: Paracentric Inversion Pericentric Inversion
Paracentric – the inverted chromosome segment does not include the centromere.
Pericentric – when the centromere is included in the inverted segment, such inversions are known pericentric inversions.
TRANSLOCATION - is the transfer of a chromosomal segment onto a non-homologous chromosome. 2 kinds of Translocation: Reciprocal Translocation Non-Reciprocal Translocation
Reciprocal Translocation – occur when chromosomal segments are exchanged between two non-homologous chromosomes and is the most typical type of translocation. It is the give and take of the chromosome which results to a balanced translocation.
Non-reciprocal Translocation - are a one-way transfer of a chromosomal segment to another chromosome.The translocation is not balanced. Another name for non-reciprocal is insertion.