Chromosomal Translocations

The most common type of chromosome rearrangement is a translocation, which is the movement of a segment of chromosome from its normal position to a new site. Reciprocal translocations involve breaks along the arms of two chromosomes and

Quadrivalent Chromosome

Figure 2.4 (a) Reciprocal translocation—reciprocal exchange of material between two nonhomologous chromosomes, (b) Cross-shaped arrangement (quadrivalent) adopted by reciprocal translocations during early meiosis allows pairing of homologous chromosomes. A, B: normal homologues; derA, derB: rearranged or derivative chromosomes

Figure 2.4 (a) Reciprocal translocation—reciprocal exchange of material between two nonhomologous chromosomes, (b) Cross-shaped arrangement (quadrivalent) adopted by reciprocal translocations during early meiosis allows pairing of homologous chromosomes. A, B: normal homologues; derA, derB: rearranged or derivative chromosomes

Figure 2.5 (a) Robertsonian translocation—fusion of two acrocentric chromosomes with varying loss of centromeric and short-arm material, (b) Pairing arrangement (trivalent) adopted by Robertsonian translocations during early meiosis allowing pairing of most homologous regions. A, B: normal homologues; der(A;B): rearranged or derivative chomosome

Figure 2.5 (a) Robertsonian translocation—fusion of two acrocentric chromosomes with varying loss of centromeric and short-arm material, (b) Pairing arrangement (trivalent) adopted by Robertsonian translocations during early meiosis allowing pairing of most homologous regions. A, B: normal homologues; der(A;B): rearranged or derivative chomosome exchange of material with reunion creating two abnormal derivatives (Figure 2.4). Robertsonian translocations involve breakage and reunion around the centromere of the 'acrocentric' chromosomes; the minute short arms of these chromosomes are normally lost in the process so that the chromosome number per cell is reduced to 45, again with no phenotypic effect (Figure 2.5). Robertsonian translocations are unusual in that identical types occur repeatedly in humans whereas reciprocal translocations are normally unique to the family. These factors make PGD much simpler for Robertsonian than for reciprocal translocations.

In the case of reciprocal translocations a group of four chromosomes is formed at meiosis in the parental carrier and this group can separate into two groups of two in three ways, only one of which will be balanced genetically (Figure 2.4b). It is also

• miscarriage

£ chromosome 13 Q chromosome 21

■ Robertsonian translocation I der(13;21)(ql0;q!0)

Translocation Down Syndrome

Down's syndrome

Down's syndrome Normal

Figure 2.6 Pedigree of a family with Robertsonian translocation between chromosomes 13 and 21. This balanced chromosome rearrangement was only detected in the healthy mother after the birth of two children with Down's syndrome

• miscarriage

£ chromosome 13 Q chromosome 21

■ Robertsonian translocation I der(13;21)(ql0;q!0)

Down's syndrome

Down's syndrome Normal

Figure 2.6 Pedigree of a family with Robertsonian translocation between chromosomes 13 and 21. This balanced chromosome rearrangement was only detected in the healthy mother after the birth of two children with Down's syndrome possible for three of the four chromosomes to go to one daughter cell and only one to the other (3:1 segregation). It is clear that there is a high risk of producing chromosomally unbalanced gametes but the exact risk depends on several factors including the chromosomes involved, the position of the breakpoints and the sex of the parent with the translocation.

In Robertsonian translocation carriers a group of three chromosomes is formed when pairing occurs in meiosis but in the same way as for reciprocals this group can segregate in three ways, only one of which is balanced (Figure 2.5b). An example of a typical family with an inherited Robertsonian translocation is shown in Figure 2.6.

Understanding And Treating Autism

Understanding And Treating Autism

Whenever a doctor informs the parents that their child is suffering with Autism, the first & foremost question that is thrown over him is - How did it happen? How did my child get this disease? Well, there is no definite answer to what are the exact causes of Autism.

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