Genetic analysis for hemophilia B should be carried out in specialist laboratories and in the context of comprehensive genetic counseling.[14'15] The reasons for referral for genetic testing are:
Nucleotide sequencing can identify a mutation in over 99% of patients tested. In addition to direct nucleotide sequencing, mutation-screening methods have also been used in factor IX analysis. Partial or complete deletions may be further investigated by Southern blotting.
Although hemophilia B can be diagnosed by measurement of factor IX coagulant activity (FIX:C), genetic analysis confirms hereditary hemophilia and may provide information on the risk of inhibitor development. Knowledge of the genetic cause of hemophilia also facilitates accurate carrier analysis for female relatives.
Figure 3 illustrates the oligonucleotide primers and conditions used in this laboratory for the amplification of the eight exons of the FIX gene, including splice junctions. 2. Carrier testing
Carrier testing should only be carried out after relevant genetic counseling. Family history may be sufficient to identify some women as obligate carriers (e.g., the daughters of affected males or women with an affected son in addition to maternal relative with hemophilia B). However, for other female relatives, or in cases of sporadic hemophilia with no previous family history (approximately 30% of cases), carrier analysis is required.
All potential carriers should have their factor IX clotting activity (FIX:C) determined as in a small percentage; this may be low enough to cause a mild bleeding disorder. However, many carriers have normal FIX:C levels. Therefore, genetic analysis is necessary to determine female carrier status and to provide appropriate genetic counseling.
Carrier analysis is generally carried out by nu-cleotide sequencing after identification of the causative mutation in an affected relative, although this approach is not suitable for the detection of large deletions.
Where there is a family history of the disorder, linkage analysis using intragenic polymorphisms may be used to
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