1. SECTIONS:
2. preface & acknowledgements
3. genetics at a glance
4. contacts, support & testing
5. testing & pregnancy
6. newborn screening
7. cancer in the family
8. cardiovascular conditions
9. chromosomal conditions
10. clotting & bleeding conditions
11. cystic fibrosis
12. diabetes
13. fragile X syndrome and other causes of developmental delay
14. haemoglobinopathies
15. hereditary haemochromatosis
16. neurofibromatosis
17. neurological conditions
18. psychiatric conditions
19. genetics in practice
20. emerging genetic technologies
21. glossary

X-linked recessive inheritance

eg haemophilia, muscular dystrophies, colour blindness

 

Figure 5

Haemophilia: X-linked recessive inheritance

 

1. Since a male inherits only one X chromosome (from his mother), in a family affected by a condition that follows a pattern of X-linked recessive inheritance there will be more affected males than affected females. Males are usually more severely affected than females (see X inactivation below).
   
2. In the family history there may be a pattern of transmission of the mutation from an affected grandfather to an affected grandson through an intermediate carrier female, who is usually unaffected by the condition
   (see X inactivation below).
   
3. It is important to correct the common misunderstanding that conditions can ’skip’ generations. In the case of X-linked inheritance, affected family members may appear to be scattered across generations.
   
4. Since a male only passes his Y chromosome to his son, there will be no male-to-male transmission present, as shown in the pedigree in Figure 5 above.
   
5. Females who are carriers for the mutation involved have a 1 in 2 chance, with each pregnancy, of passing on the mutation. Sons who inherit the mutation will be affected and daughters who inherit the mutation will be carriers like their mothers (indicated by filled circles inside symbols), as shown in the pedigree in
   Figure 5 above.
   
6. Daughters of affected males can only inherit the mutation from their father and are said to be ‘obligate carriers’.
   
7. If you notice a family history of an X-linked condition, and would like more information on the condition and/or recurrence risks, contact Genetics Services (see Contacts, support and testing).

 

X inactivation

1. Inactivation of most genes on the X chromosome in female somatic cells ensures that both males and females have the same number of X chromosome genes instructing the body to perform particular functions.
   
2. This is usually a random process, and thus a female’s body will contain a mixture of cells with respect to the inactivated X chromosomes being of maternal or paternal origin.
   
3. The usual random process of X inactivation means that female carriers for the mutation will not usually show any signs of the condition as there are enough cells with the correct copy of the gene to instruct the body to perform particular functions.
   
4. Rarely, some female carriers may be mildly symptomatic due to unequal or skewed inactivation of the
   X chromosomes.

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