Sexlinked Recessive Inheritance

It has been mentioned already that males have the "XY" configuration of sex chromosomes, whereas females have "XX". Because of the unpaired nature of much of the male sex chromosomes, some recessive genes can have an effect in males when they do not do so in the female. Certain important eye conditions are carried in this way in pathological genes on the

X chromosome and the pattern of inheritance is termed X-linked recessive. Examples of this type of inheritance are seen in ocular albinism and colour blindness. Retinitis pigmentosa can also show this pattern in some families. When inheritance is X-linked, only males are affected and there is no father-to-son transmission of the disease. Instead, it is conveyed through a carrier female to the next generation (Figure 23.3).

This description of the three important modes of inheritance should make it apparent

PARENTS

GAMETES

CHILDREN

Normal girl

Normal boy Carrier girl

Figure 23.3. X-linked inheritance.Q

Affected boy that it is possible to predict the likely disease incidence in offspring. It should also be realised that such predictions can only be based on careful and extensive investigation of the family. Although some eye diseases are known to follow a fixed pattern of inheritance, others, notably retinitis pigmentosa, can be inherited in different ways in different families. In most large centres, there are now genetic clinics in which time is devoted specifically to the investigation of families and also to the detection of carriers.

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