Thursday, April 30, 2015

X-Chromosome Recessive Disorder




          Human gene consists of 23 pairs of chromosomes; of which 22 pairs are called autosome chromosome. Usually 1 copy is inherited from the mother, while other copy is inherited from the father. 23rd pair is sex chromosome as they determine the sex of the child. Females have 2 copies of X chromosome, while males have 1 copy of X chromosome and 1 copy of Y chromosome. In males, X gene is inherited from their mother, while Y is inherited from their father.

          Genes are small packets containing messages; like colour of eyes or hair, their body structure, etc. of the parents which are copied into their children. That is why we always say child has mother’s eyes or father’s complexion. So along with positive messages if a parent is a carrier of any genetic related disorder, it gets copied into the child’s gene.

          X-linked inheritance means the gene carrying the disorder is present on X chromosome.

          X-linked disorders are common in males as the X chromosome which is inherited from mother are usually the carrier of the disorder, whereas in females it requires 2 copies of genetic disorders as females consist of 2 X chromosomes. So if a female inherits the X chromosome with the genetic disorder from her mother, she doesn’t show the features of the disorder but is labeled as carrier gene as she passes the gene to her offspring’s.

          Some of the common X-linked recessive disorders are:

  • Red-green color blindness
  • Hemophilia A
  • Hemophilia B
  • Duchenne Muscular dystrophy
  • Becker’s Muscular Dystrophy.



 Some points regarding X-linked disorder :
  • The vast majority of affected individuals are male.
  • Affected males never pass the disease to their sons because there is no male-to-male transmission of the X chromosome.
  • Affected males pass the defective X chromosome to all of their daughters, who are described as obligate carriers. This means they carry the disease-causing allele but generally show no disease symptoms since a functional copy of the gene is present on the other chromosome.

  • Female carriers pass the defective X chromosome to half their sons (who are affected by the disease) and half their daughters (who are therefore also carriers). The other children inherit the normal copy of the chromosome.


  • The overall pattern of the disease is therefore characterized by the transmission of the disease from affected males to male grandchildren through carrier daughters, a pattern sometimes described as a 'knight's move'.
  • Affected females, with two deficient X chromosomes, are the rare products of a marriage between an affected male and a carrier female. However, manifesting carrier females (with one deficient X chromosome and one normal one) may arise if there is a chromosome disorder or a problem with X-chromosome inactivation.



          It’s always better now-a-days to get all necessary tests done before conceiving, to rule out genetic problems. Since most of the genetic disorders don’t have any known cure, it’s better to know beforehand. 


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