The abnormalities or mutation on 1-22 chromosomes causes autosomal diseases. But the abnormalities on 23rd chromosome causes sex linked diseases which are gender biased. Genes on X and Y chromosomes are called sex linked gene. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes. Generally, these sex linked disorders are X- linked recessive in nature as they involve only X chromosomes. Men are most affected by these disorders as they are hemizygous (carrying only one X chromosome). Females are virtually never affected by these disorders because they are homozygous (carrying two X chromosomes). If a carrier mother and an unaffected father are mated, they produce offspring in which there is a chance of 50% of son getting affected by the disorder and 50% of the daughters can become the carrier.

A few X-linked recessive disorders include Duchenne muscular dystrophy (Fatal neuromuscular disorder) DMD gene encodes dystrophin, a huge muscle protein with about 4,000 amino acids that plays an integral role in maintaining the structural integrity of muscle cells. Cells with mutated DMD genes cannot make normal dystrophin, which means that they are unable to function appropriately. Most of the patients rely on the use of a wheelchair by their early teens, with their muscles becoming progressively weaker and more atrophied; fragile-X syndrome, a common form of mental retardation in boys; and Hemophilia A & B.- Impairs the body's ability to control coagulation or blood clotting.Lowers blood plasma clotting factor levels of coagulation factor needed for a normal clotting process.This can be fatal in the areas such as brain or inside joints.; Color blindness. - Inability to perceive difference between some of the colors.

X-linked dominant disorder is a very rare type of inherited diseases.Here both the X chromosomes are affected so females are also affected,they are not just the mere carriers.male and female are equally affected. The pattern of inheritance varies depending on whether the father or mother has the trait of interest. There is 50% chance for both the son and daughter to get affected if there mother is affected and father in unaffected. A few example are - vitamin D resistant rickets, Huntington's chorea, has been described as "the most demonic of diseases". Its symptoms include depression, dementia, exhaustion, extreme weight loss, and constant jerking movements of the limbs - the hallmark of the disease. Because it is a dominant disorder, a child of a Huntington's victim has a 50% chance of also succumbing to it.

In the case of Y- linked inheritance or holandric inheritance, phenotypic expression are determined by an allele on Y-chromosome. These are passed from father to son as Y chromosomes are present only in males. If the father is suffering from the disease, there is 100% chance of son inheriting the disorder. The example illustrated is hairy ears.

With the development and advancement in the field of Genomics, we can now sequence the genome of any individual. The screening methods can determine the information stored in the genome of an individual. Any individual with such family background (frequent occurrence of any genetic disorder from generations) should definitely undergo their genomic study. Any abnormalities or mutation found in the gene can be rectified. If the individual has the risk of genetic disorder can start there treatment beforehand. Even prenatal diagnostic testing can now determine whether a fetus carries a debilitating or fatal sex-linked mutation. Modern reproductive technologies have the opposite effect, they often result in an increased frequency of sex-linked, disease-causing mutations in a population. If a woman decides to terminate her pregnancy and then in the future tries to give birth to an unaffected child, a one-in-three chance exists that the next child will be a female carrier. This actually increases the number of X- linked recessive mutation in the is quiet debatable as even though the frequency of the lethal mutations would increase, the number of babies born with disorder would decrease.

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Geetanjali Murari
B.Tech (Biotechnology)
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