X-Inactivation in Chromosomes: Mosaicism in Females Lessens Instance of X-Linked Diseases
Chromosomes encapsulate DNA and are responsible for every physical aspect and a host of personality traits of every human body. Humans have 23 pairs of chromosomes. Men and women have all the same types of chromosomes, except one vital pair. In women the last pair will be XX. In men the last pair will be XY. Whether or not the last chromosome pair is XX or XY is determined by the male sperm.
Human Genome Project
The quest for the map of human chromosomes was started in 1990. Surprisingly, the project was fully completed in May 2006. The results have already poured mounds of new information about genetics into the biology world.
For instance, one thing determined was that the X chromosome has many more genes than the Y chromosome. Due to less genetic material from their fathers, males are 6% more genetically similar to their mothers. Male twins are also slightly more identical than female twins due to less genetic information, according to Natalie Angiers, in her book Woman: An Intimate Geography.
X and Y Chromosomes
According to the U.S. National Library of Medicine, it was determined that the Y chromosome contains about 70+ genes. The X chromosome carries about 2,500 genes, and is responsible for about 300 specific diseases, including colorblindness, prostate cancer, hemophilia, baldness, Duchenne muscular dystrophy and many mental retardations, as reported by Jai Dennison at Daily News Central, in his article, “X-Chromosome Studies Unlock Deep Mysteries About Gender Differences.” Women are less likely to suffer from these X-linked genetic diseases due to what is called X-inactivation.
X-inactivation and Mosaicism
When two X chromosomes come together to create a female embryo, X inactivation occurs. During X-inactivation, cells randomly chooses to “shut down” sections of its extra X chromosome to form a “mosaic” of genes.
Consequently, X-linked diseases are more common in men than women, since women have both Mom’s and Dad’s X genes to pull from. Scientists are still discerning just how “random” the X-inactivation process is. Philip Avner and Edith Heard’s article “X-Chromosome Inactivation: Counting, Choice and Initiation” discusses interesting research in this area. Similar to the reason that tortoise-shell cats can only be female, X-inactivation creates a mosaic of X-linked traits in women but not in men, who are subject their single X’s full expression.
Inheritance and the X Chromosome
Because men only have one X chromosome, men may inherit their mother’s genetic diseases related to the X chromosome but will never inherit their father’s X-linked genetic diseases, since men only receive their father’s Y chromosome. While men escape any X-linked diseases from their father, they are vulnerable to and likely to fully develop any of the X-linked diseases given to them on their mother’s X chromosome.
In contrast, women’s double X requires that both parents have a gene an X-linked disease and even then severity of inherited disease will likely vary due to the fact that during female development random sections of the X-chromosome are inactivated. A woman is less likely to develop X-linked diseases, and more likely to develop a light instance if a disease should develop.
In conclusion, nature appears to have a “back up plan” for women through X-inactivation, while unlucky men must suffer the full severity of X-linked diseases.