Sex linkage
In X-linked recessive inheritance, a son born to a carrier mother and an unaffected father has a 50% chance of being affected, while a daughter has a 50% chance of being a carrier, however a fraction of carriers may display a milder (or even full) form of the condition due to a phenomenon known as skewed X-inactivation, in which the normal process of inactivating half of the female body's X chromosomes preferably targets a certain parent's X chromosome (the father's in this case).
In X-linked dominant inheritance, a son or daughter born to an affected mother and an unaffected father both have a 50% chance of being affected (though a few X-linked dominant conditions are embryonic lethal for the son, making them appear to only occur in females).
In classical genetics, a mating experiment called a reciprocal cross is performed to test if an animal's trait is sex-linked.
Their children would inherit the disorder as follows: In such a case, where both parents carry and thus are affected by an X-linked dominant disorder, the chance of a daughter receiving two copies of the X chromosome with the defective gene is 50%, since daughters receive one copy of the X chromosome from both parents.
A few scholars have suggested discontinuing the use of the terms dominant and recessive when referring to X-linked inheritance, stating that the highly variable penetrance of X-linked traits in females as a result of mechanisms such as skewed X-inactivation or somatic mosaicism is difficult to reconcile with standard definitions of dominance and recessiveness.
In contrast, females have two X chromosomes and must inherit two mutated recessive X alleles, one from each parent, to be affected.
[17] Examples: female sterility in Drosophila; and many polymorphic characters in insects, especially in relation to mimicry.
A disease or trait determined by a gene on the X chromosome demonstrates X-linked inheritance, which can be divided into dominant and recessive patterns.
The first X-linked genetic disorder described on paper was by John Dalton in 1794, then later in 1910, following Thomas Hunt Morgan's experiment, more about the sex-linked inheritance was understood.
In 1961, Mary Lyon proposed the hypothesis of random X-chromosome inactivation providing the fundamental for understanding the mechanism of X-linked inheritance.
Common X-linked genetic diseases include Red-green colour blindness, which affects an individual's ability to see red or green images; X-linked agammaglobulinemia, resulting in a deficiency of immunity; Duchenne Muscular Dystrophy, causing muscle weakness and immobility; Hemophilia A, leading to blood clotting deficiency.
X-linked recessive diseases are more frequently encountered than dominant ones and predominantly affect males, with Red-green colour blindness having the highest prevalence among all.
[citation needed] Examples include: Red-green colour blindness is a type of colour vision deficiency (CVD) caused by a mutation in X-linked genes, affecting cone cells responsible for absorbing red or green light.
In milder cases, those affected exhibit reduced sensitivity to red or green light, as a result of hybridisation of the genes,[22] shifting the response of one cone towards that of the other.
[21] In the more extreme conditions, there is a deletion or replacement of the respective coding genes,[23] resulting in the absence of L or M cones photopigments and thus losing the ability to differentiate between red or green light completely.
X-linked agammaglobulinemia (XLA) is a primary immunodeficiency disorder that impairs the body’s ability to produce antibodies, which are proteins protecting us from disease-causing antigens, resulting in severe bacterial infections.
Duchenne Muscular Dystrophy (DMD) is a severe neuromuscular disease causing progressive weakness and damage of muscle tissues,[27] leading to mobility loss and difficulties in daily activities.
[30] With the loss of the dystrophin complex, the muscle cells would no longer be protected and therefore result in progressive damage or degeneration.
[32] Prenatal screening is offered to females during pregnancy, it involves both maternal blood tests and ultrasound to check for possible defect genes in developing fetus.
[33] The screening result only confirms a possibility of genetic disease, so parents would be prepared psychologically, or could consider the option of pregnancy termination.
A few drops of blood would be collected with a cotton paper from the heel of a newborn that is less than a week old,[34] samples would then be analysed for a variety of disorders.
