X-linked genes are those that are located on the X chromosome. Therefore, the X chromosome is one of two sex chromosomes found in humans. Males have an X and a Y chromosome, while females have two X chromosomes. When males are mutated, the majority of genes on the X chromosome exhibit a phenotype. That is because, while females have two X chromosomes, the chromosome that is not mutated can frequently compensate for the mutated X chromosome. Individuals may be familiar with the X-linked diseases such as haemophilia A, and Duchenne muscular dystrophy. However, certain disorders are more prevalent in females, such as Rett syndrome, which is also caused by an X chromosomal mutation.

X linked inheritance

The term “X-linked inheritance” refers to the fact that the gene responsible for the characteristic or condition is situated on the X chromosome. Males have one X and one Y chromosome; females have two X chromosomes. The X chromosome’s genes can be recessive or dominant. Their expression is not identical in females and males. The Y chromosome’s genes do not always correspond to the X chromosome’s genes. Females express X-linked recessive genes only if there are two copies of the gene (one on each X chromosome). However, males require only one copy of an X-linked recessive gene to express the characteristic or illness. For instance, a woman may inadvertently inherit a recessive gene on one of her X chromosomes and pass it on to her son, who will express the feature.

Daughters have a 50% probability of carrying the gene and passing it on to the next generation. There is a 50% possibility that a daughter will lack the gene and hence be unable to pass it on. There is a 50% chance that sons will be healthy if they do not have the gene. However, a son has a 50% chance of inheriting the gene and manifesting the characteristic or condition.

Inheritance patterns

In humans, the inheritance of X-linked recessive characteristics follows a distinct three-point pattern.

• The first is that affected men are unable to pass on X-linked recessive features to their sons because fathers give their sons Y chromosomes. This suggests that males with X-linked recessive disorders inherited the X chromosome from their mothers.
• Second, males exhibit more X-linked recessive characteristics than females.

This is because males have only one X chromosome and hence require only one mutant X to be affected. Because women have two X chromosomes, they must inherit two mutant recessive X chromosomes (one from each parent). A well-known example of this pattern of inheritance is that of Queen Victoria’s descendants and the blood illness haemophilia. 

• The last pattern seen is that X-linked recessive characteristics tend to skip generations, which means that an affected grandfather will not have an affected son but may have an affected grandson via his daughter.

Further explained, all daughters of an affected male would inherit his mutant X and will then be carriers or affected, depending on the mother. The ensuing sons will have a 50% probability of being impacted (mother is a carrier), or a 100% chance of being affected (mother is affected). Males are more frequently impacted than females as a result of these percentages.

Numerous X-linked recessive disorders

Red-green colour blindness and haemophilia A are two examples of X-linked recessive disorders.

• Colour blindness to red and green. Red-green colour blindness is a term that refers to an individual’s inability to distinguish between colours of red and green (usually blue-green). Their visual acuity is normal. There are no major issues; nevertheless, affected persons may be disqualified from certain vocations that require colour recognition, such as transportation or the Armed Forces. Males are more frequently affected than females due to the gene’s location on the X chromosome.

• Hemophilia A. Hemophilia A is a blood disorder. Hemophilia A is a blood clotting condition caused by a shortage of a clotting factor called Factor VIII. This leads to excessive bleeding that will not stop, even from a minor cut. Individuals with haemophilia A are prone to bruising and may experience internal bleeding into their joints and muscles. Hemophilia A (Factor VIII deficiency) affects around 1 in 4500 live male births. Hemophilia B (Factor IX deficiency) affects around one in every 20,000 live male births. The majority of cases are caused by haemophilia A. Factor VIII infusion therapy is available (blood transfusion). Female bearers of the gene may exhibit modest symptoms of Factor VIII insufficiency, such as bruising quickly or bleeding more slowly than normal when cut. Female carriers, on the other hand, do not always exhibit these symptoms. One-third of all instances are believed to represent novel familial mutations (not inherited from the mother).

Conclusion

X-linked recessive inheritance is a type of inheritance in which a genetic feature or condition is passed down from parent to kid via mutations (changes) in an X chromosome gene. In males (who have only one X chromosome), the disorder is caused by a mutation in the gene’s copy on the one X chromosome. To be afflicted by the disorder, females (who have two X chromosomes) must have a mutation on both X chromosomes. If either one parent or both parents have the mutant X-linked gene, the daughters are normally unaffected and are referred to be carriers since one of their X chromosomes contains the mutation but the other is normal. Sons will be affected if they inherit their mother’s mutant X-linked gene. Fathers are unable to pass on X-linked recessive diseases to their sons.