X-Linked Recessive Inheritance

Biological inheritance is the process by which a cell or organism’s progeny inherits its parents’ genetic characteristics. Genetics is the field of biology that studies heredity and focuses on genes and how they are passed down through generations. Heredity is another name for an inheritance, it is the link between organisms and evolution. The shared inheritance is usually passed down to the descendants through sexual reproduction. When different organisms of the same species mate, 50% of their genes are transferred to the new person. This leads to genetic variation and mixture.

The process of genes being passed from parents to children is referred to as inheritance. Genetic qualities are passed down through inheritance from parents to their offspring, who receive all of their material from their parents. Traits that are inherited from the parents are coded in the DNA and can be transferred to the next generations. Examples: the color of eyes, height, hair color, and skin color are traits inherited. The variations that occur from reproduction may be passed down through families, increasing the chances that entities will survive.

Sex-Linked Inheritance 

Sex is determined by the sex chromosomes, allosomes, heterosomes, and idiosomes in most animals and some plants. Characters (phenotypes) are inherited through sex-linked inheritance and influenced by genes present on a particular sex chromosome. There are hundreds of genes on the human X chromosome that are absent on the Y chromosome. Thus, the traits determined by these genes exhibit sex-related inheritance. Genes found on the X chromosome are referred to as X-linked genes, whereas genes found on the Y chromosome are referred to as holandric genes. Sex-linked inheritance refers to the transmission of these X and holandric genes.

Characteristics of Sex-linked Inheritance

• Male-to-male transmission of traits is not possible, since sons will inherit only the Y chromosome rather than the X chromosome. 
• The heterozygous carrier female has a 50 percent chance of passing the mutant gene to each of her children, sons who inherit the mutant gene will be hemizygotes, while daughters who receive the mutant gene will be unaffected carriers. 
• Males with the trait will pass the gene on to all of their daughters, who will be carriers. 
• Most of the sex-linked traits are inherited recessively so heterozygous females generally do not display the trait. 

Sex-Linked Recessive Inheritance 

One of the X or Y chromosomes can be passed down through generations to cause sex-linked disorders. These X and Y chromosomes are sex chromosomes. Recessive inheritance requires two defective genes to develop a disorder. The disease does not appear or is mild if only one of the two genes in the pair is mutated. A person is referred to as a carrier when they have one abnormal gene but don’t show any symptoms. The defective genes can be passed on to the offspring through these carriers.

X-linked recessive is frequently referred to as “Sex-Linked Recessive.” Males are more frequently affected by X-linked recessive disorders. Men possess only one X chromosome. The disease is brought on by a single recessive gene on that X chromosome.

The other half of the male XY gene pair is the Y chromosome. The majority of the X chromosome’s genes are absent on the Y chromosome. Because of this, men are more likely than women to acquire the condition. A recessive gene on the X chromosome causes disorders like hemophilia and Duchenne muscular dystrophy.

In this type of Inheritance, males are affected almost exclusively. Transmission of the disorders occurs through carrier females to their sons. Male-to-male transmission is not possible. The males that are affected are at risk of transmitting the disorder to their grandsons through their carrier daughters. However, some situations may impact female genetic carriers, and this is assumed to be due to the process of X inactivation switches off the X with the functioning gene copy in more body cells. As a result, this person is not sufficiently protected from acquiring the disorder by the working copy of the X chromosome. Males with the recessive gene variant on their X chromosome do not have a working copy of the gene on a second chromosome. Therefore, they will not be able to send the right message to the cells to make the gene product and will have the disorder. 

Female carriers often have some cells with an unaffected active X chromosome, they rarely exhibit the X-linked recessive characteristic. However, female carriers can convey the characteristic to half of their daughters and their sons, who will be affected. Affected males can transfer the character to their daughters but not to their sons. When there are only female carriers, the trait seems to skip a generation. If an affected father marries a carrier, the female may be impacted (homozygous).

Sex Linked Recessive Diseases 

Males are more frequently affected by sex (x) linked recessive disorders. Men only possess one X chromosome. The disease is brought on by a single recessive gene present on the X chromosome.

The disorder is caused by the variants in genes on the X chromosome. In males only one X chromosome is present, one altered copy of the gene in each cell is sufficient to cause the disorder. Females have two X chromosomes, a variant would have to occur in both copies of the gene to cause the disorder. Since it is unusual that females will have two copies of the gene that are mutated, In general, males are more vulnerable to X-linked recessive traits than females.

For the following scenarios, the X-linked recessive disorders are passed on to the next generation. In the following case, the mother is a carrier of an X-linked recessive gene and the father has only a working copy of the gene. There are four possibilities in every pregnancy that the child can receive from the parents. 

• 1 chance in 4 (25% chance) that a son will inherit the Y chromosome and X-linked recessive nonworking gene copy with the variant from the mother. 
• 1 chance in 4 (25% chance) that a son will inherit the Y chromosome from their father and the working copy of the X-linked gene from the mother. 
• 1 chance in 4 (25% chance) that a daughter will inherit both working copies of the X-linked genes. One copy from the father and one copy from the mother. 
• 1 chance in 4 (25% chance) that a daughter will inherit from their father the working copy of the X-linked gene and the X-linked recessive nonworking gene copy with a variant from the mother. 

If the father has an X-linked recessive non-working copy of the gene and the mother has the working copy of the gene. The possibilities are as follows. 

• 1 chance in 2 (50 % chance) that a son will inherit the Y chromosome from their father and the working copy of the X-linked gene from the mother. 
• 1 chance in 2 (50 % chance) that a daughter will inherit the working copy of the X-linked gene from their father and the X-linked recessive nonworking gene copy with a variant from the mother. This person will be a genetic carrier of the condition. 

The mother is the carrier of an X-linked recessive gene and the father is affected by the condition, he has an X-linked recessive nonworking gene copy with a variant. The possibilities are as follows. 

• 1 chance in 4 (25% chance) that a son will inherit the Y chromosome and X-linked recessive nonworking gene copy with the variant from the mother. 
• 1 chance in 4 (25% chance) that a son will inherit the Y chromosome from their father and the working copy of the X-linked gene from the mother. 
• 1 chance in 4 (25% chance) that a daughter will inherit a working copy of the X-linked gene from the mother and an X-linked recessive nonworking gene copy with the variant from the father. Here the daughter is a carrier. 
• 1 chance in 4 (25% chance) that a daughter will inherit both X-linked recessive nonworking copies of the gene with the variant. One copy from the father and one copy from the mother. 

The mother and father both are affected by the condition, a X-linked recessive nonworking gene copy is present in both. The possibilities are as follows.

• There is a 100% chance of the child inheriting the disease, whether it is a boy or a girl. 

 X-linked recessive Inheritance Examples

Color Blindness 

In color blindness, a person cannot differentiate between red and/or green colors. The cone cells present in the retina of the eye are responsible for colorful vision. There are three types of cone cells i.e. red, blue, and green. The mutation in these cells can lead to different conditions. There are three forms of color blindness found in humans

• When the red cone cells show mutation or are absent on the chromosome, then this condition is called protanopia. It is a common form of color blindness. 
• When the green cone cells show mutation, then that condition is called deuteranopia. 
• When there is an absence of functionality of red, green, and blue cone photoreceptors cells, such mutation is called as monochromatic vision. This condition is a rare one. 

Color blindness shows criss-cross inheritance like any other X-linked recessive inheritance. In this, the male transmits the character to his grandson via his daughter, while the female transmits her character to his granddaughter through her son. So the transmission occurs from one generation to its successive third generation. The traits are passed on from one sex of the offspring to the opposite sex.

Hemophilia

It is a genetically inherited disease in which the person suffering from this disease loses the ability to make blood clots (A process needed to stop bleeding). Even a minor injury results in people bleeding for a longer time. There are three types of hemophilia: 

• Hemophilia A: It is caused due to low amounts of clotting factor VIII, the mutation is shown in Factor VIII genes. They are able to form AMG (Antihemophilic Globulin). 
• Hemophilia B: It is caused due to low amounts of clotting factor IX, the mutation is shown in Factor IX genes. They are not able to form Thromboplastin. 
• Hemophilia C: This occurs due to low levels of clotting factor XI. It is a very rare condition.

Hemophilia A and Hemophilia B are both X-linked recessive disorders, females are rarely severely affected by them. Some females with a non-functional gene on one of the X chromosomes may be mildly symptomatic. Hemophilia C occurs equally in both males and females. Hemophilia is more commonly expressed in human males than human females. Females must have two deficient X-chromosomes in order to have hemophilia, they are more likely to be silent carriers. It is possible for female carriers to become mild hemophiliacs due to the inactivation of the X-chromosome. 

Lesch-Nyhan Syndrome 

This syndrome is a rare, inherited disorder caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). Lesch-Nyhan Syndrome is an X-linked recessive disease, the gene is carried by the mother and passed on to her son. It is present from birth in baby boys. The lack of HPRT causes a build-up of uric acid in all body fluids and leads to symptoms such as poor muscle control and retardation. 

Fragile-X Syndrome (FXS) 

FXS is a genetic disorder characterized by mild to moderate intellectual disability. It is caused by changes in a gene called Fragile X Messenger Ribonucleoprotein 1 (FMR1). The risk of developing FXS can be passed down from parents to children through genes. Usually, males are more severely affected by this syndrome as compared to females. 

Duchenne Muscular Dystrophy (DMD) 

DMD is an inherited disorder characterized by progressive muscle deterioration and weakening, It occurs due to changes in a protein called dystrophin. Although girls can rarely be affected, boys are the main victims of this disorder.

DMD has an X-linked recessive inheritance pattern and is passed on by the mother, who is referred to as a carrier. Generally, females are the carriers who have a normal dystrophin gene on one X-chromosome and an abnormal dystrophin gene on the other X chromosome. Most of the carriers of DMD themselves do not show any signs and symptoms of the disease, but a few do. 

Characteristics of X-Linked Recessive Pedigree Pattern

• Fathers transmit the Y chromosome to their son which has no mutated gene on it. Due to this, there is no male-to-male transmission of the trait. This means the males affected by X-linked recessive disorder inherited the X chromosome from their mother. 
• As there is only one X chromosome present in males, and therefore require only one mutated X chromosome in order to be affected. Males are more commonly affected by these X-linked recessive traits as compared to females.
• The X-linked recessive traits follow a criss-cross inheritance pattern to inherit a particular trait. It tends to skip a generation, which means an affected grandfather will not have an affected son but could have an affected grandson through his daughter who will play the role of the carrier.
• If a mother is affected by a particular disorder then all of her sons should also possess the disorder. Whereas the son of the female carrier has a 50 percent chance of having the disorder.  

FAQs on X-Linked Recessive Inheritance

Question 1: What is Sex Linked Recessive Inheritance?

Answer: 

X-linked recessive inheritance is a mode of inheritance, where a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males and in females who are homozygous. 

Question 2: State some examples of sex-linked recessive diseases.

Answer:

Duchenne muscular dystrophy, Colorblindness, hemophilia A, hemophilia B, Fabry disease, and Becker muscular dystrophy.

Question 3: How are traits inherited?

Answer:

Traits are passed using the sperm and egg. Genetic material consists of DNA, Genes, and chromosomes. Chunks of DNA form chromosomes. DNA holds the genetic code. One copy of the gene is received from each parent, creating a pair.

Question 4: Explain the causes of Sex-linked recessive disorders. 

Answer:

X-linked recessive disorders are caused by variants in genes on the X chromosome. In males, only one X chromosome is present. Due to this only one altered copy of the gene in each cell is sufficient to cause the disorder. In females, two X chromosomes are present. Hence, both chromosomes must carry the altered gene to inherit the disorder. 

Question 5: What is sex-linked Inheritance?

Answer: 

Sex-linked inheritance is an inheritance in which genes or alleles are inherited through sex chromosomes (i.e. X-chromosome or Y-chromosome).