What type of gene is considered recessive and located only on the x chromosome?

Sex linkage refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y)

  • The Y chromosome is much shorter than the X chromosome and contains only a few genes (50 million bp; 78 genes)
  • The X chromosome is longer and contains many genes not present on the Y chromosomes (153 million bp ; ~ 2,000 genes)
  • Hence, sex-linked conditions are usually X-linked - as very few genes exist on the shorter Y chromosome


X and Y chromosomes

What type of gene is considered recessive and located only on the x chromosome?


What type of gene is considered recessive and located only on the x chromosome?

Understanding:

•  The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes

    
Sex-linked inheritance patterns differ from autosomal patterns due to the fact that the chromosomes aren’t paired in males (XY)

  • This leads to the expression of sex-linked traits being predominantly associated with a particularly gender


As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous

  • Hence, X-linked dominant traits are more common in females (as either allele may be dominant and cause disease)


Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits

  • X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele


The following trends always hold true for X-linked conditions:

  • Only females can be carriers (a heterozygote for a recessive disease condition), males cannot be heterozygous carriers
  • Males will always inherit an X-linked trait from their mother (they inherit a Y chromosome from their father)
  • Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele)


Inheritance of an X-linked Recessive Disease Condition

What type of gene is considered recessive and located only on the x chromosome?


What type of gene is considered recessive and located only on the x chromosome?

Application:

•  Red-green colour blindness and haemophilia as examples of sex-linked inheritance

    
Red-green colour blindness and haemophilia are both examples of X-linked recessive conditions

  • Consequently, they are both far more common in males than in females (males cannot mask the trait as a carrier)


When assigning alleles for a sex-linked trait, the convention is to write the allele as a superscript to the sex chromosome (X)

  • Haemophilia:  XH = unaffected (normal blood clotting) ; Xh = affected (haemophilia)
  • Colour blindness:  XA = unaffected (normal vision) ; Xa = affected (colour blindness)


Haemophilia

Haemophilia is a genetic disorder whereby the body’s ability to control blood clotting (and hence stop bleeding) is impaired

  • The formation of a blood clot is controlled by a cascade of coagulation factors whose genes are located on the X chromosome
  • When one of these factors becomes defective, fibrin formation is prevented - meaning bleeding continues for a long time
  • Different forms of haemophilia can occur, based on which specific coagulation factor is mutated (e.g. haemophilia A = factor VIII)


Red-Green Colour Blindness

Red-green colour blindness is a genetic disorder whereby an individual fails to discriminate between red and green hues

  • This condition is caused by a mutation to the red or green retinal photoreceptors, which are located on the X chromosome

    If you're seeing this message, it means we're having trouble loading external resources on our website.

    If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

    X-linked recessive inheritance

    X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males (who are necessarily homozygous for the gene mutation because they have one X and one Y chromosome) and in females who are homozygous for the gene mutation, see zygosity. Females with one copy of the mutated gene are carriers.[citation needed]

    X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Females have two X chromosomes while males have one X and one Y chromosome. Carrier females who have only one copy of the mutation do not usually express the phenotype, although differences in X-chromosome inactivation (known as skewed X-inactivation) can lead to varying degrees of clinical expression in carrier females, since some cells will express one X allele and some will express the other. The current estimate of sequenced X-linked genes is 499, and the total, including vaguely defined traits, is 983.[1]

    Patterns of inheritance[edit]

    Patterns of X-linked recessive inheritance in a royal family

    In humans, inheritance of X-linked recessive traits follows a unique pattern made up of three points.

    • The first is that affected fathers cannot pass X-linked recessive traits to their sons because fathers give Y chromosomes to their sons. This means that males affected by an X-linked recessive disorder inherited the responsible X chromosome from their mothers.
    • Second, X-linked recessive traits are more commonly expressed in males than females.[2] This is due to the fact that males possess only a single X chromosome, and therefore require only one mutated X in order to be affected. Women possess two X chromosomes, and thus must receive two of the mutated recessive X chromosomes (one from each parent). A popular example showing this pattern of inheritance is that of the descendants of Queen Victoria and the blood disease hemophilia.[3]
    • The last pattern seen is that X-linked recessive traits tend to skip generations, meaning that an affected grandfather will not have an affected son, but could have an affected grandson through his daughter.[4] Explained further, all daughters of an affected man will obtain his mutated X, and will then be either carriers or affected themselves depending on the mother. The resulting sons will either have a 50% chance of being affected (mother is carrier), or 100% chance (mother is affected). It is because of these percentages that we see males more commonly affected than females.

    Pushback on recessive/dominant terminology[edit]

    A few scholars have suggested discontinuing the use of the terms dominant and recessive when referring to X-linked inheritance.[5] The possession of two X chromosomes in females leads to dosage issues which are alleviated by X-inactivation.[6] 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, scholars have suggested referring to traits on the X chromosome simply as X-linked.[5]

    Examples[edit]

    Most common[edit]

    The most common X-linked recessive disorders are:[7]

    • Red–green color blindness, a very common trait in humans and frequently used to explain X-linked disorders.[8] Between seven and ten percent of men and 0.49% to 1% of women are affected. Its commonness may be explained by its relatively benign nature. It is also known as daltonism.
    • Hemophilia A, a blood clotting disorder caused by a mutation of the Factor VIII gene and leading to a deficiency of Factor VIII. It was once thought to be the "royal disease" found in the descendants of Queen Victoria. This is now known to have been Hemophilia B (see below).[9][10]
    • Hemophilia B, also known as Christmas disease,[11] a blood clotting disorder caused by a mutation of the Factor IX gene and leading to a deficiency of Factor IX. It is rarer than hemophilia A. As noted above, it was common among the descendants of Queen Victoria.
    • Duchenne muscular dystrophy, which is associated with mutations in the dystrophin gene. It is characterized by rapid progression of muscle degeneration, eventually leading to loss of skeletal muscle control, respiratory failure, and death.
    • Becker's muscular dystrophy, a milder form of Duchenne, which causes slowly progressive muscle weakness of the legs and pelvis.
    • X-linked ichthyosis, a form of ichthyosis caused by a hereditary deficiency of the steroid sulfatase (STS) enzyme. It is fairly rare, affecting one in 2,000 to one in 6,000 males.[12]
    • X-linked agammaglobulinemia (XLA), which affects the body's ability to fight infection. XLA patients do not generate mature B cells.[13] B cells are part of the immune system and normally manufacture antibodies (also called immunoglobulins) which defends the body from infections (the humoral response). Patients with untreated XLA are prone to develop serious and even fatal infections.[14]
    • Glucose-6-phosphate dehydrogenase deficiency, which causes nonimmune hemolytic anemia in response to a number of causes, most commonly infection or exposure to certain medications, chemicals, or foods. Commonly known as "favism", as it can be triggered by chemicals existing naturally in broad (or fava) beans.[15]

    Less common disorders[edit]

    Theoretically, a mutation in any of the genes on chromosome X may cause disease, but below are some notable ones, with short description of symptoms:

    When a recessive trait is on the X chromosome?

    In X-linked recessive inheritance, a daughter inherits a single mutated gene on the X chromosome from one of her parents. The X chromosome she inherits from the other parent will usually cancel the effect of the mutation, and she most likely will not have the genetic condition.

    What is X

    Examples of X-linked recessive conditions include red-green color blindness and hemophilia A: Red-green color blindness. Red-green color blindness simply means that a person cannot distinguish shades of red and green (usually blue-green).

    What genetic disorder is X

    Duchenne muscular dystrophy, some types of colorblindness and hemophilia A are examples of X-linked recessive disorders. A man with an X-linked recessive disorder will pass his unaffected Y chromosome to his sons, and none will be affected.

    What are X chromosome genes?

    ​X Chromosome The X chromosome is one of the two sex chromosomes that are involved in sex determination. Humans and most other mammals have two sex chromosomes (X and Y) that in combination determine the sex of an individual. Females have two X chromosomes in their cells, while males have one X and one Y.