X-linked dominant
Encyclopedia
X-linked dominant inheritance, sometimes referred to as X-linked dominance, is a mode of genetic inheritance
by which a dominant gene is carried on the X chromosome
. As an inheritance pattern, it is less common than the X-linked recessive
type. In medicine, X-linked dominant inheritance indicates that a gene responsible for a genetic disorder
is located on the X chromosome, and only one copy of the allele is sufficient to cause the disorder when inherited from a parent who has the disorder.
X-linked dominant traits do not necessarily affect males more than females (unlike X-linked recessive
traits). The exact pattern of inheritance varies, depending on whether the father or the mother has the trait of interest. All daughters of an affected father will also be affected but none of his sons will be affected (unless the mother is also affected). In addition, the mother of an affected son is also affected (but not necessarily the other way round).
), X-linked
inheritance is determined by the gender
of the parent carrying a specific gene and can often seem complex. This is due to the fact that, typically, female
s have two copies of the X-chromosome, while male
s have only one copy. The difference between dominant and recessive inheritance patterns also plays a role in determining the chances of a child inheriting an X-linked disorder from their parentage.
Males are normally hemizygous for the X chromosome, having only one copy. As a result, X-linked dominant disorders usually show higher expressivity
in males than females.
In X-linked dominant inheritance, when the mother alone is the carrier
of a mutated, or defective gene associated with a disease or disorder; she herself will have the disorder. Her children will inherit the disorder as follows:
When the father alone is the carrier of a defective gene associated with a disease or disorder, he too will have the disorder. His children will inherit the disorder as follows:
If both parents were carriers of a defective gene associated with a disease or disorder, they would both have the disorder. 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. Were this to occur with an X-linked dominant disorder, that daughter would likely experience a more severe form.
Some X-linked dominant conditions such as Aicardi Syndrome
are fatal to boys, therefore only girls with these conditions survive. Similarly, individuals with Klinefelter's Syndrome
are referred to as "47,XXY Males".
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
by which a dominant gene is carried on the X chromosome
X chromosome
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...
. As an inheritance pattern, it is less common than the X-linked recessive
X-linked recessive
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 expressed in males and in females who are homozygous for the gene mutation X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on...
type. In medicine, X-linked dominant inheritance indicates that a gene responsible for a genetic disorder
Genetic disorder
A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....
is located on the X chromosome, and only one copy of the allele is sufficient to cause the disorder when inherited from a parent who has the disorder.
X-linked dominant traits do not necessarily affect males more than females (unlike X-linked recessive
X-linked recessive
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 expressed in males and in females who are homozygous for the gene mutation X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on...
traits). The exact pattern of inheritance varies, depending on whether the father or the mother has the trait of interest. All daughters of an affected father will also be affected but none of his sons will be affected (unless the mother is also affected). In addition, the mother of an affected son is also affected (but not necessarily the other way round).
Genetics
As the X chromosome is one of the sex chromosomes (the other being the Y chromosomeY chromosome
The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
), X-linked
inheritance is determined by the gender
Gender
Gender is a range of characteristics used to distinguish between males and females, particularly in the cases of men and women and the masculine and feminine attributes assigned to them. Depending on the context, the discriminating characteristics vary from sex to social role to gender identity...
of the parent carrying a specific gene and can often seem complex. This is due to the fact that, typically, female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
s have two copies of the X-chromosome, while male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
s have only one copy. The difference between dominant and recessive inheritance patterns also plays a role in determining the chances of a child inheriting an X-linked disorder from their parentage.
Males are normally hemizygous for the X chromosome, having only one copy. As a result, X-linked dominant disorders usually show higher expressivity
Expressivity
Expressivity is a term used in genetics to refer to variations in a phenotype among individuals carrying a particular genotype. The term can be used to characterize qualitatively or quantitatively the extent of phenotypic variation given a particular genotype. The term is analogous to the...
in males than females.
Inheritance
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| X-linked dominant inheritance works differently depending upon whether the mother (left image) or father (right image) is the carrier of a gene that causes a disease or disorder. | |
In X-linked dominant inheritance, when the mother alone is the carrier
Genetic carrier
A genetic carrier , is a person or other organism that has inherited a genetic trait or mutation, but who does not display that trait or show symptoms of the disease. They are, however, able to pass the gene onto their offspring, who may then express the gene...
of a mutated, or defective gene associated with a disease or disorder; she herself will have the disorder. Her children will inherit the disorder as follows:
- Of her daughters and sons: 50% will have the disorder, 50% will be completely unaffected. Children of both genders have an even chance of receiving either of their mother's two X chromosomes, one of which contains the defective gene in question.
When the father alone is the carrier of a defective gene associated with a disease or disorder, he too will have the disorder. His children will inherit the disorder as follows:
- Of his daughters: 100% will have the disorder, since all of his daughters will receive one copy of his single X chromosome.
- Of his sons: none will have the disorder; sons do not receive an X chromosome from their father.
If both parents were carriers of a defective gene associated with a disease or disorder, they would both have the disorder. Their children would inherit the disorder as follows:
- Of their daughters: 100% will have the disorder, since all of the daughters will receive a copy of their father's X chromosome.
- Of the sons: 50% will have the disorder, 50% will be completely unaffected. Sons have an equal chance of receiving either of their mother's X chromosomes.
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. Were this to occur with an X-linked dominant disorder, that daughter would likely experience a more severe form.
Some X-linked dominant conditions such as Aicardi Syndrome
Aicardi syndrome
Aicardi syndrome is a rare genetic malformation syndrome characterized by the partial or complete absence of a key structure in the brain called the corpus callosum, the presence of retinal abnormalities, and seizures in the form of infantile spasms...
are fatal to boys, therefore only girls with these conditions survive. Similarly, individuals with Klinefelter's Syndrome
Klinefelter's syndrome
Klinefelter syndrome, 46/47, XXY, or XXY syndrome is a condition in which human males have an extra X chromosome. While females have an XX chromosomal makeup, and males an XY, affected individuals have at least two X chromosomes and at least one Y chromosome...
are referred to as "47,XXY Males".