X-linked

X-linked refers to the pattern of inheritance for genes that are located on the X chromosome. In humans, as well as many other species, there are two types of sex chromosomes: X and Y. Females typically have two X chromosomes, while males have one X and one Y. This means that males are more likely to express traits that are controlled by genes on the X chromosome, as they do not have a second X chromosome to potentially mask the effect of these genes. This is the basis for X-linked inheritance.

X-linked inheritance

X-linked inheritance is a mode of genetic inheritance by which a trait is passed from parent to child through the X chromosome. This form of inheritance is dominant in males, as they have only one X chromosome, and any gene present on it, even if recessive, will manifest as there is no corresponding gene on the Y chromosome to counteract it. In females, who have two X chromosomes, a recessive gene on one X chromosome would need to be present on the other X chromosome as well to manifest.

X-linked diseases

There are many X-linked diseases, including Hemophilia, Duchenne muscular dystrophy, and Fragile X syndrome. These diseases are often more severe in males, as they only have one X chromosome. Females can also be affected by X-linked diseases, but they are typically carriers and do not show symptoms unless both of their X chromosomes carry the mutation.

X-linked recessive inheritance

In X-linked recessive inheritance, a mutation in a gene on the X chromosome causes the phenotype to be expressed in males (who are hemizygous for the gene mutation because they have one X and one Y chromosome) and in females who are homozygous for the gene mutation (i.e., they have a copy of the gene mutation on each of their two X chromosomes).

X-linked dominant inheritance

X-linked dominant inheritance occurs when a gene responsible for a genetic disorder is located on the X chromosome, and the phenotype is expressed in the heterozygous state in both males and females. In the case of X-linked dominant inheritance, the chance of passing on an X-linked dominant disorder differs between men and women. The sons of a man with an X-linked dominant disorder will not be affected (since they receive their father's Y chromosome), but his daughters will all inherit the condition.