The physical characteristics, which are also known as physical/phenotypic traits in the field of genetics, that are determined by the genes present on the sex chromosomes (allosomes) i.e. the X and Y chromosomes are known as sex linked characteristics or sex linked traits. The primary criteria for any of the traits to be sex linked characters are that they should be a physical aspect of the functional product of a gene that is present on either of the two sex chromosomes. Hence, sex linked characters are those characters that can be linked to the inheritance and presentation of a gene that is present on a sex chromosome rather than an autosome.
Sex Linkage and Inheritance Pattern
It is clear from the introduction that sex linked characters mean those characters or traits that are due to the expression of a gene located on the sex chromosome. In humans, such characters are said to be either X-linked recessive, X-linked dominant or Y-linked. Because of these differences in the two chromosomes and the traits being mainly linked to two types of specific chromosomes, the inheritance pattern of the sex linked characters differs from the autosomal inheritance pattern and the dominance and recessive properties of it.
Here, the sex linkage is dominant and recessive for genes located on the X chromosome because, in the case of humans, the sex linked definition is more applicable to the genes that are located in the X chromosome. This is due to the number of genes that are present and are primarily and secondarily responsible for sex linked characters are present mostly on the X chromosome which is large when compared with the smaller Y chromosome.
Because of these differences of specific genes being present on X and Y chromosomes, certain traits are such that only women can act as carriers and so even if the genes are recessive they are going to be expressed in men because of the lack of another X chromosome to compensate for the healthy copy of the gene. For example, in an X linked recessive inheritance, when a son is born it will carry any of the two X chromosomes from the mother and the Y chromosome from the father and if the father does not have a particular trait for which the mother is a carrier then the son has a 50% chance of being affected and the daughter will have a 50% chance of being a carrier.
On the other hand, in the case of X linked dominant inheritance when either a son or a daughter is born, both have a 50% probability of expressing the sex linked trait even if the trait or character is obtained from an affected mother and if the father is not affected. In case the father carries the copy of a particular trait on the X chromosome then only and only the daughter might get affected by a particular trait or else be a carrier for it depending on whether the trait is dominant or recessive. And lastly in the case of Y linked inheritance a particular trait is passed only from father to son and will always affect every generation.
In the case of animals as well, a similar pattern of sex link traits and pattern of inheritance is observed while determining the sex based on the XY system. For certain animals, the sex linked definition and inheritance pattern will be applicable differently while using a sex determination system other than XY. In the ZW sex-determination system, the pattern given above is reversed because a male is the homogametic sex i.e. male is only produced when two Z chromosomes come together in a fertilised egg and so having ZZ indicates male sex chromosomes and so correspondingly the female is the heterogametic having ZW sex chromosomes. For studying sex link in between the traits from parents to child, an experimental methods known as the reciprocal cross is performed in classical genetics.
Pedigree analysis charts are usually used to understand the inheritance pattern of the traits. The transfer of traits from the parents to children that are sex linked according to sex linked definition can be understood from the following pedigree analysis which shows an example of a female carrier of a trait:
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Examples of Sex Linked Inheritance
An example of sex linked inheritance as per the sex linked trait definition can include either an X-linked dominant or recessive trait carried by an affected mother and an X-linked dominant or recessive trait carried by an affected father and Y-linked traits which are definitely transferred from father to son only.
X Linked Recessive Inheritance:
Typically, in this case, a female possessing only one X linked recessive trait will manifest any of its effects because of the presence of another possible alternative in the form of another allele present on another X chromosome, in case the particular gene is faulty. When such a female gives birth to a son along with an unaffected father, the son will definitely express the physical characteristics whereas the daughter will only show the trait when the father is also affected by the same trait as it will lead to a combination of homozygous recessive traits. Otherwise, the daughter will act only as a carrier of the recessive trait when the father is unaffected.
In certain cases, such as X chromosome inactivation or skewed X inactivation, this pattern of inheritance is not followed in the case of daughters because the other copy of the gene (apart from the faulty one) is inactivated. In such a case, the daughters will also manifest the effects of the trait to a varying degree.
The rate of X linked recessive inheritance in females is the square of the rate in males. For example, in a male, if 1 out of 20 male has certain colour blindness, then the same will be 1 in 400 for females.
Common sex linked trait examples that follow X linked recessive inheritance include: colour blindness, haemophilia (inability of blood clotting), Araskog-skot syndrome, Hunter syndrome, etc.
X Linked Dominant Inheritance:
In this case, the probability of each child of a mother having X linked dominant trait will have a 50% chance of getting affected by the dominant trait. In case, the father has the affected trait then the daughter will have a 100% chance of getting affected by the dominant trait and a son will have 0% chance of inheriting the dominant because the son inherits the Y chromosome. There are fewer X linked dominant cases as compared to X linked recessive because the dominance in X linked inheritance requires the particular gene to be present in the females with a fraction of the reduction of the gene expressed in the case of autosomal chromosome because approximately half (or as much as 90% in some cases) of a particular parent’s X chromosome are inactivated in the females.
Sex linked trait examples that follow X linked dominant inheritance include Vitamin D resistant rickets, Alport syndrome (characterised by hearing loss, end-stage kidney disease, etc.), Fragile X syndrome (characterised by mild to moderate intellectual activity), etc.
Y Linked Inheritance:
This is the most simplistic form of sex-linked inheritance. It is transferred from father to son only and can affect every generation. The most common examples of this type of inheritance include failures in the SRY gene that produces the sex-determining region Y protein. Mutations and changes in this gene can cause disorders in the male child.