Dictionary > Sex-linked trait

Sex-linked trait

Sex-linked trait
n., singular: sex-linked traits
[sɛks lɪŋkt tɹeɪt]
Definition: trait controlled by its presence on the sex chromosome

Definition of Sex-Linked Traits

A sex-linked trait is an observable characteristic of an organism that is influenced by the genes on the chromosomes that determine the organism’s sex. There are two sex chromosomes in each person, one of which is inherited from each parent.

Sex-linked or sex-linkage (related terms) qualities are characteristics that are present on either the sex-determining chromosome or the sex chromosomes. The X or Y chromosomes are responsible for this in humans. This is because sex determination in humans depends on the chromosomes that are obtained. Humans usually only obtain one sex, which is either female or male.

Nonsex chromosomes are called autosomes.

Each of the three has a unique pattern of inheritance and presentation depending on the sex of the parent and the child. They distinguish themselves from autosomal dominance and recessiveness in this way.

Humans have many times more genes on the X chromosome than the Y chromosome, hence there are many more X-linked diseases than Y-linked diseases. Males tend not to be “carriers” but exhibit X-linked illnesses; they will always be affected by them because they lack a second X chromosome with a healthy copy of the gene. They only have one X chromosome, which when it has the aberrant gene, this gene will tend to be expressed.

Only females can be carriers for X-linked conditions. As a result, males are affected by X-linked recessive diseases significantly more frequently than females. However, both females and males inherit and can experience sex-linked disorders.

The SRY gene provides instructions for producing the sex-determining region Y protein. The formation of male-typical sex, which often follows a specific pattern based on a person’s chromosomes, is influenced by this protein.

You might like to read: Men could go extinct? Y chromosome disappearing slowly 

SRY gene position on the chromosome
Figure 1. SRY gene position on the chromosome. Image Credit: Research Gate.


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Biology definition:
A sex-linked trait is a trait genetically determined by an allele located on the sex chromosome. In humans, the sex chromosomes are the X chromosome and the Y chromosome. A trait that is determined by the allele on the X chromosome is particularly described as X-linked whereas that trait determined by the allele on Y chromosome is said to be Y-linked. Since traits are sex-linked, there is usually a distinct pattern. Some traits would therefore be linked to the sex (or gender) of an individual. The manifestation of certain traits as an outcome of the expression of particular genes in the sex chromosome is referred to as sex linkage. For example, color blindness is a sex-linked trait whose allele is recessive and located on the X chromosome. When the mother is color blind and the father is not, all sons are going to be color blind. The daughters are going to be normal but carriers of the gene. Conversely, when the father is color blind and the mother is not, all sons and daughters are normal but the daughters will be carriers of the gene. See also: Sex-linked disorder

Sex-Linked Genes

Sex-linked genes are often identified by the chromosome, either X or Y, followed by a superscript to identify the relevant allele. For instance, a female might have the genetic code XAXa for the X-linked gene A, where ‘X’ stands for the X chromosome, ‘A’ for the dominant allele of the gene, and ‘a’ for the recessive allele of the gene. In this case, the female will therefore possess one copy of each allele, that is, one of the dominant allele and one of the recessive allele.

Sex-related features are determined by sex-related genes. There are three possible inheritance patterns for sex-linked genes:

  1. Y-linked inheritance
  2. X-linked inheritance (recessive or dominant inheritance)
    • X-linked recessive inheritance
    • X-linked dominant inheritance
  • X-linked Dominant Genes

Some genes are X-linked dominant genes and function similarly to autosomal genes that have dominant traits in that they require only one copy of the allele to express the desired feature. They, however, are not the same genes as autosomal genes. The person will exhibit the trait of interest if there is just one copy of the X-linked dominant allele.

    • X-linked Dominant Genes in Females

Since approximately half (or as much as 90% in some cases) of a particular parent’s X chromosomes are inactivated in females, there are fewer X-linked dominant conditions than X-linked recessive conditions. This is because dominance in X-linkage requires the condition to present in females with only a fraction of the reduction in gene expression of autosomal dominance.

    • X-linked Dominant Genes in Males

A mother with an X-linked dominant trait has a 50% probability of passing the mutation to each of her children, who then have a 25% risk of developing the condition. Since daughters inherit their father’s X chromosome and sons their father’s Y chromosome, if just the father is impacted, then 100% of the daughters and 0% of the sons will be affected.

  • X-linked Recessive Genes

X-linked recessive alleles are covered up by a dominant allele, in contrast to X-linked dominant genes. Consequently, for the X-linked recessive characteristic to manifest, a dominant allele must be lacking.

X-linked Recessive Inheritance
Figure 2. X-linked Recessive Inheritance. Image Credit: Vedantu.


    • X-linked Recessive Genes in Females

Females who have one X-linked recessive disorder are known as carriers and typically do not show clinical symptoms of the disorder. However, since some cells express one X allele while others express the other, differences in X chromosome inactivation can result in varying degrees of clinical expression in carrier females.

Given that daughters carry their father’s X chromosome, all female children of an affected father will be carriers (presuming the mother is not affected or a carrier).

    • X-linked Recessive Genes in Males

Since males only have one X chromosome and a single copy of each X-linked gene, all males with an X-linked recessive mutation will be impacted. If the father does not possess the recessive allele (or recessive alleles), then all of the offspring of a carrier female or carrier mother has a 50% chance of acquiring the mutation.

Because boys solely inherit their father’s Y chromosome, no male children of an afflicted father will be impacted if the mother is not a carrier.

  • Y-linked Genes

The term “Y linkage” refers to features that are brought about by genes on the Y chromosome. It is an instance of sex linking. Y connection might be challenging to find. This is partially due to the fact that the Y chromosome is smaller and has few genes, fewer than the X or autosomal chromosomes. There are thought to be 200 genes in it.

Before, it was believed that the human Y chromosome wasn’t very significant. Not all sex-determining genes are Y-linked, despite the fact that the Y-chromosome determines sex in humans and some other species. Only a few tiny areas, known as pseudoautosomal regions, on the Y chromosome typically experience genetic recombination.

ZW Sex-determination System

Birds, some fish, and crustaceans like the giant river prawn, some insects (including butterflies and moths), the schistosome family of flatworms, and some reptiles, including the majority of snakes, lacertid lizards, monitors, and Komodo dragons, use the ZW sex-determination system to determine the sex of their offspring.

It is also utilized by some plants, where it most likely underwent multiple independent evolutionary occurrences. This system is distinguished from the XY sex-determination system by the letters Z and W, hence the Z chromosomes and W chromosomes. In this arrangement, males have two Z chromosomes that are comparable and females have two ZW chromosomes that are different from one another.

Males are the homogametic sex, as opposed to females, who are the heterogametic sex (ZZ) (ZW).

The Z chromosome is larger and has more genes than the X chromosome in the XY system.

Common Sex-Linked Traits

The eye color in the fruit fly, scientifically known as the Drosophila, is one of the most common sex-linked trait examples.

The fruit fly sex-linked gene was first discovered in fruit flies by Thomas Hunt Morgan. He initially discovered fruit flies with a recessive mutation that caused their eyes to turn white. Mendel predicted that the offspring of a red-eyed female and a white-eyed man would all have red eyes based on his theory of segregation. As predicted by Mendel’s law of segregation, the F1 generation’s progeny all had red eyes.

Morgan expected to see a 3:1 ratio of red eyes to white eyes in the F1 offspring, a red-eyed female with a red-eyed male, as suggested by Mendel’s law of segregation. He noted that all of the female fruit flies had red eyes, whereas only half of the male fruit flies had white eyes, when this 3:1 ratio was being watched. Hence, it was evident that the inheritance of eye color in male and female fruit flies differed.

Because the patterns of eye color in males and females of fruit flies varied, he hypothesized that eye color in fruit flies must be on the X chromosome. Reviewing Morgan’s Punnett square experiments reveals that eye color was X-linked.

Sex-Linked Traits in Humans

Hemophilia, red-green color blindness, congenital night blindness, several high blood pressure genes, Duchenne muscular dystrophy, and Fragile X syndrome are some of the most well-known sex-linked abnormalities.

What’s also very intriguing is that you can probably imagine that for people who are XY or males, having these various mutations on the genes, on the X chromosome, is particularly problematic because, unlike females, there are not two X chromosomes that give you the potential to carry a normal gene on the X chromosome. It is for this reason that males are frequently more affected by these sex-related diseases.

Symptoms of Fragile X syndrome
Figure 3. Symptoms of Fragile X syndrome. Image Credit: Asuragen.


Examples of Sex-Linked Traits

Fragile X syndrome and vitamin D-resistant rickets are two examples of X-linked dominant characteristics. The symptoms of each of these illnesses can be seen in boys and females with just one copy of the dominant allele.

The hemophilia gene and red-green color blindness are two examples of X-linked recessive traits. In these circumstances, males will express features with just one copy of the recessive gene while females require two copies.

Sex-Linked Traits – Key Takeaways

  • A trait that is sex-linked is one that is governed by an allele or gene on the sex chromosome. The X and Y chromosomes are considered to be the sex chromosomes in humans.
  • Males have a copy of each chromosome, one X and one Y, whereas females have two copies of the Y chromosome.
  • Y-dominant, X-linked recessive inheritance, and X-linked dominant are the three main ways that sex inheritance occurs.
  • Mendal’s law is not followed by X-linked genes as males carry only one x chromosome (copy of the X gene) and females carry no copies of the Y gene.
  • X-linked recessive genes are those that are present on the X chromosome; in biological females, two alleles (both) are required for the trait to manifest itself, whereas, in biological males, only one allele is required.
  • A single allele of an X-linked dominant gene is sufficient to cause the trait to manifest. These genes can be discovered on the X chromosome (x-linked dominant allele).
  • The Y-chromosome carries Y-linked traits or genes which can only be expressed by a biological male in a species.
  • Hemophilia, fragile X syndrome, and red-green color blindness are a few conditions that frequently have sex-related genes in humans.

Frequently Asked Questions about Sex-Linked Traits

Since “sex-linked traits” is such an interesting topic, there are often numerous questions that get asked often about it. Here are some of the most-asked questions about sex-linked traits and the answers to them.

  • What is an example of a sex-linked trait?

Being color blind, hemophilia (the inability of the blood to coagulate), Araskog-Skot syndrome, Hunter syndrome, etc. are examples of common sex-linked traits that are inherited by X-linked recessive inheritance.

  • How are sex-linked traits inherited?

These traits are classified as  X-linked recessive, X-linked dominant, or Y-linked in humans. The inheritance pattern of the sex-linked characteristics differs from the autosomal inheritance pattern and the dominance and the recessive features of it due to these variances in the two chromosomes and the traits being largely related to two types of unique chromosomes.

  • Why are sex-linked traits more common in males?

Male organisms are more prone than females to suffer from a sex-related illness because they only have one copy of the X chromosome. In a female, a second, unaltered copy may produce different, softer, or no symptoms of a sex-related sickness.

  • Is baldness a sex-linked trait?

Male pattern baldness is a sex-linked recessive characteristic, which shows that it is carried on the X chromosome. A female inherits one X chromosome from her father. The woman only possesses one copy of the pattern baldness allele because her mother does not have it.

Grades of male pattern baldness
Figure 4: Grades of male pattern baldness. Image Credit: NBCI


Note it!

Only males may carry and pass on webbed toes and numerous other minor features since they are dictated by genes on the Y chromosome.

Take the Sex-linked traits – Biology Quiz!


Choose the best answer. 

1. What is a sex-linked trait?

2. In humans, SRY gene is located in which chromosome?

3. Which of the following is true about X-linked dominant trait?

4. If inherited, there's a tendency that a daughter becomes a "carrier"

5. If a trait is coded by the Y gene, who will likely express the trait?

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