What Are Sex Linked Traits? Definition, Inheritance Patterns & Examples
Sex-linked inheritance refers to the transmission of certain traits from parents to offspring through the Sex Chromosomes. Humans typically have two types of sex chromosomes, X and Y. Females have XX, and males have XY. Any trait or gene located on these chromosomes may display unique inheritance patterns compared to those found on non-sex chromosomes (autosomes).
Types of Sex-linked Inheritance
There are two main types of sex-linked inheritance in human biology: X-linked and Y-linked.
X-linked Inheritance: Traits are carried on the X chromosome.
Y-linked Inheritance: Traits are carried on the Y chromosome, which is exclusive to males.
X-linked Inheritance
The X chromosome is larger and carries more genes compared to the Y chromosome. X-linked traits can be X-linked recessive or X-linked dominant.
X-linked Recessive Inheritance
More common in males because males have only one X chromosome (XY).
A single copy of a recessive allele on the X chromosome (in males) is enough to express the trait.
Haemophilia and Colour Blindness are well-known sex-linked inheritance disease examples.
Females (XX) need two copies of the recessive allele to be affected; with one defective allele, they become carriers.
X-linked Dominant Inheritance
Occurs if a single copy of a dominant allele on the X chromosome causes the trait to appear.
Both males and females can be affected, but females might show more varied symptoms due to having two X chromosomes.
Incontinentia Pigmenti is an example of an X-linked dominant condition.
Y-linked Inheritance
Traits linked to genes on the Y chromosome are passed strictly from father to son. Since only males possess the Y chromosome, any mutation here will exclusively affect males. One sex-linked disease example of Y-linkage is Hypertrichosis, which involves excessive hair growth on certain parts of the body (such as the ears).
Characteristics of Sex-linked Inheritance
Males are often more affected by X-linked recessive traits because they have only one X chromosome.
Females can pass X-linked recessive traits to both sons and daughters. Sons receiving the affected X typically show the condition, whereas daughters may become carriers if they inherit one affected X.
Sex-linked inheritance in humans can sometimes skip generations, especially in the case of X-linked recessive traits.
Common Sex-linked Inheritance Diseases
Haemophilia (Recessive X-linked disorder):
Also known as Bleeder’s disease.
Characterised by the inability of blood to clot properly.
A small injury can lead to excessive bleeding.
Colour Blindness (Recessive X-linked disorder):
Commonly involves difficulty in distinguishing between green, red, or blue.
Males are more frequently affected due to their single X chromosome.
Incontinentia Pigmenti (Dominant X-linked disorder):
Abnormalities in skin pigmentation, hair, nails, and teeth.
Mainly affects females as it is usually lethal for males.
Quiz Time
Question: Which chromosome carries more genes, X or Y?
Answer: X chromosome
Question: Name one sex-linked trait example in humans that is Y-linked.
Answer: Hypertrichosis
Question: Why are males more prone to X-linked recessive disorders?
Answer: Males have only one X chromosome, so one defective allele is enough to express the disorder.
Mnemonic for X-linked Recessive Disorders
Remember “B-HaC” for some common X-linked recessive disorders:
B – Becker’s Muscular Dystrophy
H – Haemophilia
aC – Colour Blindness
This simple mnemonic can help you recall important sex-linked inheritance disease examples.
Sex-linked Traits in Animals
Apart from humans, there are sex-linked traits examples in animals, too:
Calico Cats: The distinct patches of fur colours (black, orange, white) are associated with X-linked genes. Usually observed in female cats (XX).
Barred Plumage in Chickens: A pattern in feathers linked to the Z chromosome (in birds, the ZW system determines sex, where males are ZZ and females are ZW).
These sex-linked traits examples show that sex chromosomes play significant roles across different species.
Key Takeaways
Sex-linked inheritance involves genes on the X or Y chromosomes.
X-linked traits can be recessive or dominant, more commonly affecting males when recessive.
Y-linked traits pass exclusively from father to son.
Familiar examples include Haemophilia, Colour Blindness, and Hypertrichosis.
Related Topics
FAQs on Sex Linked Characters: Inheritance, Types, and Disorders
1. What is sex-linked inheritance as per the Class 12 Biology syllabus?
Sex-linked inheritance refers to the transmission of traits determined by genes located on the sex chromosomes (X and Y). As per the CBSE syllabus, understanding this concept is crucial for differentiating it from Mendelian inheritance. The expression of these traits is linked to the biological sex of an individual because males (XY) and females (XX) have different combinations of these chromosomes.
2. What are the main types of sex-linked inheritance?
There are two primary types of sex-linked inheritance studied in human genetics:
- X-linked inheritance: Involves genes located on the X chromosome. It can be further divided into X-linked dominant and X-linked recessive patterns.
- Y-linked inheritance (Holandric inheritance): Involves genes located on the Y chromosome. These traits are exclusively passed from father to son.
3. What are the classic examples of X-linked and Y-linked disorders in humans?
Classic examples of sex-linked disorders frequently discussed in the NCERT curriculum include:
- X-linked Recessive Disorders: Haemophilia, a blood clotting disorder, and red-green colour blindness are key examples.
- X-linked Dominant Disorders: A rarer example is Incontinentia Pigmenti, which affects the skin, hair, and teeth.
- Y-linked Traits: Hypertrichosis, characterized by excessive hair growth on the ear pinna, is a well-known example of a Y-linked trait.
4. How does X-linked recessive inheritance differ from autosomal recessive inheritance?
The key difference is the chromosomal location and its impact on gender. X-linked recessive traits are carried on the X chromosome and are more frequently expressed in males, who only need one copy of the recessive allele (on their single X chromosome) to be affected. In contrast, autosomal recessive traits are on non-sex chromosomes, affecting males and females with equal probability, as both require two copies of the recessive allele to express the trait.
5. Why are males more frequently affected by X-linked recessive disorders like colour blindness?
Males are more susceptible to X-linked recessive disorders because they have only one X chromosome (genotype XY). If this single X chromosome carries a recessive allele for a disorder like haemophilia or colour blindness, the trait will be expressed. Females (genotype XX), however, have a second X chromosome that can carry a dominant, healthy allele, which typically masks the recessive one, making them an unaffected carrier instead.
6. Can a father pass an X-linked trait to his son? Explain why or why not.
No, a father cannot pass an X-linked trait to his son. This is because a son always inherits his Y chromosome from his father and his X chromosome from his mother. Therefore, any genetic information on the father's X chromosome can only be passed down to his daughters, as they inherit one X chromosome from each parent.
7. What is the significance of a female being a 'carrier' in sex-linked inheritance?
A female 'carrier' is heterozygous for an X-linked recessive trait, meaning she has one normal allele and one recessive allele. The significance is that she typically does not show symptoms of the disorder but has a 50% probability of passing the recessive allele to her children. Any son who inherits it will be affected, and any daughter who inherits it will also become a carrier. This mechanism explains how X-linked recessive disorders can persist in a family line, often appearing to skip generations.
8. How is a Punnett square used to demonstrate the inheritance of sex-linked traits?
A Punnett square is a visual tool used in genetics to predict the probable outcomes of a cross. For sex-linked traits, the alleles are shown as superscripts on the X and Y chromosomes (e.g., XH for normal, Xh for haemophilia). By mapping the parental gametes (e.g., XhY from an affected father and XHXH from a normal mother), the Punnett square clearly illustrates the potential genotypes and phenotypes of their sons and daughters, making it easier to calculate inheritance probabilities.
9. Can a sex-linked trait like haemophilia skip a generation?
Yes, X-linked recessive traits like haemophilia can appear to skip a generation. This occurs when an affected father passes the allele to his daughter, making her a carrier. Since she has a normal allele on her other X chromosome, she likely won't have the disorder. However, she can then pass the recessive allele to her son, who will be affected. In this scenario, the trait is not expressed in the daughter's generation but reappears in her son's, thus 'skipping' her generation.
10. What is the difference between X-linked dominant and X-linked recessive inheritance?
The main difference lies in how many copies of the allele are needed for the trait to be expressed.
- In X-linked recessive inheritance, a female needs two copies of the recessive allele to be affected, while a male only needs one. This is why these disorders are more common in males.
- In X-linked dominant inheritance, just one copy of the dominant allele on an X chromosome is enough to cause the disorder in both males and females. Affected fathers will pass the disorder to all of their daughters but none of their sons.
