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.

FAQs on Sex-linked Inheritance in Humans: Types, Examples and Disorders

1. How do X-linked recessive disorders differ from autosomal recessive disorders?

X-linked recessive disorders are carried on the X chromosome, making males more prone to the condition since they have only one X chromosome. Autosomal recessive disorders are inherited through non-sex chromosomes (autosomes), and both males and females are affected equally.

2. Why do females rarely show X-linked recessive disorders?

Females have two X chromosomes (XX). They would need both copies of a recessive allele to fully express the disorder. If only one X carries the recessive allele, they typically become carriers without showing severe symptoms.

3. Can Y-linked traits be passed to daughters?

No. Y-linked traits are carried on the Y chromosome, which is found only in males. Hence, these traits are transmitted strictly from father to son.

4. What are some sex-linked traits examples in humans other than haemophilia and colour blindness?

Other examples include Duchenne Muscular Dystrophy (X-linked recessive) and Incontinentia Pigmenti (X-linked dominant).

5. What is a sex-linked trait punnett square, and why is it useful?

A punnett square is a visual tool used in genetics to predict the probability of offspring inheriting certain traits. When dealing with sex-linked inheritance, a punnett square helps illustrate how genes on the X or Y chromosomes can be passed on, showing possible genotypes and phenotypes in sons and daughters.