Define sex linked inheritance. Explain it with haemophilia and colourblindness.

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Q. Define sex linked inheritance. Explain it with haemophilia and colourblindness.

Uttar Pradesh UpmspUP Board (UPMSP) Intermediate 2023Subjective· 3mImportance★★★★★

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Sex-linked inheritance involves genes on the sex chromosomes (mostly X). Haemophilia and colour blindness are X-linked recessive, so they show up mainly in males, with females acting as carriers.

Definition: Sex-linked inheritance is the transmission of characters whose genes are located on the sex chromosomes (X or Y). Because males (XY) and females (XX) differ in their X-chromosome number, such traits are inherited differently in the two sexes. Genes on the X chromosome show X-linked inheritance.

Key point for X-linked recessive traits: A male has only one X chromosome, so a single recessive allele on it is fully expressed (males are hemizygous). A female needs two copies of the recessive allele to show the trait; with one copy she is a carrier. Hence such disorders are much more common in males.

1. Haemophilia (bleeder's disease):

An X-linked recessive disorder in which blood fails to clot (a clotting factor is missing), so even minor injuries cause prolonged bleeding.
Alleles: $X^H$ (normal) dominant over $X^h$ (haemophilic).
A carrier mother $X^H X^h$ $X^{H} X^{h}$ × normal father $X^H Y$ $X^{H} Y$ :
- Sons: ½ $X^H Y$ normal, ½ $X^h Y$ haemophilic.
- Daughters: ½ $X^H X^H$ normal, ½ $X^H X^h$ carrier.
Famous example: it ran through the royal families of Europe from Queen Victoria (a carrier).

2. Colour blindness (red–green):

An X-linked recessive defect in which a person cannot distinguish red and green colours (defective cone pigment genes on the X chromosome).
Alleles: $X^C$ (normal) dominant over $X^c$ (colour-blind). …

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CBSE 2026Set ANNUAL1 mark
$Q. Which colours cannot be discriminated by person in colour blindness?$
›Reveal solutionSolution

Colour blindness is most commonly red-green colour blindness, a sex-linked recessive disorder affecting the ability to distinguish red from green.

Colour blindness is a sex-linked (X-linked) recessive disorder involving a defect in one or more of the three cone-cell photopigments in the retina. The most common form is red-green colour blindness, in which the affected individual cannot discriminate bet …

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CBSE 2025Set KH1 mark
$Q. Write name of any one sex-linked recessive disorder.$
›Reveal solutionSolution

Haemophilia (or colour blindness) is a sex-linked recessive disorder carried on the X chromosome.

Concept. In sex-linked recessive inheritance the mutant recessive allele sits on the X chromosome. A male (XY) with a single recessive allele on his lone X is affected, while a female (XX) must be homozygous recessive to be affected — so such disorders appear more frequently in males.

…

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CBSE 2025Set F1 markMCQ
$Q. Which of the following is a sex-linked character?$
(a) Anaemia
(b) Colourblindness
(c) Baldness
(d) All of these
›Reveal solutionSolution

Colourblindness is a sex-linked (X-linked) trait.

Colourblindness (red-green) is caused by a recessive gene located on the X chromosome, making it a true sex-linked trait: it appears far more often in males (XY) because a single recessive allele is expressed, whereas females (XX) must inherit it on both X chromosomes. It follows the same X-linked recessive inheritance as haemophilia. Ordinary anaemia is gener …

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CBSE 2025Set ANNUAL1 mark
$Q. A haemophillic man marries a normal homozygous woman. What is the probability in percentage that their daughter will be haemophillic?$
›Reveal solutionSolution

Haemophilia is an X-linked recessive trait. A haemophilic father (XhY) crossed with a normal homozygous mother (XHXH) produces daughters who are all carriers (XHXh) but phenotypically normal - so the probability that a daughter is haemophilic is 0%.

Haemophilia is caused by a recessive allele located on the X chromosome. The father, being haemophilic, has the genotype XhY (he has only one X chromosome, so the trait is fully expressed). The mother is normal and homozygous, genotype XHXH.

…

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CBSE 2025Set ANNUAL1 mark
$Q. Select the genetic disorder in which a blood clotting protein is affected leading to a non-stop bleeding even through a simple wound. (Phenylketonuria, Haemophilia, Thalassemia, Sickle-cell Anaemia)$
›Reveal solutionSolution
Haemophilia is the X-linked recessive genetic disorder in which a blood-clotting protein (clotting factor) is defective/absent, so even a minor wound causes prolonged, non-stop bleeding.

Among the given options:

Phenylketonuria — an autosomal recessive metabolic disorder caused by lack of the enzyme phenylalanine hydroxylase; leads to accumulation of phenylalanine, affecting the brain. (Not related to clotting.)

Haemophilia — an X-linked recessive disorder in which one of the protein clotting factors (e.g. Factor VIII) needed for normal blood clotting is missing or defective. In an affected person, a simple cut fails to clot normally and bleeding continues without stopping, which can be life-threatening. This matches the question exactly.

Thalassemia — an autosomal recessive blood disorder caused by defective synthesis of one of the globin chains of haemoglobin, leading to reduced haemoglobin/anaemia — not primarily a clotting defect. …
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CBSE 2025Set ANNUAL1 markMCQ
$Q. Haemophilia is a disease that usually appears in male but rarely in female. However, female can become haemophilic if -$
(i) Mother is haemophillic and father is normal
(ii) Mother is a carrier and father is normal
(iii) Mother is normal and father is haemophillic
(iv) Mother is a carrier and father is haemophillic
›Reveal solutionSolution
Haemophilia is X-linked recessive; a female is affected only if she is homozygous recessive, which requires a recessive allele from BOTH parents — a carrier mother and a haemophilic father.

Haemophilia is caused by a recessive allele carried on the X-chromosome ( $X^h$ ). Males (XY) need only one copy of the recessive allele to be haemophilic, which is why the disease appears far more often in males. A female (XX) becomes haemophilic ( $X^hX^h$ ) only if she inherits the recessive allele from BOTH of her parents:

One $X^h$ from her father, which means the father himself must be haemophilic ( $X^hY$ ).

One $X^h$ from her mother, which means the mother must carry at least one recessive allele — i.e., she is a carrier ( $X^HX^h$ ), since a fully haemophilic mother is rare and not required, only a carrier is. …
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CBSE 2022Set HE2201 markMCQ
$Q. Choose the correct answer: Haemophilia is a disease -$
(a) X Linked gene
(b) Y Linked gene
(c) XY Linked gene
(d) XXY Linked gene
›Reveal solutionSolution

Haemophilia is caused by a recessive allele carried on the X chromosome, so it shows a characteristic sex-linked (criss-cross) pattern of inheritance.

Haemophilia is a blood-clotting disorder in which a mutated allele fails to produce a functional clotting factor (Factor VIII in haemophilia A), so even a minor injury can cause prolonged bleeding. The gene for this clotting factor is located on the X chromosome, and the disease allele is recessive. Because males have only one X chromosome (XY), a single copy of the defective allele is enough to make a male haemophilic; females (XX), …

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CBSE 2022Set HE2201 mark
$Q. Match the following (Column I with Column II): Colour blindness — choose the matching term from:$
(a) Obelia
(b) Hydra
(c) Transgenic
(d) Gender linked disease
(e) Amoebiasis
(f) Antimicrobial substance
(g) Male sterilization
(h) Genetic disorder.
›Reveal solutionSolution

Colour blindness is a classic X-linked (sex-linked) recessive genetic disorder.

Colour blindness (typically red-green colour blindness) is caused by a recessive allele located on the X chromosome, affecting the cone photoreceptor pigments needed to distinguish red and green light. Because the gene is carried on the X chromosome, it follows a sex-linked (criss-cross) inheritance pattern — it is far more common in males (who need only one …

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CBSE 2021Set ANNUAL1 mark
$Q. Name the genetic disorder in which a blood clotting protein is affected leading to non-stop bleeding even through a simple wound.$
›Reveal solutionSolution
The genetic disorder in which a blood-clotting protein is affected, causing non-stop bleeding, is Haemophilia.

Haemophilia is a sex-linked (X-linked) recessive genetic disorder.

It occurs due to a mutation in the gene coding for a blood-clotting protein - most commonly clotting Factor VIII (Haemophilia A), or Factor IX (Haemophilia B).

Because the clotting factor is missing or defective, the blood of an affected person does not clot normally.

As a result, even a simple/minor cut or wound leads to continuous, non-stop bleeding, which can be life-threatening. …
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CBSE 2021Set NC1 markMCQ
$Q. Which of the following is a Mendelian disorder?$
(a) Down syndrome
(b) Klinefelter syndrome
(c) Haemophilia
(d) Genital herpes
›Reveal solutionSolution
Mendelian disorders arise from mutation in a single gene; haemophilia (X-linked recessive) fits this, while Down/Klinefelter syndromes are chromosomal aneuploidies and genital herpes is a viral infection.

Classifying the options

Mendelian disorders are determined by alteration/mutation in a single gene and are transmitted to offspring according to Mendelian principles of inheritance (dominant/recessive, autosomal/X-linked). Examples: haemophilia, sickle-cell anaemia, colour blindness, thalassemia, cystic fibrosis, phenylketonuria.

Haemophilia: an X-linked recessive disorder in which a single defective gene fails to produce a functional blood-clotting factor (Factor VIII or IX), leading to excessive bleeding even from minor injuries. It is passed from carrier mothers to sons in the classic Mendelian criss-cross pattern — a textbook Mendelian disorder. …
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CBSE 2020Set ANNUAL1 markMCQ
$Q. A person suffering from colour blindness cannot differentiate which of the following colours?$
(a) Blue and green colour
(b) Red and green colour
(c) Red and yellow colour
(d) Red and blue colour
›Reveal solutionSolution

Colour blindness is a sex-linked (X-linked) recessive genetic disorder that most commonly causes an inability to distinguish red and green colours.

Colour blindness results from a defect in one or more of the red-, green-, or blue-sensitive cone cells (photoreceptors) in the retina. The gene for red-green colour perception is located on the X chromosome, so this disorder is X-linked recessive — it is more common in males (who have only one X chromosome, so a single recessive allele is enough to cause it) than in females (who need two copies to be affected). The most common form o …

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CBSE 2020Set ANNUAL1 markMCQ
$Q. Haemophilia in man is due to:$
(a) Sex linked inheritance
(b) Sex limited inheritance
(c) Non disjunction
(d) Sex influenced inheritance
›Reveal solutionSolution
Haemophilia is caused by a recessive allele carried on the X chromosome, so its inheritance pattern is sex-linked.

Haemophilia is a classic X-linked recessive disorder in which the blood fails to clot normally because of a deficiency of a clotting factor (Factor VIII or IX). The gene for haemophilia is located on the X chromosome, and the disease allele (h) is recessive to the normal allele (H).

Females (XX) need the recessive allele on both X chromosomes to be haemophilic; a single copy makes them unaffected carriers (XHXh) who can pass the disease to sons.

Males (XY) have only one X chromosome, so a single copy of the recessive allele (XhY) is enough to make them haemophilic — this is why haemophilia is far more common in males, and why it characteristically passes from a carrier mother to her sons ('criss-cross' pattern typical of X-linked genes). …
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Q.Define sex linked inheritance. Explain it with haemophilia and colourblindness.

Concept understanding — X-linked Inheritance

Q.Define sex linked inheritance. Explain it with haemophilia and colourblindness.

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