Sex Determination Linkage

8 MCQs9-step worked example
Source: NCERT Structural Organisation in AnimalsPYQ coverage: NEET 2020, 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

The trap that costs marks in sex-linked inheritance questions: confusing carrier status with affected status in females for X-linked recessive conditions.

Sex determination in humans follows the XX-XY system. Females carry two X chromosomes (XX); males carry one X and one Y (XY). The father's gamete determines the sex of offspring — X-bearing sperm produces daughters, Y-bearing sperm produces sons. The Y chromosome carries the SRY gene (sex-determining region), which triggers male development (NCERT Class 12 Biology Chapter 4, page 84).

Sex-linked inheritance refers to genes located on sex chromosomes. Most NEET-relevant examples involve X-linked recessive traits: haemophilia, colour-blindness, and Duchenne muscular dystrophy.

The critical inheritance logic:

  • An affected father (X^a Y) passes his X^a to ALL daughters → all daughters become carriers (X^A X^a), none affected.
  • A carrier mother (X^A X^a) passes X^a to 50% of sons → those sons are affected (X^a Y).
  • For a daughter to be AFFECTED, she needs X^a from BOTH parents (X^a X^a) — requires an affected father AND a carrier/affected mother.

The high-frequency trap: When a pedigree shows an affected male, students incorrectly conclude his daughters will also be affected. They forget that the daughter receives a normal X from her mother (assuming mother is homozygous dominant), making the daughter a carrier only. The recessive allele on one X is masked by the dominant allele on the other.

Watch-out for NEET: Questions often present a cross between a carrier female and a normal male, then ask for the probability of an affected child. The answer is 1/4 of sons (not 1/4 of all children) — because only males express the single-copy recessive.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

In humans, sex of the offspring is determined by:

MCQ 2Direct ApplicationPractice

Haemophilia is an X-linked recessive disorder. A carrier woman marries a normal man. What proportion of their SONS will be affected?

MCQ 3Direct ApplicationPractice

A colour-blind man marries a woman who is homozygous normal for colour vision. What is the genotype of their daughters?

MCQ 4Easy RecallPractice

In the XX-XY system of sex determination, which parent contributes the sex-determining chromosome to produce a male offspring?

MCQ 5Direct ApplicationPractice

A haemophilic man (X^h Y) marries a carrier woman (X^H X^h). What is the probability that their daughter will be haemophilic?

MCQ 6Easy RecallPractice

Which of the following statements about X-linked recessive disorders is CORRECT?

MCQ 7CalculationPractice

In a cross between a carrier female for colour-blindness (X^C X^c) and a normal male (X^C Y), what fraction of ALL offspring will be colour-blind?

MCQ 8Easy RecallPractice

The SRY gene responsible for male sex determination in humans is located on:

Worked Example

  1. 1

    Given

    A woman who is a carrier for haemophilia (X^H X^h) marries a normal man (X^H Y).

  2. 2

    Required

    (a) Probability of a haemophilic child. (b) Probability of a haemophilic son. (c) Probability that a randomly chosen son is haemophilic.

  3. 3

    Concept

    X-linked recessive inheritance. Males express recessive allele on single X. Females need homozygous recessive to express.

  4. 4

    Formula/Logic

    Punnett square for X-linked cross. Each gamete combination is equally probable (1/4 each).

  5. 5

    Substitution

    | | X^H (from father) | Y (from father) | |---|---|---| | X^H (from mother) | X^H X^H (normal daughter) | X^H Y (normal son) | | X^h (from mother) | X^H X^h (carrier daughter) | X^h Y (haemophilic son) |

  6. 6

    Calculation

    - Four equally likely outcomes: X^H X^H, X^H X^h, X^H Y, X^h Y. - Affected offspring: only X^h Y = 1/4 of all children. - Among sons only (X^H Y and X^h Y): affected = 1/2 of sons. - Among daughters: 0 affected (one is carrier, but not affected).

  7. 7

    Final answer

    (a) P(haemophilic child) = 1/4 (b) P(haemophilic son being born) = 1/4 (same as (a) since only sons can be affected here) (c) P(a son is haemophilic | child is son) = 1/2

  8. 8

    Common trap

    Students confuse (a) and (c). "What fraction of sons are affected?" = 1/2. "What is the probability of an affected child?" = 1/4. The word "son" as a condition changes the denominator. This is the X-linked inheritance carrier-vs-affected confusion at its most exam-costly.

  9. 9

    Similar NEET-style question

    A colour-blind woman (X^c X^c) marries a man with normal vision (X^C Y). What proportion of their children will be colour-blind? [Answer: 1/2 — all sons (X^c Y) are affected, all daughters (X^C X^c) are carriers. Sons = half of all children.] ---

Before solving, remember these

Key Fact

Sex determination

XY system: humans, Drosophila — male XY, female XX. ZW: birds — female ZW, male ZZ. Haplodiploidy: bees — male haploid (parthenogenesis), female diploid. Y-chromosome carries SRY gene.

-- NCERT Class 12 Biology, Ch. 4, p. 84

Formulas

Mendel's monohybrid ratio

F2 ratio in monohybrid cross — products of independent assortment of two alleles per locus.

SymbolQuantitySI Unit
ratioF2 progeny ratio-

Valid when

  • Single gene with complete dominance
  • Pure-bred parents

Mendel's dihybrid ratio

F2 ratio in dihybrid cross with two independently-segregating loci, complete dominance, no linkage.

SymbolQuantitySI Unit
ratioF2 phenotype ratio-

Valid when

  • Two unlinked loci
  • Complete dominance

Hardy-Weinberg equation

In an idealised population (no mutation, drift, selection, gene flow, random mating), allele and genotype frequencies remain constant.

SymbolQuantitySI Unit
pfreq of dominant allele A-
qfreq of recessive allele a-

Valid when

  • Idealised population
  • All five conditions met
  • Diploid, autosomal, biallelic locus

Recombination frequency (genetic mapping)

Proportion of recombinant offspring measures genetic distance between linked loci. Capped at 50% (independent assortment).

SymbolQuantitySI Unit
RFrecombination frequency%

Valid when

  • Linked loci on same chromosome

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

5' → 3' synthesis polarity

Category: Sign Convention

DNA polymerase synthesises only 5'→3'. Leading strand: continuous, same direction as fork. Lagging: discontinuous (Okazaki), opposite to fork.

When it triggers

Question on Okazaki, leading vs lagging, primer direction.

How to avoid

Reading template 3'→5'; synthesising 5'→3'. Lagging strand needs short fragments because it can't run continuously against fork direction.

Trap

Hardy-Weinberg disequilibrium causes

Category: Negative Marking

Five forces disturb HW: mutation, gene flow, drift, selection, non-random mating. ANY of these violates equilibrium.

When it triggers

Question asks which factor maintains/disturbs HW.

How to avoid

Random mating + no other forces → equilibrium. ANY of mutation/migration/drift/selection/assortative mating → disequilibrium.

Trap

Genotype vs phenotype ratio confusion

Category: Similar Terms

Monohybrid: genotype 1:2:1 (AA:Aa:aa); phenotype 3:1.

When it triggers

Question asks for one ratio while presenting cross details.

How to avoid

Always note dominance: phenotype merges Aa + AA; genotype keeps them separate.

Trap

X-linked recessive: father→daughter expression

Category: Similar Terms

X-linked recessive (haemophilia, colour-blindness): affects males predominantly; carrier mother → 50% sons affected; affected father → all daughters carriers but not affected.

When it triggers

Pedigree question; carrier vs affected.

How to avoid

Sex chromosomes: XX vs XY. Recessive on X needs both copies (XaXa) in female, only one (XaY) in male.

Trap

Sympatric vs allopatric speciation

Category: Similar Terms

Allopatric: geographic isolation. Sympatric: same area, no physical barrier (e.g. polyploidy in plants, host-shift).

When it triggers

Question gives speciation scenario and asks which type.

How to avoid

If geographic barrier mentioned → allopatric. If population overlaps → sympatric.

Past Year Questions

51 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Identify the statement that is NOT correct.

1Constant region of heavy and light chains are located at C-terminus of antibody molecules
2Each antibody has two light and two heavy chains.
3The heavy and light chains are held together by disulfide bonds.
4Antigen binding site is located at C-terminal region of antibody molecules.
NTA Answer: Option 4(final)
NEET 2025

Given below are two statements : Statement I : In the RNA world, RNA is considered the first genetic material evolved to carry out essential life processes. RNA acts as a genetic material and also as a catalyst for some important biochemical reactions in living systems. Being reactive, RNA is unstable. Statement II : DNA evolved from RNA and is a more stable genetic material. Its double helical strands being complementary, resist changes by evolving repairing mechanism. In the light of the above statements, choose the most appropriate answer from the options given below :

1Statement I is incorrect but statement II is correct
2Both statement I and statement II are correct
3Both statement I and statement II are incorrect
4Statement I is correct but statement II is incorrect
NTA Answer: Option 2(final)
NEET 2024

Which one of the following can be explained on the basis of Mendel's Law of Dominance? A. Out of one pair of factors one is dominant and the other is recessive. B. Alleles do not show any expression and both the characters appear as such in F generation. 2 C. Factors occur in pairs in normal diploid plants. D. The discrete unit controlling a particular character is called factor. E. The expression of only one of the parental characters is found in a monohybrid cross. Choose the correct answer from the options given below:

1A, B and C only
2A, C, D and E only
3B, C and D only
4A, B, C, D and E
NTA Answer: Option 2(final)
NEET 2024

Which of the following statement is correct regarding the process of replication in E.coli?

1The DNA dependent DNA polymerase catalyses polymerization in one direction that is 3’ → 5’
2The DNA dependent RNA polymerase catalyses polymerization in one direction, that is 5’ → 3’
3The DNA dependent DNA polymerase catalyses polymerization in 5’ → 3’ as well as 3’ → 5’ direction
4The DNA dependent DNA polymerase catalyses polymerization in 5’ → 3’ direction
NTA Answer: Option 4(final)
NEET 2023

The phenomenon of pleiotropism refers to

1Presence of two alleles, each of the two genes controlling a single trait
2A single gene affecting multiple phenotypic expression
3More than two genes affecting a single character
4Presence of several alleles of a single gene controlling a single crossover
NTA Answer: Option 2(final)
NEET 2023

Expressed Sequence Tags (ESTs) refers to

1All genes that are expressed as proteins.
2All genes whether expressed or unexpressed.
3Certain important expressed genes.
4All genes that are expressed as RNA.
NTA Answer: Option 4(final)
NEET 2023

Which of the following statements are correct about Klinefelter’s Syndrome? A. This disorder was first described by Langdon Down (1866). B. Such an individual has overall masculine development. However, the feminine developement is also expressed. C. The affected individual is short statured. D. Physical, psychomotor and mental development is retarded. E. Such individuals are sterile. Choose the correct answer from the options given below:

1C and D only
2B and E only
3A and E only
4A and B only
NTA Answer: Option 2(final)
NEET 2022

Given below are two statements : Statement I : Mendel studied seven pairs of contrasting traits in pea plants and proposed the Laws of Inheritance. Statement II : Seven characters examined by Mendel in his experiment on pea plants were seed shape and colour, flower colour, pod shape and colour, flower position and stem height. In the light of the above statements, choose the correct answer from the options given below :

1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 2(final)
NEET 2022

Statements related to human Insulin are given below. Which statement(s) is/are correct about genetically engineered Insulin? (a) Pro-hormone insulin contain extra stretch of C-peptide (b) A-peptide and B-peptide chains of insulin were produced separately in E.coli, extracted and combined by creating disulphide bond between them. (c) Insulin used for treating Diabetes was extracted from Cattles and Pigs. (d) Pro-hormone Insulin needs to be processed for converting into a mature and functional hormone. (e) Some patients develop allergic reactions to the foreign insulin. Choose the most appropria

1(c), (d) and (e) only
2(a), (b) and (d) only
3(b) only
4(c) and (d) only
NTA Answer: Option 3(final)
NEET 2021

Match List-I with List-II. (2) (a)-Replication; (b)-Transcription; (c)-Transduction; (d)-Protein (3) (a)-Translation; (b)-Replication; (c)-Transcription;(d)-Transduction (4) (a)-Replication; (b)-Transcription; (c)-Translation; (d)-Protein Answer (4) Choose the correct answer from the options given 127. Which of the following are not secondary below. metabolites in plants? (a) (b) (c) (d)

1Rubber, gums (1) (ii) (i) (iv) (iii)
2Morphine, codeine (2) (ii) (iv) (i) (iii)
3Amino acids, glucose (3) (iv) (iii) (ii) (i)
4(iii) (i) (iv) (ii) (4) Vinblastin, curcumin
NTA Answer: Option 2(final)
NEET 2020

Which of the following refer to correct example(s) of organisms which have evolved due to changes 97. ÁŸêŸ ◊ ∑§ÊÒŸ, ∞ ‚ ¡ËflÊ ∑ § ‚„Ë ©ŒÊ„⁄UáÊÊ ∑§Ê ‚ ŒÁ÷¸Ã ∑§⁄UÃÊ „Ò in environment brought about by anthropogenic ¡Ê ◊ÊŸfl ∑§Ë Á∑˝§ÿÊ•Ê mÊ⁄UÊ flÊÃÊfl⁄UáÊ ◊ ’Œ‹Êfl ∑ § ∑§Ê⁄UáÊ action ? Áfl∑§Á‚à „È∞ „Ò? (a) Darwin’s Finches of Galapagos islands. (a) ªÒ‹Ê¬ÒªÊ mˬ ◊ «UÊÁfl¸Ÿ ∑§Ë Á» §ø (b) Herbicide resistant weeds. (b) π⁄U¬ÃflÊ⁄UÊ ◊ ‡ÊÊ∑§ŸÊ‡ÊË ∑§Ê ¬˝ÁÃ⁄UÊ œ (c) Drug resistant eukaryotes. (c) ‚‚Ë◊∑ §ãŒ˝∑§Ê ◊ ŒflÊßÿÊ ∑§Ê ¬˝ÁÃ⁄UÊ œ (d) Man-created breeds of domesticated animal

1∑ §fl‹ (d) (1) only (d)
2∑ §fl‹ (a) (2) only (a)
3(a) ∞fl (c) (3) (a) and (c)
4(b), (c) ∞fl (d) (4) (b), (c) and (d) Hindi+English 23 H3
NTA Answer: Option 4(final)
NEET 2020

•ŸÈ‹ πŸ ∑ § ‚◊ÿ «UË.∞Ÿ.∞. ∑§Ë ∑È §«U‹Ë ∑§Ê πÊ ‹Ÿ ◊ ∑§ÊÒŸ‚Ê

1RNA polymerase ∞ ¡Êß◊ ◊ŒŒ ∑§⁄UÃÊ „Ò?
2DNA ligase (1) •Ê⁄U.∞Ÿ.∞. ¬ÊÚÁ‹◊⁄ U$¡
3DNA helicase
4DNA polymerase (2) «UË.∞Ÿ.∞. ‹Êߪ $¡ (3) «UË.∞Ÿ.∞. „Ò‹Ë∑ §$¡ 117. Snow-blindness in Antarctic region is due to : (4) «UË.∞Ÿ.∞. ¬ÊÚ‹Ë◊⁄ U$¡ (1) Damage to retina caused by infra-red rays (2) Freezing of fluids in the eye by low
NTA Answer: Option 1(final)
NEET 2020

¡ËŸ ‘I’ ¡Ê ABO ⁄UÄà flª¸ ∑§Ê ÁŸÿ òÊáÊ ∑§⁄UÃÊ „Ò ©‚∑ § ‚ Œ÷¸ 121. Identify the wrong statement with reference to the gene ‘I’ that controls ABO blood groups. ◊ ª‹Ã ∑§ÕŸ ∑§Ê ¬„øÊÁŸ∞–

1Allele ‘i’ does not produce any sugar. (1) ‘i’ ∞ ‹Ë‹ ∑§Ê ߸ ÷Ë ‡Ê∑¸§⁄UÊ ©à¬ÛÊ Ÿ„Ë ∑§⁄UÃÊ–
2The gene (I) has three alleles. (2) ¡ËŸ (I) ∑ § ÃËŸ ∞ ‹Ë‹ „Ê Ã „Ò –
3A person will have only two of the three (3) ∞∑§ √ÿÁÄà ◊ ÃËŸ ◊ ‚ ∑ §fl‹ ŒÊ ∞ ‹Ë‹ „Ê ª – alleles.
4¡’ IA ∞fl IB ŒÊ ŸÊ ß∑§_ „Ê Ã „Ò , ÿ ∞∑§ ¬˝∑§Ê⁄U ∑§Ë (4) When IA and IB are present together, they ‡Ê∑¸§⁄UÊ •Á÷√ÿÄà ∑§⁄Uà „Ò – express same type of sugar.
NTA Answer: Option 4(final)
NEET 2020

≈˛UÊ ‚‹ ‡ÊŸ (•ŸÈflÊŒŸ/SÕÊŸÊ Ã⁄UáÊ) ∑§Ë ¬˝Õ◊ •flSÕÊ ∑§ÊÒŸ ‚Ë 123. The first phase of translation is : „Ê ÃË „Ò?

1Recognition of an anti-codon (1) ∞∑§ ∞ ≈UË-∑§Ê «UÊÚŸ ∑§Ë ¬„øÊŸ
2Binding of mRNA to ribosome (2) ⁄UÊß’Ê ‚Ê ◊ ‚ mRNA ∑§Ê ’㜟
3Recognition of DNA molecule (3) «UË.∞Ÿ.∞. •áÊÈ ∑§Ë ¬„øÊŸ
4Aminoacylation of tRNA (4) tRNA ∑§Ê ∞ ◊ËŸÊ ∞‚Ë‹ ‡ÊŸ
NTA Answer: Option 4(final)
NEET 2020

◊ «U‹ Ÿ Sflà òÊ M§¬ ‚ ¬˝¡ŸŸ ∑§⁄UŸ flÊ‹Ë ◊≈U⁄U ∑ § ¬ÊÒœ ∑§Ë 130. How many true breeding pea plant varieties did Mendel select as pairs, which were similar except Á∑§ÃŸË Á∑§S◊Ê ∑§Ê ÿÈÇ◊Ê ∑ § M§¬ ◊ øÈŸÊ ¡Ê Áfl¬⁄UËà Áfl‡Ê ∑§Ê in one character with contrasting traits ? flÊ‹ ∞∑§ ‹ˇÊáÊ ∑ § •‹ÊflÊ ∞∑§ ‚◊ÊŸ ÕË?

18 (1) 8
24 (2) 4
32 (3) 2
414 (4) 14 H3 30 Hindi+English
NTA Answer: Option 4(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Sources

NCERT refs: Class 12 Biology Chapter 4, p.84

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