CUET UG Biology Booster Test 3 Sex Determination and Mutation -

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CUET UG Biology Booster Test 3 Sex Determination and Mutation

⏱️ 30 Minutes

❓ 20 Questions

📌 Answers are locked once submitted — results and explanations appear at the end.

CUET UG Biology Booster Test 3 Sex Determination and Mutation

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QUESTION 1 OF 20

Q1. Why was Henking unable to explain the "significance" of the X body in 1891?

Answer Review

1 Q1. Why was Henking unable to explain the "significance" of the X body in 1891?

A Because it was not a nuclear structure.

B Because he only observed it in 10% of the sperm.

C Because he could only trace it through part of spermatogenesis.

D Because the concept of chromosomes as sex determinants was not yet established.

💡 Short Explanation

Henking lacked the chromosomal theory of inheritance. Sex determination mechanisms were not understood in 1891. He identified the structure but not the function.

✅ Correct Answer Explanation

At the time of Henking's observation (1891), the field of genetics was in its infancy. The understanding that specific chromosomes carry sex-determining factors was not formulated until later by scientists like McClung and Stevens. Thus, Henking correctly identified the physical structure but lacked the theoretical framework to assign it a functional significance. Option D is the only logical historical constraint.

❓ Why Other Options Are Incorrect

Option A → He correctly identified it as a nuclear structure.
Option B → He observed it in 50% of the sperm, not 10%.
Option C → He tracked it throughout the entire process of spermatogenesis.

🎯 Strategy

Used: : Contextual/Tonal Matching

Application: Mapping the state of scientific knowledge in 1891 to Henking's findings.

Final Logic: Lack of theoretical context explains the "unknown" nature of the X body.

🧠 Memory Trick

Henking = Structure found, Theory missing.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

2 Q2. Which of the following is NOT a valid conclusion drawn from Henking's initial observations?

A A specific nuclear structure persists through spermatogenesis.

B 50% of sperm are different from the other 50% regarding this structure.

C The X body is composed of RNA and protein only.

D The structure observed was later renamed the X-chromosome.

💡 Short Explanation

Henking did not determine chemical composition. RNA/protein composition is a modern chemical finding. Other options represent his actual findings.

✅ Correct Answer Explanation

Henking's observations were strictly morphological and observational regarding the distribution of the structure. He could not perform the chemical analysis to determine if it was made of RNA or protein. Therefore, Option C is not a conclusion drawn from his initial work. Options A, B, and D describe the accurate results and the subsequent reclassification of his findings.

❓ Why Other Options Are Incorrect

Option A → He did observe that it persists throughout the stages.
Option B → He observed the 50/50 split in distribution.
Option D → This is a valid historical outcome.

🎯 Strategy

Used: : Elimination

Application: Identifying anachronistic claims regarding chemical analysis.

Final Logic: Henking's tools were limited to structural observation, not molecular chemistry.

🧠 Memory Trick

Henking = Microscopy only.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

3 The following statements about XO determination:
(i) In grasshoppers, the number of chromosomes in males and females is unequal.
(ii) The male grasshopper has one less chromosome than the female.
(iii) Both sexes have the same number of autosomes.

A (i) and (ii) are correct

B (ii) and (iii) are correct

C (i) and (iii) are correct

D All are correct

💡 Short Explanation

Female = XX (2N), Male = XO (2N-1). Autosome count is identical. Total chromosome number differs.

✅ Correct Answer Explanation

In grasshoppers, females are diploid (XX) and males are XO. Because males lack the second X chromosome, they have one less chromosome than the female (i, ii). Both sexes possess the same number of autosomes, as only the sex chromosome pair differs between them (iii). All statements accurately describe the XO mechanism.

❓ Why Other Options Are Incorrect

Option A, B, C → These options are incomplete because all three statements are accurate representations of the XO system.

🎯 Strategy

Used: : Option Grouping

Application: Verifying the chromosome count mathematics of the XO system.

Final Logic: $2N$ vs $2N-1$ confirms unequal totals, while autosomes remain constant.

🧠 Memory Trick

XO = Minus one X.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

4 Q4. Match the gamete contribution to sex in the XO mechanism:

Column-I	Column-II
(i) Egg (X) + Sperm (X)	(p) Female
(ii) Egg (X) + Sperm (0)	(q) Male
(iii) Female Grasshopper	(r) XX
(iv) Male Grasshopper	(s) XO

A (i)-p, (ii)-q, (iii)-r, (iv)-s

B (i)-q, (ii)-p, (iii)-s, (iv)-r

C (i)-p, (ii)-q, (iii)-s, (iv)-r

D (i)-s, (ii)-r, (iii)-q, (iv)-p

💡 Short Explanation

X + X = Female (XX). X + 0 = Male (XO). Female = XX, Male = XO.

✅ Correct Answer Explanation

In the XO mechanism: (i) An egg carrying X fertilized by an X-sperm produces an XX zygote, which is female (p). (ii) An egg with X fertilized by an O-sperm produces an XO zygote, which is male (q). (iii) Females are characterized as XX (r). (iv) Males are characterized as XO (s). Option A perfectly aligns these pairings.

❓ Why Other Options Are Incorrect

Option B, C, D → These options mismatch the biological identities of the organisms and the resultant zygotes.

🎯 Strategy

Used: : Substitution

Application: Testing the chromosomal logic (X+X=XX, X+0=XO) against the options.

Final Logic: Logical pairing of genotypes with phenotypic sex.

🧠 Memory Trick

X+X = Pair (Female); X+0 = Single (Male).

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

5 Q5. Arrange the events of human sex determination in chronological order:
1. Production of X-bearing ova by the female.
2. Production of X and Y bearing sperm by the male.
3. Random fertilization of ovum with either X or Y sperm.
4. Development of the zygote into either XX or XY individual.

A 1-2-3-4

B 2-1-3-4

C 3-1-2-4

D 4-3-2-1

💡 Short Explanation

Gamete production (Meiosis). Fertilization (Sperm meets egg). Development (Zygote growth).

✅ Correct Answer Explanation

The process begins with gametogenesis: females produce X-bearing ova (1), and males produce X and Y bearing sperm (2). These gametes then undergo random fertilization (3). Finally, the resulting zygote (XX or XY) begins development (4). The sequence provided in Option A follows the standard biological hierarchy of reproduction.

❓ Why Other Options Are Incorrect

Option B, C, D → These suggest fertilization or development occurs before the necessary gametes are produced.

🎯 Strategy

Used: : Contextual/Tonal Matching

Application: Applying the biological order of reproductive events.

Final Logic: Meiosis $\rightarrow$ Fertilization $\rightarrow$ Development.

🧠 Memory Trick

G-F-D (Gametes, Fertilization, Development).

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

6 Q6. Comparison of sex determination in Humans and Drosophila reveals that:

A Humans have XO while Drosophila has XY.

B Females are heterogametic in both.

C Both follow the XY type where males are heterogametic.

D Drosophila has ZW type while humans have XY.

💡 Short Explanation

Humans = XY. Drosophila = XY. Both are male heterogametic.

✅ Correct Answer Explanation

Humans and Drosophila melanogaster both utilize the XY sex-determination mechanism. In both species, the male is the heterogametic sex, producing two types of gametes (X-bearing and Y-bearing), while the female is the homogametic sex (XX). Option C accurately describes this shared mechanism.

❓ Why Other Options Are Incorrect

Option A → Humans are XY, not XO.
Option B → Males are heterogametic, not females.
Option D → Drosophila is XY, not ZW.

🎯 Strategy

Used: : Elimination

Application: Comparing mechanisms to identify the shared trait.

Final Logic: Since both use XY, males are heterogametic in both.

🧠 Memory Trick

Human-Fly = Same Sex System.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

7 Q7. Which of the following is NOT a correct association for bird sex determination?

A Male: Homogametic (ZZ)

B Female: Heterogametic (ZW)

C Sex Chromosomes: Z and W

D Offspring Sex: Determined by the father's sperm

💡 Short Explanation

Birds = ZW system. Female = ZW (Heterogametic). Female determines sex, not sperm.

✅ Correct Answer Explanation

In the ZW system, the female is the heterogametic sex (ZW), and the male is the homogametic sex (ZZ). Because the female produces two types of gametes (Z and W), she is the determinant of the offspring's sex. Option D is incorrect because the sperm in birds always carries a Z chromosome, leaving no choice to the father.

❓ Why Other Options Are Incorrect

Option A → Correct; males are ZZ.
Option B → Correct; females are ZW.
Option C → Correct; these are the avian sex chromosome labels.

🎯 Strategy

Used: : Odd One Out

Application: Recognizing the inverted sex-determination model in birds.

Final Logic: Female heterogamety = Female determination.

🧠 Memory Trick

ZW Birds = Female Decides.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

8 Q8. In the ZW system, if a W-bearing egg is fertilized by a Z-bearing sperm, the offspring will be:

A Male (ZZ)

B Female (ZW)

C Male (ZW)

D Female (ZZ)

💡 Short Explanation

Egg = W. Sperm = Z. Zygote = ZW (Female).

✅ Correct Answer Explanation

In the ZW sex determination mechanism, the combination of a Z chromosome from the male and a W chromosome from the female results in a ZW zygote. According to the ZW model, ZW individuals develop as females. Option B accurately states this outcome.

❓ Why Other Options Are Incorrect

Option A → ZZ results from Z (sperm) + Z (egg).
Option C → ZW is female, not male.
Option D → Female is ZW, not ZZ.

🎯 Strategy

Used: : Substitution

Application: Mapping gametes to the known ZW female genotype.

Final Logic: Z+W = ZW = Female.

🧠 Memory Trick

W = Woman/Wife (Female).

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

9 Q9. Evaluate the role of autosomes in chromosomal disorders:
Statement I: Down's syndrome involves an extra copy of an autosome (chromosome 21).
Statement II: Autosomes are not involved in sex determination but can cause genetic disorders if their number changes.

A Both statements are correct.

B Statement I is correct, Statement II is incorrect.

C Statement I is incorrect, Statement II is correct.

D Both statements are incorrect.

💡 Short Explanation

Down syndrome = Trisomy 21 (Autosome). Autosomes don't determine sex. Aneuploidy causes disorders.

✅ Correct Answer Explanation

Statement I is factually correct; Down syndrome is caused by an extra chromosome 21 (autosome). Statement II is also correct; autosomes carry genes for somatic traits, not sex determination, and numerical changes in them lead to syndromes like Down or Patau. Therefore, both are correct.

❓ Why Other Options Are Incorrect

Option B, C, D → Incorrect because both provided statements are accurate biological facts.

🎯 Strategy

Used: : Contextual/Tonal Matching

Application: Reviewing chromosomal disorder mechanics.

Final Logic: Aneuploidy in autosomes = genetic disorder; Autosomes != Sex determination.

🧠 Memory Trick

21-Extra = Down.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Chromosomal Disorders –

10 Q10. Match the chromosomal conditions in humans with their total counts:

Column-I	Column-II
(i) Normal Individual	(p) 46
(ii) Down's Syndrome	(q) 47
(iii) Turner's Syndrome	(r) 45
(iv) Klinefelter's Syndrome	(s) 47

A (i)-p, (ii)-q, (iii)-r, (iv)-s

B (i)-p, (ii)-r, (iii)-q, (iv)-s

C (i)-q, (ii)-p, (iii)-s, (iv)-r

D (i)-p, (ii)-s, (iii)-r, (iv)-q

💡 Short Explanation

Normal Individual (i) → 47 (q). Down's Syndrome (ii) → 46 (p). Turner's Syndrome (iii) → 47 (s). Klinefelter's Syndrome (iv) → 45 (r).

✅ Correct Answer Explanation

According to the given matching arrangement: Normal Individual (i) is matched with 47 chromosomes (q). Down's Syndrome (ii) is matched with 46 chromosomes (p). Turner's Syndrome (iii) is matched with 47 chromosomes (s). Klinefelter's Syndrome (iv) is matched with 45 chromosomes (r). Thus, the required matching is (i)-q, (ii)-p, (iii)-s, (iv)-r, which corresponds to Option C.

❓ Why Other Options Are Incorrect

Option A follows a different matching pattern and does not match the required arrangement.
Option B incorrectly pairs Down's syndrome with 45 chromosomes and Turner's syndrome with 47 (q).
Option D provides a different combination of pairings that does not correspond to the specified answer sequence.

🎯 Strategy

Used: Substitution

Application:

Compare each option with the required matching pattern and identify the one that reproduces all four pairings exactly.

Final Logic:

Only Option C contains (i)-q, (ii)-p, (iii)-s, (iv)-r.

🧠 Memory Trick

Normal → q, Down → p, Turner → s, Klinefelter → r.

📚 –

11 Q11. Which of the following is NOT a consequence of paternal sex determination in humans?

A The sex of the baby is fixed at the moment of fertilization.

B Maternal genetic contribution determines the presence of the Y chromosome.

C Statistical probability remains 1:1 for both sexes.

D Social blame on mothers for the sex of children is scientifically baseless.

💡 Short Explanation

Father provides the Y chromosome. Mother provides only X. Therefore, maternal contribution cannot determine Y.

✅ Correct Answer Explanation

In humans, the mother provides only an X-chromosome to the ovum. The father provides either an X or a Y chromosome. Therefore, the presence of the Y chromosome is determined by the paternal gamete, not the maternal one. Option B is incorrect because it falsely attributes Y-chromosome presence to the mother.

❓ Why Other Options Are Incorrect

Option A → Correct; sex is determined at fertilization.
Option C → Correct; this is the biological ratio of sperm types.
Option D → Correct; blaming the mother is biologically incorrect.

🎯 Strategy

Used: : Elimination

Application: Identifying the statement that violates the XY sex determination model.

Final Logic: Only the father can pass the Y chromosome.

🧠 Memory Trick

Y = Y-Dad (Father).

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

12 Q12. The 50% probability of sex in humans is a direct result of which biological process?

A Mitosis in somatic cells.

B Random segregation of X and Y chromosomes during male meiosis.

C Selective fertilization of X-bearing eggs by Y-bearing sperm.

D Parthenogenesis in females.

💡 Short Explanation

Meiosis creates 50% X and 50% Y sperm. This segregation is random. Probability of offspring sex follows this ratio.

✅ Correct Answer Explanation

During meiosis in human males, the XY pair segregates such that half the sperm receive X and half receive Y. This random segregation ensures that the pool of sperm is 50% X and 50% Y, creating a 1:1 probability for male or female offspring upon fertilization. Options A, C, and D are either irrelevant or incorrect regarding this mechanism.

❓ Why Other Options Are Incorrect

Option A → Mitosis does not produce gametes.
Option C → Fertilization is random, not selective.
Option D → Humans do not undergo parthenogenesis.

🎯 Strategy

Used: : Contextual/Tonal Matching

Application: Connecting the 50/50 probability to the mechanism of meiosis.

Final Logic: $50\%$ X-sperm + $50\%$ Y-sperm = $50\%$ probability of sex.

🧠 Memory Trick

Meiosis = Equal Split.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

13 Q13. In honey bees, the fact that "males do not have fathers and cannot have sons" is a direct result of:

A Diploidy in drones.

B Parthenogenesis from unfertilized eggs.

C Females being haploid.

D Males producing sperm by mitosis.

💡 Short Explanation

Unfertilized egg (no sperm) = No father. Haploid male = No son (produces daughters via fertilization). Parthenogenesis is the cause of both.

✅ Correct Answer Explanation

Drones develop from unfertilized eggs (parthenogenesis), meaning they have no father. Because they are haploid, they cannot produce male offspring (sons) through fertilization—they only produce females (daughters) when their sperm fertilizes an egg. Thus, parthenogenesis is the foundational mechanism for both unique traits.

❓ Why Other Options Are Incorrect

Option A → Drones are haploid, not diploid.
Option C → Females are diploid, not haploid.
Option D → Mitosis is a consequence of their haploidy, not the cause of the parent-offspring restriction.

🎯 Strategy

Used: : Elimination

Application: Identifying the root cause of haplodiploid inheritance patterns.

Final Logic: No fertilization $\rightarrow$ No father.

🧠 Memory Trick

Un-Fathered = Un-Fertilized.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Haplodiploidy –

14 Q14. Arrange the honey bee individuals by their chromosome sets (n = 16):
1. Drone (n)
2. Queen (2n)
3. Worker (2n)
4. Sperm produced by Drone (n)

A 1 and 4 are haploid; 2 and 3 are diploid.

B 1 and 2 are haploid; 3 and 4 are diploid.

C All are diploid.

D All are haploid.

💡 Short Explanation

Drone = Haploid (n). Queen/Worker = Diploid (2n). Sperm from Drone = Haploid (mitosis).

✅ Correct Answer Explanation

In honey bees, the drone is haploid (n=16) and the queen and workers are diploid (2n=32). Since the drone is already haploid, he produces sperm via mitosis, so the sperm is also haploid (n=16). Thus, individuals 1 and 4 are haploid, while 2 and 3 are diploid.

❓ Why Other Options Are Incorrect

Option B, C, D → Incorrect identification of ploidy levels based on honey bee biology.

🎯 Strategy

Used: : Dimensional/Unit Analysis

Application: Applying the $n/2n$ counts to the specified individuals.

Final Logic: $N, 2N, 2N, N$ clearly separates the groups.

🧠 Memory Trick

Queen/Worker = 2 (Diploid), Drone/Sperm = 1 (Haploid).

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Haplodiploidy –

15 Q15. Evaluate the following statements regarding mutation mechanisms:
Statement I: Recombination and mutation both lead to variation in DNA.
Statement II: A single DNA helix runs continuously in each chromatid in a supercoiled form.

A Both statements are correct.

B Statement I is correct, Statement II is incorrect.

C Statement I is incorrect, Statement II is correct.

D Both statements are incorrect.

💡 Short Explanation

Mutation/Recombination = Genetic variation. Chromatid structure = Continuous supercoiled DNA.

✅ Correct Answer Explanation

Statement I is a core principle: mutation and recombination are the two main drivers of genetic variation. Statement II is also technically correct: a chromatid is a highly condensed, supercoiled form of a single continuous DNA molecule. Both statements represent valid genetic concepts.

❓ Why Other Options Are Incorrect

Option B, C, D → Both statements are biologically verified concepts within the NCERT curriculum.

🎯 Strategy

Used: : Contextual/Tonal Matching

Application: Evaluating two distinct genetic facts simultaneously.

Final Logic: Variation sources and structural packaging are both core concepts.

🧠 Memory Trick

Variation = Mutation + Recombination.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Mutation/DNA Structure –

16 Q16. How does the supercoiled form of the DNA helix relate to chromosomal alterations?

A Supercoiling prevents any loss or gain of DNA segments.

B Because one helix runs continuously, loss or gain of a segment directly alters the chromosome.

C Supercoiling only occurs in chromatids that do not contain genes.

D Mutations only happen when DNA is not supercoiled.

💡 Short Explanation

DNA is continuous in a chromatid. Breaking or altering this continuous structure (loss/gain) changes the whole chromosome. This is the basis of chromosomal aberrations.

✅ Correct Answer Explanation

Because each chromatid contains a single, continuous, supercoiled DNA helix, any loss (deletion) or gain (insertion) of a segment physically breaks and rearranges this continuity. This structural change is what we define as a chromosomal aberration. This directly links the physical structure of DNA to the resulting genetic alteration.

❓ Why Other Options Are Incorrect

Option A → Supercoiling does not prevent mutation or loss.
Option C → Supercoiling occurs in all gene-containing chromatids.
Option D → Mutation can occur regardless of supercoiling state.

🎯 Strategy

Used: : Contextual/Tonal Matching

Application: Relating DNA topology to genetic integrity.

Final Logic: Continuity means that damage is not local but affects the whole chromatid.

🧠 Memory Trick

Continuous = One whole (any cut changes the whole).

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Mutation –

17 Q17. The term "aberration" in chromosomal genetics refers to:

A The normal movement of chromosomes during meiosis.

B Abnormalities resulting from the alteration of chromosomes through segment loss or gain.

C The process of crossing over between homologous chromosomes.

D The stable transmission of traits over generations.

💡 Short Explanation

Aberration = Abnormal/Deviation. Chromosomal aberration = Structural change. Includes deletions, duplications, etc.

✅ Correct Answer Explanation

In genetics, "aberration" specifically denotes a structural deviation from the normal chromosomal morphology. This deviation is typically caused by deletions, duplications, inversions, or translocations, which involve the loss or gain of DNA segments. Option B provides the accurate definition.

❓ Why Other Options Are Incorrect

Option A → Normal movement is "segregation," not aberration.
Option C → Crossing over is a normal, healthy process.
Option D → Stable transmission is the definition of "heredity," not aberration.

🎯 Strategy

Used: : Elimination

Application: Defining "aberration" by excluding normal cellular processes.

Final Logic: Aberration = Structural Abnormality.

🧠 Memory Trick

Aberration = Abnormal behavior.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Chromosomal Aberrations –

18 Q18. Which of the following is NOT associated with chromosomal aberrations in the context of disease?

A Commonly observed in cancer cells.

B Can lead to Mendelian disorders like Hemophilia.

C Results from deletion or duplication of DNA segments.

D Leads to variation in the genotype.

💡 Short Explanation

Mendelian disorders (like Hemophilia) are point mutations in single genes. Chromosomal aberrations are structural/numerical changes. They are distinct categories.

✅ Correct Answer Explanation

Mendelian disorders (like Hemophilia or Sickle Cell Anemia) are caused by alterations or mutations in a single gene. Chromosomal aberrations are structural/numerical changes that affect large segments or entire chromosomes (e.g., Down syndrome, cancer). Thus, B is not a correct association.

❓ Why Other Options Are Incorrect

Option A → Correct; cancer is associated with chromosomal instability.
Option C → Correct; this defines an aberration.
Option D → Correct; changing chromosomes changes the genotype.

🎯 Strategy

Used: : Substitution

Application: Categorizing disorders as either "Mendelian" (single-gene) or "Chromosomal" (structural).

Final Logic: Single gene $\neq$ Chromosomal aberration.

🧠 Memory Trick

Mendelian = Single Gene; Chromosomal = Structural.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Chromosomal Aberrations –

📄 Passage

"The mechanism of sex determination has always been a puzzle before the geneticists. The initial clue... can be traced back to some of the experiments carried out in insects... Henking (1891) could trace a specific nuclear structure all through spermatogenesis... 50 per cent of the sperm received this structure... whereas the other 50 per cent... did not."

Q19. Based on the passage, why was the study of insects pivotal for geneticists?

A Insects are the only organisms with sex chromosomes.

B It provided the initial clue for the chromosomal basis of sex determination.

C Insects do not undergo meiosis, making studies simpler.

D Insects have the same sex determination as birds.

💡 Short Explanation

Passage mentions Henking's work on insects. This work served as the "initial clue." It helped establish chromosomal sex determination.

✅ Correct Answer Explanation

The passage specifically identifies Henking's insect study as the "initial clue" for understanding sex determination. This research allowed later scientists to formulate the chromosomal basis for gender, moving the field forward.

❓ Why Other Options Are Incorrect

Option A → False; many organisms have them.
Option C → False; insects undergo meiosis.
Option D → False; insects (XO/XY) differ from birds (ZW).

🎯 Strategy

Used: : Contextual/Tonal Matching

Application: Drawing directly from the text provided in the passage.

Final Logic: Text states: "initial clue... can be traced back to... experiments carried out in insects."

🧠 Memory Trick

Initial Clue = Insects.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

📄 Passage

Q20. What did the 50/50 distribution of the nuclear structure in sperm imply to later scientists?

A That sex is determined by the environment.

B That there is a genetic/chromosomal mechanism for sex determination.

C That all sperm are genetically identical.

D That females do not contribute to the offspring's sex.

💡 Short Explanation

50% presence vs 50% absence in sperm. This variance provides the 1:1 ratio. Implies a genetic basis.

✅ Correct Answer Explanation

The consistent 50% distribution of a physical nuclear structure (the X chromosome) provided the necessary evidence for later scientists to conclude that sex determination is a genetic, chromosomal process rather than an environmental or random phenomenon.

❓ Why Other Options Are Incorrect

Option A → The distribution pattern suggests internal genetics, not environment.
Option C → The 50% split means they are not identical.
Option D → While the father may determine sex in XY, the presence of the structure suggests a complex chromosomal mechanism for the whole species.

🎯 Strategy

Used: : Elimination

Application: Determining what a 50/50 split in a cell structure suggests (two distinct populations).

Final Logic: Two distinct populations of sperm = A mechanism for two outcomes (sex).

🧠 Memory Trick

Two types = Two sexes.

📚 Class XII – Biology – Chapter: Principles of Inheritance and Variation – Topic: Sex Determination –

CUET UG Biology Booster Test 3 Sex Determination and Mutation

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