આ MCQ મોડ્યુલ આના પર આધારિત છે: Chromosomal Theory Linkage
ટોપિક 17 / 64
Chromosomal Theory Linkage
🎓 Class 12
Biology
CBSE
Theory
Ch 4 – Principles of Inheritance and Variation
⏱ ~14 min
🌐 ભાષા:
🧠 AI-આધારિત MCQ મૂલ્યાંકન
▲
📝 વર્કશીટ / મૂલ્યાંકન
▲
આ મૂલ્યાંકન આના પર આધારિત હશે: Chromosomal Theory Linkage
મૂલ્યાંકન બનાવવામાં તેમની સામગ્રી સામેલ કરવા ચિત્રો, PDF અથવા Word દસ્તાવેજ અપલોડ કરો.
Chromosomal Theory Linkage
5.7 Chromosomal Theory of Inheritance
Mendel's laws were rediscovered in 1900, but no one knew where genes physically resided. By that time, biologists had discovered chromosomes in the nucleus and had observed their behaviour during meiosis. In 1902, Walter Sutton and Theodor Boveri independently noticed a striking parallel: chromosomes behave during meiosis exactly like Mendel's 'factors' (genes).
Chromosomal Theory of Inheritance: Genes are located on chromosomes, and chromosomes serve as the physical vehicles of Mendel's laws. The pairing and separation of chromosomes during meiosis is the cellular basis of segregation and independent assortment.
| Mendel's Factor (Gene) | Behaviour of Chromosomes |
|---|---|
| Factors come in pairs | Chromosomes are in homologous pairs (2n) |
| One factor per gamete (Law of Segregation) | Homologous chromosomes separate during meiosis I |
| Different factor pairs assort independently | Non-homologous chromosomes assort independently in metaphase I |
| Factors recombine at fertilisation | Sperm + egg restore the diploid (2n) state |
5.8 Linkage and Recombination — Morgan's Work
Thomas Hunt Morgan chose Drosophila melanogaster (fruit fly) for genetic experiments because it has a short life cycle, breeds in laboratory milk bottles, produces hundreds of offspring per cross, has only 4 pairs of chromosomes, and shows clear sex differences.
When Morgan crossed flies with two pairs of traits located on the same chromosome, he did NOT get the expected 9:3:3:1 ratio. Instead, parental combinations were over-represented. This phenomenon is called linkage.
However, some offspring carried new combinations not seen in either parent. Morgan called these recombinants, produced by crossing-over between homologous chromosomes during meiosis I.
Morgan's student Alfred Sturtevant used recombination frequencies to construct the first genetic map — a diagram showing the relative positions of genes on a chromosome. The greater the distance between two genes, the more often crossing-over occurs between them.
5.9 Sex Determination
The mechanism by which an organism's sex is determined depends on a special pair of chromosomes called sex chromosomes. There are several distinct sex-determination systems in nature.
5.9.1 The XY System (Humans, Drosophila)
In humans, the sex chromosomes are X and Y. Females have two X chromosomes (XX); males have one X and one Y (XY). This is also called male heterogamety — males produce two types of gametes (X-bearing and Y-bearing).
- Female (XX) produces eggs with X only.
- Male (XY) produces sperm with X (50%) or Y (50%).
- Outcome: X-egg + X-sperm → XX (girl); X-egg + Y-sperm → XY (boy). Sex is determined by the father's sperm.
5.9.2 The XO System (Grasshoppers)
In grasshoppers and many insects, females are XX and males are XO — males have only one sex chromosome (X) with no partner. Males produce sperm with X (50%) or no sex chromosome (50%). This is also a case of male heterogamety.
5.9.3 The ZW System (Birds)
In birds, butterflies, and some fish, the system is reversed: males are ZZ (homogametic) and females are ZW (heterogametic). The W chromosome is found only in females — so the female determines the offspring's sex! This is female heterogamety.
| System | Female | Male | Heterogametic sex | Examples |
|---|---|---|---|---|
| XY | XX | XY | Male | Humans, mammals, Drosophila |
| XO | XX | XO | Male | Grasshoppers, cockroaches |
| ZW | ZW | ZZ | Female | Birds, butterflies, some fish |
5.10 Sex-linked Inheritance
Genes located on the X chromosome are called X-linked. Because males have only one X, X-linked recessive traits show up more often in males. Classic examples: haemophilia and colour blindness.
🧬 Interactive: Pedigree Predictor — X-linked Inheritance
Choose parental genotypes to see the chance of haemophilia in offspring (XH = normal, Xh = haemophilic):
📐 Activity 5.3 — Pedigree Analysis
Setup: A normal-looking woman has a haemophilic father. She marries a normal-looking man (no family history of haemophilia).
Predict: (a) What is the genotype of this woman? (b) What is the chance their son has haemophilia? (c) What about their daughter?
(a) Her father is XhY. Father gives daughter his X. So she gets Xh from father + XH from mother → genotype XHXh (carrier).
(b) Sons: Mother contributes XH (50%) or Xh (50%); Father contributes Y. So sons are XHY (normal, 50%) or XhY (haemophilic, 50%). 50% chance.
(c) Daughters: Mother contributes XH or Xh; Father contributes XH. So daughters are XHXH (normal, 50%) or XHXh (carrier, 50%). 0% will have haemophilia, but 50% will be carriers.
Worked Examples
Worked Example 1: Recombination Frequency
In a Drosophila test cross, 1000 offspring were observed: 380 yellow-vestigial, 380 grey-normal, 120 yellow-normal, 120 grey-vestigial. Calculate the recombination frequency (map distance).
Step 1: Identify parental vs recombinant. Most common = parental → 380 + 380 = 760 (parental). Least common = recombinant → 120 + 120 = 240.
Step 2: Recombination frequency = (recombinant offspring / total) × 100
RF = (240 / 1000) × 100 = 24%
Step 3: Map distance = 24 map units (centiMorgan, cM).
Conclusion: The two genes are 24 cM apart on the chromosome.
Step 2: Recombination frequency = (recombinant offspring / total) × 100
RF = (240 / 1000) × 100 = 24%
Step 3: Map distance = 24 map units (centiMorgan, cM).
Conclusion: The two genes are 24 cM apart on the chromosome.
Worked Example 2: Predict Sex of Child
What is the probability that a couple's third child is female if their first two children are male?
Each fertilisation is an independent event — the previous outcomes do not affect the next.
P(female) = P(X-bearing sperm fertilises egg) = 50% (1/2).
The "gambler's fallacy" says past pattern affects the future — biology disagrees! Each child has independent 50:50 chance.
P(female) = P(X-bearing sperm fertilises egg) = 50% (1/2).
The "gambler's fallacy" says past pattern affects the future — biology disagrees! Each child has independent 50:50 chance.
🎯 Competency-Based Questions
Q1. The chromosomal theory of inheritance was proposed by:L1 Remember
Answer: (b). Walter Sutton and Theodor Boveri (1902) independently proposed that chromosomes are the physical carriers of Mendel's genes — based on observed parallels between chromosome behaviour in meiosis and Mendel's laws.
Q2. Fill in the blank: In humans, the sex of a child is determined by the _____. L2 Understand
Answer: Sperm of the father (specifically the X- or Y-bearing chromosome it carries). Mothers contribute only X chromosomes through eggs. Fathers produce both X-bearing and Y-bearing sperm — so the father's sperm determines whether the child is XX (girl) or XY (boy).
Q3. A colour-blind man marries a woman heterozygous for colour blindness. What is the probability their daughter is colour-blind? L3 Apply
Answer: Father is XcY (colour-blind). Mother is XCXc (carrier). Daughter receives Xc from father (always) and either XC or Xc from mother (50%/50%). So daughter is XCXc (carrier, 50%) OR XcXc (colour-blind, 50%). P(colour-blind daughter) = 50%.
Q4. Analyse: Why are X-linked recessive disorders much more common in males than females? L4 Analyse
Reason — single X in males:
- Males have only ONE X chromosome (XY). A single recessive allele on that X is unmasked — it expresses immediately.
- Females have TWO X chromosomes (XX). A single recessive allele can be masked by a normal allele on the other X — so females are usually only carriers.
- For a female to express the disorder, she must inherit the recessive allele from BOTH parents (rare event).
Q5. HOT (Create): Design a hypothetical genetic cross to test whether a newly discovered fly trait is X-linked or autosomal. L6 Create
Reciprocal cross strategy:
- Cross 1: Mutant female × Wild-type male → observe F1 sons and daughters.
- Cross 2 (reciprocal): Wild-type female × Mutant male → observe F1.
- Predicted outcome if AUTOSOMAL recessive: Both reciprocal crosses give same F1 result (all heterozygous, mutant phenotype hidden).
- Predicted if X-LINKED recessive:
- Cross 1 (mutant ♀ × WT ♂): all sons are mutant (get X from mother), all daughters are heterozygous carriers (look wild-type).
- Cross 2 (WT ♀ × mutant ♂): all sons are wild-type (get X from mother only), all daughters are heterozygous carriers (look wild-type).
- The DIFFERENCE between reciprocal cross results in F1 reveals X-linkage.
🧠 Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Linked genes show recombination frequencies less than 50%.
R: Crossing-over between linked genes occurs in only some meioses.
Answer: (A). Both true; R explains A. Linked genes inherit together more often than not, so recombinant frequency < 50%. Independently assorting (unlinked) genes give 50% recombination — equal parental and recombinant offspring.
A: Sex of an offspring is determined by the mother in the human XY system.
R: Mother contributes only X chromosomes through her eggs.
Answer: (D). A is FALSE — sex is determined by the FATHER, not the mother. R is TRUE — mother indeed contributes only X. The father is heterogametic (X or Y sperm), so his sperm decides offspring sex.
A: In birds, female heterogamety operates.
R: Female birds are ZW while male birds are ZZ.
Answer: (A). Both true; R explains A. ZW females produce two types of gametes (Z or W eggs), while ZZ males produce only Z sperm — so the female determines offspring sex in birds.
Frequently Asked Questions - Chromosomal Theory Linkage
What is the main concept covered in Chromosomal Theory Linkage?
In NCERT Class 12 Biology Chapter on Principles of Inheritance and Variation, "Chromosomal Theory Linkage" covers the core biological structures, processes, and pathways students need for board exam success. The MyAiSchool lesson explains the topic with definitions, labelled diagrams, comparison tables, and interactive simulations. Scientific terminology and physiological/genetic significance are highlighted throughout to build conceptual depth aligned with CBSE 2025-26 syllabus.
How is Chromosomal Theory Linkage useful in real-life or applied biology?
Real-life applications of "Chromosomal Theory Linkage" from NCERT Class 12 Biology Principles of Inheritance and Variation include medical diagnostics, agriculture, biotechnology, public health, evolutionary insights, and ecological monitoring. The MyAiSchool lesson links every biological concept to a tangible application so students see biology as a problem-solving framework for living systems and real-world challenges.
What are the key terms students should memorize for Chromosomal Theory Linkage?
Key terms in "Chromosomal Theory Linkage" (NCERT Class 12 Biology Principles of Inheritance and Variation) are tabulated in the MyAiSchool key-terms grid. Students should memorize each term with its precise definition, function, and example. Terminology is high-yield in CBSE board exams — 1-mark MCQs and 2-mark short answers test definitions directly. The Summary section provides a printable quick-reference card.
How does this part connect to other parts of the chapter?
NCERT Class 12 Biology Principles of Inheritance and Variation is structured so each part builds biological understanding sequentially. "Chromosomal Theory Linkage" connects to neighbouring parts via shared mechanisms, structural hierarchies, and physiological processes. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected biological story rather than disconnected fragments.
What types of CBSE board questions come from Chromosomal Theory Linkage?
CBSE board questions from "Chromosomal Theory Linkage" typically include: (1) 1-mark MCQs on definitions and processes, (2) 2-mark short-answer differences/comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining mechanism + diagram + significance. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Chromosomal Theory Linkage" lesson allows students to explore biological processes, classifications, or pathways using selectors and sliders, with live visual feedback. To use it effectively: (1) explore each option/state, (2) compare with textbook diagrams, (3) note the function/outcome changes, (4) try the integrated practice quiz. The simulation reinforces visual-spatial understanding that pure text-based study cannot.
AI ટ્યુટર
Biology Class 12 – NCERT (2025-26)
તૈયાર
🎁 Join our community and get free AI credits!