Science
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Science — CBSE Class X Sample Paper 1 (2025-26)
Section A · Section B · Section C
આ MCQ મોડ્યુલ આના પર આધારિત છે: Sex Determination and Inherited vs Acquired Traits
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Sex Determination and Inherited vs Acquired Traits
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આ મૂલ્યાંકન આના પર આધારિત હશે: Sex Determination and Inherited vs Acquired Traits
મૂલ્યાંકન બનાવવામાં તેમની સામગ્રી સામેલ કરવા ચિત્રો, PDF અથવા Word દસ્તાવેજ અપલોડ કરો.
8.2.3 How Do These Traits Get Expressed?
We said that each gene controls one trait. But what is the physical link between a DNA segment and a visible feature like tallness? The answer lies in proteins.
A gene is a set of instructions for making one specific protein. That protein goes on to do a particular job inside the cell or body — and the sum of such jobs is what we recognise as the trait.
Example — tallness in pea plants: the T allele codes for an efficient enzyme that helps the plant make a growth hormone in the normal quantity → the plant grows tall. The t allele codes for a less efficient enzyme → less hormone is produced → the plant stays dwarf. The gene itself has no "height"; it decides height only through the protein it creates.
Gene → Protein → Trait. A change (mutation) in a gene changes the protein it codes for, and so can change the trait. This simple chain is the physical basis of all inheritance.
8.2.4 Sex Determination
Most human traits follow Mendel's laws, but one pair of chromosomes gets special treatment — the sex chromosomes. They decide whether the child will be biologically female or male.
- Female: two identical sex chromosomes — XX. All her eggs carry one X.
- Male: two different sex chromosomes — XY. Half his sperms carry an X, the other half carry a Y.
At fertilisation:
- Egg (X) + Sperm (X) → XX → girl
- Egg (X) + Sperm (Y) → XY → boy
Notice that the mother always contributes an X (she has nothing else to give). It is the father's sperm — whether X-carrying or Y-carrying — that decides the sex of the child. The chance is 50 % for each.
Important social point: Since the mother can only give an X chromosome, it is genetically impossible for her to "decide" the sex of the child. Blaming a mother (or a wife) for the birth of a daughter is unscientific. The chromosome theory of sex determination clearly shows that the father's sperm decides the child's sex and the chance is exactly 50 : 50.
Other species: Sex determination is not the same everywhere. In some reptiles (turtles, crocodiles) the temperature at which the eggs are incubated decides the sex — high temperatures give rise to more of one sex, low of the other.
8.3 Inherited vs. Acquired Traits — Why Evolution Builds on Genes
Not every feature of an individual is passed to its offspring. Traits come in two kinds:
Inherited Traits
Inherited traits are written into the DNA. They are passed on through the egg and sperm from parent to child, generation after generation.
Examples: eye colour, natural skin colour, blood group (A, B, AB, O), attached/free earlobes, widow's peak, sickle-cell condition, the natural hair form.
Acquired Traits
Acquired traits are developed during the lifetime of an individual in response to the environment — through use, training, diet or accidents. They are not coded in the DNA and therefore are not inherited.
Examples: muscles built by a weightlifter, tanning of skin in the sun, scars or burns on the body, a pierced earlobe, the ability to swim, fluency in English, knowledge of a song.
Why are acquired traits not passed on? Only changes that take place in the DNA of the reproductive cells (eggs and sperms) can reach the next generation. Changes in the body cells (muscles, skin, brain) stay with that individual alone and die with them. A weightlifter's children are not born with bigger muscles; they inherit only the DNA he was born with.
Connection with evolution: Because only inherited variations reach the next generation, evolution can act only on those. When the environment changes, individuals carrying a useful inherited variation survive better, have more offspring and pass that variation on. Over many generations the whole population shifts. Acquired characters — however useful to the individual — are a dead end for the species.
Activity 8.2 — Family Trait SurveyL3 Apply
Aim: To observe how traits are distributed in a family and distinguish inherited from acquired features.
Procedure:
- Choose any five family members (parents, grandparents, siblings).
- For each person note: (a) attached or free earlobes, (b) ability to roll the tongue, (c) straight or curly hair, (d) any scar or tattoo, (e) ability to ride a bicycle.
- Tabulate the results. Mark the first three columns as "likely inherited" and the last two as "acquired".
Predict: Will the earlobe type of a child usually match at least one parent? Will the bicycle-riding ability match at least one parent?
The first three traits are mostly genetic. A child usually shows a trait found in one of the parents or grandparents — you can often trace it on a family tree. The last two (scar, bicycle riding) are purely acquired — a child is not born with a parent's scar, nor with the ability to cycle. The survey makes the distinction between inherited and acquired traits concrete and memorable.
Interactive — "Inherited or Acquired?"
Click each trait to classify it. Green = Inherited, Blue = Acquired. Try to score 8/8.
Blood group A
Tanned skin after summer
Attached earlobes
Muscles of a weightlifter
Natural curly hair
Ability to play the guitar
Tongue-rolling ability
A surgical scar
Score: 0 / 8
Competency-Based Questions
In a village, people still blame a woman if she gives birth to only daughters. A Class 10 student, Arjun, decides to explain the science of sex determination to his relatives, using chromosomes and a Punnett square.
Q1. Using sex chromosomes, explain why blaming the mother for the child's sex is scientifically wrong. L2 Understand
A mother is XX, so every egg she produces carries an X chromosome — she has nothing else to give. A father is XY; half his sperms carry X and half carry Y. If an X-sperm fertilises the egg, the child is XX (girl); if a Y-sperm does, the child is XY (boy). Therefore the father's sperm decides the sex — a 50 : 50 chance. The mother cannot be held responsible.
Q2. (MCQ) A human male produces four types of gametes with respect to sex chromosomes. The statement is L1 Remember
(b) False. A human male produces only two types of sperm with respect to the sex chromosome: X-carrying and Y-carrying, in roughly equal numbers.
Q3. A bodybuilder trains for ten years and develops huge muscles. Will his son be born with big muscles? Give a reason. L3 Apply
No. Muscle size gained by exercise is an acquired trait — it is a change in body (muscle) cells, not in the DNA of the gamete-producing cells. The son inherits only the father's original DNA; he will need to exercise himself to build such muscles.
Q4. Fill in the blank: The transfer of traits from parents to offspring is called __________ and the differences produced during reproduction are called __________. L1 Remember
Heredity; variation.
Q5. Why does evolution depend on inherited variations and not on acquired ones? L4 Analyse
Only variations that are coded in the DNA of the reproductive cells are transferred to the next generation. Acquired changes affect only the individual's body cells and die with the individual. Evolution, which is a change in the inherited make-up of a population over many generations, can therefore be driven only by inherited (genetic) variations that natural selection can act upon.
Assertion–Reason Questions
Options: (A) Both A & R true, R correctly explains A. (B) Both A & R true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
Assertion (A): The sex of a human child is determined by the father.
Reason (R): The father can contribute either an X- or a Y-bearing sperm, while the mother's eggs all carry an X chromosome.
(A) — Both true; R correctly explains A.
Assertion (A): Muscles developed by a weightlifter are not passed on to his children.
Reason (R): Acquired traits cause changes in body cells but not in the DNA of reproductive cells.
(A) — Both true; R correctly explains A.
Assertion (A): Genes express traits by producing proteins.
Reason (R): Proteins act as enzymes, hormones or structural units that carry out the task responsible for the trait.
(A) — Both true; R correctly explains A.
Frequently Asked Questions — Sex Determination & Inherited vs Acquired Traits
What is sex determination & inherited vs acquired traits in Class 10 Science (CBSE board)?
Sex Determination & Inherited vs Acquired Traits is a key topic in NCERT Class 10 Science Chapter 8 — Heredity. It explains how sex is determined in humans and the difference between inherited and acquired traits. Core ideas covered include sex chromosomes, X chromosome, Y chromosome, sex determination. Mastering this subtopic is essential for scoring well in the CBSE Class 10 Science board exam because board papers repeatedly test these concepts through MCQs, short answers and long-answer questions. This part gives a complete, exam-ready explanation with activities, diagrams and competency-based practice aligned to NCERT.
Why is sex chromosomes important in NCERT Class 10 Science?
Sex chromosomes is important in NCERT Class 10 Science because it forms the foundation for understanding sex determination & inherited vs acquired traits in Chapter 8 — Heredity. Without a clear idea of sex chromosomes, students cannot answer higher-order CBSE board questions involving X chromosome, Y chromosome, sex determination. Board papers regularly include 2-mark and 3-mark questions on this concept, and competency-based questions often link sex chromosomes to real-life situations. Building clarity here pays off directly in board marks.
How is sex determination & inherited vs acquired traits tested in the Class 10 Science CBSE board exam?
The CBSE Class 10 Science board exam tests sex determination & inherited vs acquired traits through a mix of 1-mark MCQs, 2-mark short answers, 3-mark explanations with examples, 5-mark descriptive questions (often with diagrams or balanced equations) and 4-mark competency-based questions. Expect direct questions on sex chromosomes, X chromosome, Y chromosome and application-based questions drawn from NCERT activities. Students who follow NCERT thoroughly and practice this chapter's questions consistently score in the 90%+ range.
What are the key terms to remember for sex determination & inherited vs acquired traits in Class 10 Science?
The key terms to remember for sex determination & inherited vs acquired traits in NCERT Class 10 Science Chapter 8 are: sex chromosomes, X chromosome, Y chromosome, sex determination, inherited trait, acquired trait. Each of these concepts carries exam weightage and regularly appears in the CBSE board paper. Write clear one-line definitions of every term in your revision notes and revisit them before the exam. Linking these terms visually through a flowchart or concept map makes recall easier during the Class 10 Science board exam.
Is Sex Determination & Inherited vs Acquired Traits included in the Class 10 Science syllabus for 2025–26 CBSE board exam?
Yes, Sex Determination & Inherited vs Acquired Traits is a part of the NCERT Class 10 Science syllabus (2025–26) prescribed by CBSE. It falls under Chapter 8 — Heredity — and is examined in the annual board paper. The current syllabus retains the full treatment of sex chromosomes, X chromosome, Y chromosome as per the NCERT textbook. Because CBSE bases every board question on NCERT, studying this part thoroughly ensures complete syllabus coverage and guarantees marks from this chapter.
How should I prepare sex determination & inherited vs acquired traits for the CBSE Class 10 Science board exam?
Prepare sex determination & inherited vs acquired traits for the CBSE Class 10 Science board exam in three steps. First, read this NCERT part carefully, highlighting definitions and diagrams of sex chromosomes, X chromosome, Y chromosome. Second, solve every in-text question and end-of-chapter exercise — CBSE questions often come directly from NCERT. Third, practice competency-based and assertion-reason questions to sharpen reasoning. Write answers in the exam-style format (point-wise with diagrams) and time yourself. This method delivers confidence and full marks in the board exam.
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