Q.(a) Analyse the following two cases of sex determination in different organisms : Case I : Males have (XO) sex chromosomes and females have two copies of same sex chromosome (XX). Case II : Females have two different sex chromosomes (ZW) and males have two copies of same sex chromosome (ZZ). Identify the type of heterogamety in each case, giving one example of each.
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Sex Determination — Ethics and Social Misuse
Sex Determination Ethics: A First Look
Imagine a family waiting for a baby. Relatives whisper, "Hope it's a boy" or "Thank God it's a girl." That casual reaction carries a heavy ethical weight. Sex determination ethics is the study of the moral questions surrounding the practice of choosing or knowing the sex of a child before birth, and the social consequences that follow.
The Core Idea
At its simplest, sex determination ethics examines whether it is morally right to use medical technology to find out whether an unborn baby is male or female, and then to act on that information — especially by aborting a fetus of the "unwanted" sex. The ethical problem is not about the technology itself (ultrasound, amniocentesis), but about what people do with that knowledge.
The NCERT textbook for Class 12 (Biology, Chapter 4: Reproductive Health) states this clearly: "Amniocentesis — a technique to determine the sex of the unborn child — is being misused to kill the female foetuses." The textbook calls this a "severe" misuse and links it directly to the declining child sex ratio in India.
Why Does This Become an Ethical Issue?
Three things make sex determination an ethical problem, not just a medical one:
- Discrimination against females. The practice is overwhelmingly used to abort female fetuses. This treats being female as a defect or a burden, which is a form of gender-based violence before birth.
- Social imbalance. When many more boys are born than girls, society faces a shortage of women. This leads to forced marriages, trafficking, and increased violence against women. The NCERT notes that the child sex ratio in India has dropped alarmingly — from 945 females per 1000 males in 1991 to 919 in 2011.
- Violation of the child's right to exist. The fetus is not given a chance to live simply because of its sex. This raises deep questions about equality and the value of every human life.
The Indian government made sex determination illegal under the Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act, 1994. This law bans doctors from telling parents the sex of the fetus, and bans any advertisement or test done solely for sex selection. Violation can lead to imprisonment and heavy fines.
The Everyday Intuition
Think of it this way: if you were told that your school would only admit boys next year, you would immediately see that as unfair. Sex determination ethics extends that same intuition to the womb. It asks: Is it fair to decide that a girl should not be born, simply because she is a girl?
The answer, from an ethical standpoint, is no. Every child — boy or girl — has equal moral worth. Using technology to eliminate an entire sex is a form of systematic discrimination that harms individuals and destabilises society.
Why It Matters for a Commerce/Humanities Student
You might think this is a "science topic," but it is deeply connected to what you study:
- Economics: A skewed sex ratio affects labour markets, marriage markets, and long-term economic growth. Fewer women mean fewer workers, consumers, and entrepreneurs.
- Political Science: Laws like the PCPNDT Act are examples of the state intervening to protect fundamental rights (right to life, right to equality). You can analyse how effective such laws are.
- Sociology: Sex determination reflects deep-rooted patriarchy, son preference, and dowry practices. It is a case study in how social norms shape — and are shaped by — technology.
- Ethics (if you study it): This is a classic dilemma: individual choice (parents wanting a son) versus social good (gender balance). Where do you draw the line?
A Few Key Points to Remember …
Part (b)Concept understanding — Nitrogenous Bases vs Nucleosides
Let’s start with something you already know: the alphabet. The English alphabet has 26 letters. You can string them together to make words, sentences, whole books. But a letter by itself is just a symbol — it has no meaning until it’s part of a word.
In the world of biology, nitrogenous bases are like those letters. They are the fundamental chemical units that carry genetic information. A nucleoside is like a single letter that has been given a “handle” — a sugar molecule attached to it — so it can be picked up and used to build the real genetic material.
What is a Nitrogenous Base?
A nitrogenous base is a nitrogen-containing molecule that acts as the “information-carrying” part of DNA and RNA. There are five main ones, but you only need to remember two families:
- Purines (double-ringed): Adenine (A) and Guanine (G)
- Pyrimidines (single-ringed): Cytosine (C), Thymine (T), and Uracil (U)
Think of purines as the larger, heavier letters (like ‘A’ and ‘G’ in a font), and pyrimidines as the smaller ones (C, T, U). In DNA, A pairs with T, and G pairs with C. In RNA, U replaces T.
The NCERT textbook (Class 11, Chapter 9) defines nitrogenous bases as “nitrogen-containing heterocyclic compounds.” That’s a fancy way of saying they are ring-shaped molecules that contain nitrogen atoms. You don’t need to memorise the rings — just know they are the “letters” of the genetic code.
What is a Nucleoside?
A nucleoside is simply a nitrogenous base plus a sugar molecule (ribose in RNA, deoxyribose in DNA). The sugar acts like a handle or a backbone attachment point.
So:
Nucleoside = Nitrogenous base + Sugar
For example:
- Adenine + ribose = Adenosine
- Guanine + deoxyribose = Deoxyguanosine
- Cytosine + ribose = Cytidine
Notice the naming: purine bases end in “-osine” (adenosine, guanosine), and pyrimidine bases end in “-idine” (cytidine, thymidine, uridine). That’s a small but useful pattern.
A nucleoside is not the same as a nucleotide. A nucleotide is a nucleoside plus a phosphate group. That phosphate is what allows nucleotides to link together into long chains — the actual DNA or RNA strand. So:
Base → add sugar → nucleoside → add phosphate → nucleotide → link together → nucleic acid (DNA/RNA)
Why Does This Distinction Matter?
If you’re studying this for a commerce or humanities exam, you won’t be asked to draw chemical structures. But you will be asked to differentiate between these terms, and to understand their roles.
Here’s the key takeaway:
- Nitrogenous bases are the “letters” — they store the genetic information.
- Nucleosides are the “letters with a handle” — they are the building blocks that get activated (by adding phosphate) to become nucleotides.
- Nucleotides are the actual “bricks” that build DNA and RNA.
In NCERT, the distinction is clearly stated: “A nucleoside is composed of a nitrogenous base and a pentose sugar. A nucleotide is a nucleoside with a phosphate group.” That’s the exact line you should remember.
A Quick Comparison
| Feature | Nitrogenous Base | Nucleoside |
|---|---|---|
| What it contains | Only the base (A, G, C, T, U) | Base + sugar (ribose or deoxyribose) |
| Role | Carries genetic information | Intermediate building block |
| Example | Adenine | Adenosine |
| Found freely in cells? | Yes, as free bases | Rarely; usually converted to nucleotides |
One Common Mistake to Avoid …
Part (a)
Case I (XO males, XX females): the male is heterogametic (produces two kinds of sperm — X-bearing and no-sex-chromosome), the female is homogametic. This is male heterogamety (XO type). Example: grasshopper/insects. …
Part (a): XO (Case I) is male heterogamety (e.g., grasshopper); ZW (Case II) is female heterogamety (e.g., birds).
Part (b): DNA nucleotide has deoxyribose + thymine while RNA has ribose + uracil; each nucleotide contains an N-glycosidic bond and a phosphoester bond.
Part (a)
Concept-first idea: The sex that produces two different kinds of gametes is heterogametic; the other, producing one kind, is homogametic.
Case I — XO / XX. Females are XX (produce only X eggs → homogametic). Males are XO and produce two kinds of sperm: X-bearing and those with no sex chromosome (O). Therefore the male is heterogametic → male heterogamety (XO type).
Example: grasshopper (and many insects).
Case II — ZW / ZZ. Males are ZZ (produce only Z sperm → homogametic). Females are ZW and produce two kinds of eggs: Z-bearing and W-bearing. Therefore the female is heterogametic → female heterogamety (ZW type). …
Showing the 12 most recent of 18 on this concept.
- CBSE 2026Set V11 markMCQQ.Statement I : A nitrogenous base is linked to the -OH of IC pentose sugar through N-glycosidic linkage to form a nucleoside. Statement II : Two nucleosides are linked through 3'-5' phosphodiester linkage to form a dinucleoside.(a) Both statement I and statement II are correct(b) Both statement I and statement II are incorrect(c) Statement I is correct, but statement II is incorrect(d) Statement I is incorrect but statement II is correct
›Reveal solutionSolution
A nucleoside forms by an N-glycosidic bond (I is correct); but a phosphodiester bond joins nucleotides to give a dinucleotide, not nucleosides to give a dinucleoside (II is wrong).
Statement I is correct: a nitrogenous base linked to the -OH of the 1'C of the pentose sugar by an N-glycosidic linkage forms a nucleoside. …
- CBSE 2026Set ANNUAL1 markMCQQ.Which nitrogenous base is found in both DNA and RNA?(a) Cytosine(b) Thymine(c) Uracil(d) All of the above
›Reveal solutionSolution
DNA and RNA share adenine, guanine and cytosine; thymine is unique to DNA and uracil is unique to RNA.
Both DNA and RNA contain the purines adenine and guanine, and the pyrimidine cytosine. The other pyrimidine differs between the two: DNA uses thymine, while RNA use …
- CBSE 2026Set ANNUAL1 markQ.State whether True or False: Nitrogenous base Uracil present in DNA.
›Reveal solutionSolution
False - DNA contains thymine, not uracil; uracil occurs in RNA.
Both DNA and RNA contain the bases adenine, guanine and cytosine. The difference is in the fourth base: DNA contains thymine (T), whereas RNA contains uracil (U) in place of thymine. The …
- CBSE 2025Set KH1 markQ.What is nucleoside?
›Reveal solutionSolution
Nucleoside = nitrogenous base + pentose sugar (no phosphate); adding phosphate gives a nucleotide.
Concept. The building blocks of nucleic acids are nucleotides, which have three parts: a nitrogenous base, a pentose sugar, and a phosphate group.
Definition. When only a nitrogenous base links to a pentose sugar (ribose in RNA, deoxyribose in DNA) by an N-glycosidic bond, the unit is called a nucleoside. Examples: adenosine, guanosine, cytidine, uridine/thymidine.
…
- CBSE 2024Set ANNUAL1 markQ.Write the name of bond between phosphate group and nucleoside in DNA.
›Reveal solutionSolution
Successive nucleotides in a DNA strand are joined by phosphodiester bonds linking the sugar of one nucleotide to the phosphate of the next.
A nucleotide is formed when a phosphate group is attached, via an ester (phosphoester) linkage, to the 5'-OH of the sugar of a nucleoside (base + sugar). Successive nucleotides are then joined together to form the DNA strand's backbone by a bond between the 5'-phosphate of one nucleotide and the 3'-OH of the sugar of the next nucleotide — since thi …
- CBSE 2023Set ANNUAL1 markQ.Write name of components of nucleoside.
›Reveal solutionSolution
A nucleoside = nitrogenous base + pentose sugar (joined by an N-glycosidic bond); adding a phosphate group to a nucleoside makes it a nucleotide.
A nucleoside is formed by the attachment of a nitrogenous base (adenine, guanine, cytosine, thymine or uracil) to the 1' carbon of a pentose sugar (ribose in RNA, deoxyribose in DNA) through an N-glycosidic linkage.
…
- CBSE 2022Set ANNUAL1 markMCQQ.Which of the following nitrogenous bases is not present in DNA?(a) Thymine(b) Adenine(c) Guanine(d) Uracil
›Reveal solutionSolution
Uracil is the pyrimidine base found in RNA, not DNA, which uses thymine instead.
DNA is built from four nitrogenous bases: the purines adenine (A) and guanine (G), and the pyrimidines cytosine (C) and thymine (T). RNA carries the same bases except that thymine is replaced by uracil (U), which pairs with adenine in the same way thymine does but lacks t …
- CBSE 2022Set ANNUAL1 markMCQQ.Purine nitrogenous base is -(a) Cytosine(b) Adenine(c) Uracil(d) Thymine
›Reveal solutionSolution
Purines have a fused double-ring structure; adenine is a purine, the other three options are pyrimidines.
Nitrogenous bases in nucleic acids are of two chemical classes:
- Purines (double fused ring): Adenine (A) and Guanine (G). …
- CBSE 2022Set ZOOLOGY1 markMCQQ.Which type of sex determination is found in humans?(i) XX-XY(ii) ZW-ZZ(iii) XX-XO(iv) ZZ-ZO
›Reveal solutionSolution
Humans show the XX-XY type of sex determination, with the male being heterogametic.
In humans, each individual has 22 pairs of autosomes plus one pair of sex chromosomes. Females carry two X chromosomes (XX) and are homogametic, producing only X-bearing eggs. Males carry one X and one Y (XY) and are heterogametic, producing X-bearing and Y-bearing sperm in equal numbers. The sperm therefore determines the …
- CBSE 2022Set ZOOLOGY1 markMCQQ.Which is the genotype of Turner's syndrome?(i) XO(ii) XXY(iii) XYY(iv) XXX
›Reveal solutionSolution
Turner's syndrome has the genotype XO (45, X0) - a single X and no second sex chromosome.
Turner's syndrome arises from monosomy of the sex chromosome due to non-disjunction during gamete formation. The affected individual has 45 chromosomes: 44 autosomes + a single X (XO / 45, X0). Such individuals are sterile females with underdeveloped ovaries, short stature and lack of secondary sexual …
- CBSE 2021Set D1 markMCQQ.Pyrimidines present in RNA are(a) Cytosine and Thymine(b) Adenine and Guanine(c) Cytosine and Uracil(d) Thymine and Uracil
›Reveal solutionSolution
RNA contains the pyrimidines cytosine and uracil (uracil replaces DNA's thymine), so the answer is (C).
Nitrogenous bases are of two types: purines (adenine and guanine, each with a double ring) and pyrimidines (single ring).
…
- CBSE 2021Set D1 markMCQQ.Nucleoside is(a) Sugar + a nitrogenous base(b) Sugar + Phosphate(c) Nitrogenous base + Phosphate(d) None of these
›Reveal solutionSolution
Nucleoside = sugar + nitrogenous base; adding a phosphate makes it a nucleotide.
A nucleoside is formed when a nitrogenous base (purine or pyrimidine) links to a pentose sugar (ribose in RNA, deoxyribose in DNA) through an N-glycosidic bond. When a phosphate group is further attached to the sugar, the molecule becomes a nucleotide. Therefore 'sugar …
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