Q.What is heterogamety ? Explain the mechanism of sex determination in Drosophila.
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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 — Gene Expression Regulation
Imagine a library with thousands of books. Every cell in your body has the same library — the same complete set of genes in your DNA. But a skin cell does not need to read the book on "how to make stomach acid," and a stomach cell does not need the book on "how to make skin pigment." If every cell tried to read every book at once, the library would be chaos. Gene expression regulation is the system that decides which books are opened, which are kept closed, and when to put a book back on the shelf.
At its simplest, gene expression is the process by which information from a gene is used to make a functional product — usually a protein. Regulation means this process is not automatic; it is controlled. Cells turn genes on or off, or adjust how much product is made, depending on what the body needs at that moment.
Why does this matter? Without regulation, every cell would be identical and useless. Regulation is what makes a muscle cell different from a nerve cell, even though both contain the same DNA. It also allows your body to respond to changes — like producing more red blood cells when you move to high altitude, or repairing damage after a cut.
Think of gene regulation like a dimmer switch, not just an on/off button. Some genes are turned up high, some are turned down low, and many are in between. This fine-tuning is essential for health.
The NCERT textbook explains that regulation can happen at several stages. The most important stage in bacteria (like E. coli) is at the start of transcription — when the gene is first copied into RNA. In higher organisms, regulation is more complex and can occur at multiple points: before transcription, during RNA processing, during translation (making protein), and even after the protein is made.
Key points to remember:
- All cells have the same DNA, but different sets of genes are active in different cells.
- Regulation is dynamic — genes can be turned on and off in response to signals from inside or outside the cell.
- Mistakes in regulation can lead to diseases like cancer, where genes that should be off stay on, or genes that should be on stay off. …
Part (a)
Heterogamety is the condition in which one sex produces two different kinds of gametes with respect to sex chromosomes, while the other sex produces only one kind.
Sex determination in Drosophila: Males are heterogametic (XY → produce X-bearing and Y-bearing sperm); females are homogametic (XX). Sex is actually decided by the ratio of X chromosomes to autosome sets (X : A), not by the Y:
- XX / 2A → ratio 1.0 → female
- XY (X / 2A) → ratio 0.5 → male
- intermediate ratios (e.g. 2X/3A = 0.67) → intersex. …
Part (a): heterogamety = one sex forms two gamete types; Drosophila males are XY (heterogametic) and sex is set by the X:A ratio (1.0 female, 0.5 male), Y only for fertility.
Part (b): hnRNA → mRNA by capping + tailing + splicing, all in the nucleus.
Part (a)
Heterogamety
Heterogamety is the situation where one sex produces two different types of gametes (differing in the sex chromosome they carry), while the other sex (homogametic) produces only one type. The heterogametic parent's gamete therefore decides the offspring's sex. In humans and Drosophila the male is heterogametic (XY → X- or Y-bearing sperm); in birds it is the female (ZW).
Mechanism of sex determination in Drosophila melanogaster
Although males are XY, the deciding factor is not the Y but the ratio of X chromosomes to sets of autosomes (X : A) (Bridges' genic-balance mechanism):
| Genotype | X : A ratio | Sex |
|---|---|---|
| XX / 2A | 1.0 | Female |
| XY (X / 2A) | 0.5 | Male |
| 2X / 3A | ~0.67 | Intersex |
- A ratio of 1.0 → female, 0.5 → male, and intermediate values give intersexes. …
Showing the 12 most recent of 33 on this concept.
- AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQQ.Choose the incorrect statement of the following (A) Operator is the region of DNA where RNA polymerase binds and initiate transcription (B) A group of closely placed structural genes and regulatory element is called promoter (C) Process of turning genes on and off so that appropriate genes are expressed. This phenomenon is called gene regulation (D) Heterogenous nucleus RNA present in nucleus which becomes mRNA is called hn RNA
›Reveal solutionSolution
This tests the classic promoter-vs-operator mix-up: RNA polymerase binds the promoter to start transcription, while the operator is where the repressor binds to control it — statement (A) incorrectly assigns the promoter's role to the operator.
Concept and Intuition
In an operon (e.g. the lac operon), three key DNA elements act together: the promoter (p) is the RNA-polymerase binding site that initiates transcription; the operator (o), located between the promoter and the structural genes, is where a repressor protein binds to switch transcription off; and the structural genes are the genes actually transcribed. Confusing the promoter's binding partner (RNA polymerase) with the operator's binding partner (the repressor) is a very common conceptual trap.
Step-by-Step Solution
- Statement (A) claims the operator is where RNA polymerase binds to initiate transcription — but that is the definition of the promoter, not the operator. So (A) is factually wrong.
- Statement (C) correctly describes gene regulation as switching genes on/off so appropriate genes are expressed — matches standard usage, true. …
- AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQQ.Study the following statements regarding genic balance theory and identify incorrect statements. A) The production of gametes with abnormal number of chromosomes is due to non-disjunction. B) In Drosophila 'XO' males produce motile sperms. C) Sex index < 0.5 indicates intersexes in Drosophila. D) 'Y' chromosome in Drosophila lacks testis determining factor. (A) A & C (B) A & D (C) B & C (D) B & D
›Reveal solutionSolution
The genic balance theory (Bridges) uses the X:autosome ratio to fix sex in
Drosophila; XO flies are sterile (not fertile) males, and ratio <0.5 gives
super males, not intersexes — so B and C are the incorrect statements.
Answer: (C).
Concept and Intuition
Bridges proposed that sex in Drosophila is decided not just by presence of Y,
but by the ratio of X chromosomes to sets of autosomes (X:A, the "sex index"):
- Sex index = 1.0 → normal female (XX)
- Sex index = 0.5 → normal male (XY or XO)
- 0.5 < index < 1.0 → intersex
- index < 0.5 → super male (metamale)
- index > 1.0 → super female (metafemale)
Crucially, in Drosophila the Y chromosome does not determine maleness (that
job is done by the X:A ratio) — but the Y chromosome IS still needed for
fertility: XO males look and behave male but are sterile because normal
sperm maturation requires Y-linked fertility genes.
Step-by-Step Solution
- A: Non-disjunction → unequal chromosome distribution in gametes → gametes with abnormal chromosome numbers. This is a correct, well-established fact.
- D: Y chromosome in Drosophila lacks a testis-determining factor (sex is set by X:A ratio, not by Y) — correct statement.
- B: XO males — the X:A ratio (0.5) still makes them male, but they lack the Y chromosome's fertility genes, so they are sterile, not "produce motile …
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.If sex index ratio of a Drosophila is 0.66, its sexual phenotype is (A) Intersex (B) Meta male (C) Meta female (D) Triploid male
›Reveal solutionSolution
A sex index of 0.66 lies strictly between the male (0.5) and female (1.0) values, which by Bridges' genic balance theory in Drosophila defines an intersex.
Concept and Intuition
Calvin Bridges' genic balance theory explains Drosophila sex determination by the ratio of X chromosomes to sets of autosomes (X:A ratio, or 'sex index'). A ratio of 1.0 (2X : 2 autosome-sets) gives a normal female; 0.5 (1X : 2 autosome-sets) gives a normal male. Ratios above 1.0 give metafemales (superfemales), ratios below 0.5 give metamales (supermales), and ratios strictly between 0.5 and 1.0 give intersexes — flies with an ambiguous mix of male and female characteristics because the genic balance is intermediate.
Step-by-Step Solution
- Recall the reference points: female index = 1.0, male index = 0.5.
- Given index = 0.66 — this lies between 0.5 and 1.0. …
- AP EAPCET 2026Set ap-2026-05-20-AN1 markMCQQ.Sex determination in butter flies is (A) XX-XY type (B) XX-XO type (C) ZZ-ZW type (D) ZZ-ZO type
›Reveal solutionSolution
Sex determination in butterflies (Lepidoptera) follows the ZW system, where the female is heterogametic (ZW) and the male is homogametic (ZZ).
Concept and Intuition
Sex-chromosome systems aren't universal across animals. In the familiar XX–XY system (mammals, and also Drosophila), the male is the heterogametic sex. But several groups have the opposite arrangement, called the ZW system, where the female is heterogametic (ZW) and the male is homogametic (ZZ). Birds are the textbook example of ZW; Lepidoptera (moths and butterflies) also use this system.
Step-by-Step Solution
- Identify the taxonomic group: butterflies belong to order Lepidoptera.
- Recall the sex-determination systems taught: XX–XY (e.g., humans, Drosophila, male heterogametic), XX–XO (e.g., grasshoppers, male has only one X, no Y), ZZ–ZW (birds, most Lepidoptera; female heterogametic), and ZZ–ZO (some other insects).
- Lepidoptera (which includes butterflies) is one of the classic examples, alongside birds, of the ZZ–ZW system. …
- AP EAPCET 2025Set ap-2025-05-19-AN1 markMCQQ.Match the following Method of sex determination - Example A) XX - XY I) Grasshoppers B) XX - XO II) Birds C) ZW - ZZ III) Human beings D) ZZ - ZO IV) Moths V) Honey bees Identify the correct match (A) A-II, B-III, C-IV, D-V (B) A-III, B-IV, C-I, D-II (C) A-V, B-II, C-III, D-IV (D) A-III, B-I, C-II, D-IV
›Reveal solutionSolution
This tests recall of chromosomal sex-determination systems across different animal groups. The answer is (D) A-III, B-I, C-II, D-IV.
Concept and Intuition
Different animal groups use different combinations of sex chromosomes to determine sex, and whether the male or the female is the heterogametic sex varies by group:
- XX-XY: the female is XX (homogametic), the male is XY (heterogametic) — the system found in human beings and most mammals.
- XX-XO: the female is XX, the male has only a single X and no partner (XO) — classically illustrated by grasshoppers.
- ZW-ZZ: here it is the female that is heterogametic (ZW), while the male is ZZ (homogametic) — found in birds.
- ZZ-ZO: a rarer variant where the female is ZO (lacking a chromosome partner) and the male is ZZ — found in certain moths.
Step-by-Step Solution
- A) XX-XY → matches Human beings (III), the standard mammalian system.
- B) XX-XO → matches Grasshoppers (I), the textbook example of this system. …
- AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQQ.The Karyotype AA + XXY leads to (A) Turner syndrome (B) Kleinfelter syndrome (C) Down syndrome (D) Cushing syndrome
›Reveal solutionSolution
An extra X chromosome in an otherwise male (XY) individual, giving karyotype 44+XXY, causes Klinefelter syndrome. Answer: (B).
Concept and Intuition
Sex-chromosome aneuploidies are common examples in genetics of nondisjunction during gamete formation:
- Klinefelter syndrome (karyotype 47, XXY — i.e., 44 autosomes + XXY): affects males, who have an extra X chromosome, leading to slightly feminized development (like gynaecomastia, i.e., breast development), tallness, and reduced fertility.
- Turner syndrome (karyotype 45, X0 — i.e., 44 autosomes + a single X, no Y): affects females, who are missing one sex chromosome, leading to short stature, webbed neck, and underdeveloped ovaries.
- Down syndrome: an autosomal trisomy (extra chromosome 21), not a sex-chromosome disorder.
- Cushing syndrome: caused by excess cortisol (an endocrine disorder), not a chromosomal abnormality.
Step-by-Step Solution
- Identify the karyotype given: AA + XXY, meaning normal autosome number plus an extra X chromosome alongside a Y.
- Recognize this matches the definition of Klinefelter syndrome (47, XXY), which specifically involves an extra X in a genetically male individual. …
- AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQQ.Match the following : Karyotype of Drosophila/Phenotype: A) AA + XO I) Female; B) AA + XXY II) Intersex; C) AA + XXX III) Metamale; D) AAA + XX IV) Metafemale; V) Male (A) A-V, B-III, C-IV, D-II (B) A-III, B-I, C-II, D-IV (C) A-II, B-IV, C-I, D-V (D) A-V, B-I, C-IV, D-II
›Reveal solutionSolution
In Drosophila, sex is set by the X:autosome ratio, not by the Y chromosome: AA+XO
(ratio 0.5) is male, AA+XXY (ratio 1.0) is female, AA+XXX (ratio 1.5) is metafemale, and
AAA+XX (ratio ≈0.67) is intersex. Answer: (D).
Concept and Intuition
Unlike humans, where the Y chromosome is strongly male-determining, sex in Drosophila
is determined by the genic balance system — the ratio of the number of X chromosomes
to the number of haploid sets of autosomes (X:A ratio):
- Ratio = 1.0 (e.g., 2X:2A) → female (Y chromosome present or absent does not change this; it only affects fertility).
- Ratio = 0.5 (e.g., 1X:2A) → male.
- Ratio between 0.5 and 1.0 (intermediate) → intersex.
- Ratio > 1.0 (e.g., 3X:2A) → metafemale (superfemale), typically weak/sterile.
- Ratio < 0.5 → metamale (supermale), typically weak/sterile.
Step-by-Step Solution
- A) AA + XO: X:A = 1X : 2A = 0.5 → this is the male ratio → matches V) Male.
- B) AA + XXY: X:A = 2X : 2A = 1.0 → this is the female ratio (the extra Y does not push it toward male since Drosophila's Y is not male-determining) → matches I) Female. …
- AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQQ.Haplo - diploidy type of sex determination is found in (A) Wasp (B) Grass hopper (C) Butter fly (D) Bug
›Reveal solutionSolution
Haplo-diploid sex determination, in which males develop from unfertilised haploid eggs
and females from fertilised diploid eggs, is the hallmark sex-determination system of
Hymenoptera such as wasps. Answer: (A).
Concept and Intuition
Most sex-determination systems rely on a pair of sex chromosomes (like XX/XY or ZZ/ZW).
Haplodiploidy is a distinct system, found notably in the insect order Hymenoptera
(bees, wasps, ants), in which sex is determined by ploidy rather than chromosome type:
unfertilised eggs (which are haploid, carrying only the mother's chromosomes) develop
into haploid males, while fertilised eggs (diploid, carrying chromosomes from both
parents) develop into diploid females. This is quite different from the
X-chromosome-dosage system of Drosophila or the XX/XY system of humans.
Step-by-Step Solution
- Recall which organism groups use haplodiploidy as their sex-determination mechanism — this is a signature feature of the order Hymenoptera. …
- AP EAPCET 2025Set ap-2025-05-20-FN1 markMCQQ.The number of genes located on human Y chromosome. (A) 2968 (B) 2698 (C) 231 (D) 239
›Reveal solutionSolution
Per Human Genome Project data, the human Y chromosome contains about 231 genes (far fewer than the X chromosome's ~1098). Answer: (C).
Concept and Intuition
The Human Genome Project sequenced and catalogued the genes on every human chromosome, revealing that the sex chromosomes are strikingly different not just in size but in gene content. The X chromosome, being much larger and carrying genes for many essential (non-sex-related) functions, has a relatively high gene count, while the Y chromosome — much smaller, and largely specialised for male-determining and male-specific functions (like the SRY gene) — carries comparatively very few genes.
Step-by-Step Solution
- Recall the specific figures reported from the Human Genome Project for the sex chromosomes: the X chromosome contains about 1098 genes, and the Y chromosome contains about 231 genes.
- Since the question asks specifically for the Y chromosome's gene count, the answer is 231. …
- AP EAPCET 2024Set ap-2024-05-16-AN1 markMCQQ.Haplodiploidy is a mechanism of sex determination that is common in (A) Cockroach (B) Birds (C) Bugs (D) Wasps
›Reveal solutionSolution
Haplodiploid sex determination (fertilized = diploid female, unfertilized = haploid male) is characteristic of Hymenopterans like wasps.
Concept and Intuition
Most animals determine sex via chromosome pairs (XX/XY, XX/XO, ZZ/ZW), but in the haplodiploid system found in the order Hymenoptera (bees, wasps, ants), sex is determined by ploidy rather than a distinct sex chromosome: fertilized eggs (diploid, with a full set of chromosomes from both parents) develop into females, while unfertilized eggs (haploid, containing only the mother's chromosome set) develop into males. Cockroaches use an XX/XO system, birds use a ZW system (with females being the heterogametic ZW sex), and bugs (Hemiptera) typically use XX/XO as well — none of these are haplodiploid.
Step-by-Step Solution
- Recall haplodiploidy is defined by ploidy-based (not chromosome-based) sex determination.
- Identify that this mechanism is the hallmark of Hymenoptera — bees, ants, and wasps. …
- AP EAPCET 2024Set ap-2024-05-16-AN1 markMCQQ.Assertion (A): The Y Chromosome has no role in the determination of male sex in Drosophila. Reason (R): In XO males, sperms develop but are non-motile. (A) A and R are true. R is correct explanation for A (B) A and R are true. But R is not correct explanation for A (C) A is true. But R is false (D) A is false. But R is true
›Reveal solutionSolution
Tests the Drosophila sex-determination mechanism (X:A ratio, not Y presence) versus the
separate fact that Y carries male-fertility genes; both statements are true, but R (a fertility
fact) doesn't explain A (a sex-determination fact).
Concept and Intuition
In Drosophila melanogaster, sex is decided by the ratio of the number of X chromosomes to the number of autosome sets (X:A ratio) — a ratio of 1.0 gives a female, 0.5 gives a male, and
intermediate ratios give intersexes. The Y chromosome is essentially irrelevant to this decision:
an XO fly (only one X, no Y) still develops as a male, because its X:A ratio is 0.5, same as a
normal XY male.
However, the Y chromosome in Drosophila is not entirely without function — it carries genes
required for the male to be fertile. Without a Y (as in XO males), spermatogenesis proceeds
abnormally: sperm cells form but fail to complete proper development/motility, rendering the male
sterile.
Step-by-Step Solution
- Assertion: "The Y chromosome has no role in determination of male sex in Drosophila." True — sex is set by the X:A ratio; XO flies are still phenotypically male.
- Reason: "In XO males, sperms develop but are non-motile." True — this reflects the Y chromosome's real (but separate) role in male fertility, not sex determination.
- Does R explain A? No — R is a statement about fertility (a consequence of lacking Y-linked …
- AP EAPCET 2024Set ap-2024-05-16-FN1 markMCQQ.Assertion (A): The male ants are developed by parthenogenesis from unfertilized eggs Reason (R): The sex of offspring depends on the type of sperm that fertilizes the ovum (A) A and R are true. R is correct explanation for A (B) A and R are true. But R is not correct explanation for A (C) A is true. But R is false (D) A is false. But R is true
›Reveal solutionSolution
Ants use haplodiploidy (unfertilized egg → haploid male by parthenogenesis; fertilized egg → diploid female) — not sperm-type-based sex determination, so A is true and R is false. Answer: (C).
Concept and Intuition
Most animals with chromosomal sex determination (like humans, XY system) decide the sex of offspring based on which type of sperm (X-bearing or Y-bearing) fertilizes the egg. Hymenopteran insects (ants, bees, wasps) use a fundamentally different system called haplodiploidy (arrhenotoky):
- A fertilized (diploid, 2n) egg develops into a female (queen or worker).
- An unfertilized (haploid, n) egg develops, by parthenogenesis, into a male (drone). Sex is therefore determined by whether fertilization occurs at all, not by any qualitative difference between sperm types — indeed, since males are haploid, all the sperm they eventually produce are genetically identical clones of a single haploid genome, so there is no "X-sperm vs Y-sperm" distinction to select from in the first place.
Step-by-Step Solution
- Assess A: male ants (drones) develop from unfertilized eggs via parthenogenesis — this is the accepted haplodiploid sex-determination mechanism in ants — TRUE.
- Assess R: it claims sex depends on "the type of sperm" fertilizing the ovum — this describes chromosomal (XY-type) sex determination, not haplodiploidy. …
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