Q.A couple in West Bengal has had three daughters and no sons. A relative blames the mother for 'failing' to have a son. Using your knowledge of the XX-XY mechanism of sex determination in humans, explain why this is genetically incorrect, and state which parent's gamete actually decides the sex of a child.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Sex Determination in Humans and Birds
Sex determination is the genetic mechanism fixing an individual's sex at fertilisation, and it differs between major animal groups. In humans (and most mammals), sex follows the XX-XY system: females are homogametic (XX, every egg carries one X), while males are heterogametic (XY, producing equal numbers of X-bearing and Y-bearing sperm). Since every egg always contributes an X, it is which type of sperm fertilises it that decides the child's sex — genetically, the father's gamete, not the mother's, determines sex, with each outcome equally probable. Birds show the reversed ZW system: the FEMALE is heterogametic (ZW, producing Z-bearing and W-bearing eggs), while the male is homogametic (ZZ, producing only Z-bearing sperm) — …
Genetically incorrect: the father's sperm (X- or Y-bearing), not the mother's egg (always X), determines a child's sex. …
Step 1. In the human XX-XY system, the mother is homogametic — every single egg she produces carries exactly one X chromosome, with no variation.
Step 2. The father is heterogametic, producing two kinds of sperm in equal number: X-bearing and Y-bearing.
Step 3. The sex of a child is decided by which TYPE of sperm happens to fertilise the egg: an X-bearing sperm gives a daughter (XX), a Y-bearing sperm gives a son (XY). …
Explain that the mother's gamete is invariant (always X) while the father's gamete varies (X or Y), so responsibility for sex determination lies wi …
- Repeating the common but genetically false social belief that blames the mother.
- Assuming three daughters in a row implies some bias — each conception is an indepe …
Showing the 12 most recent of 14 on this concept.
- CBSE 2026Set SEM31 markMCQQ.In the following question, a statement of Assertion (A) is followed by a statement of Reason (R). Read the two statements and select the correct answer : Assertion (A) : Female birds are heterogametic and male birds are homogametic. Reason (R) : The sex chromosomes of female birds are ZW and the sex chromosomes of male birds are ZZ.(a) Both (A) and (R) are true and (R) is the correct explanation of (A)(b) Both (A) and (R) are true but (R) is not the correct explanation of (A)(c) (A) is true, but (R) is false(d) Both (A) and (R) are false
›Reveal solutionSolution
In birds, sex is determined by the ZW system: females are ZW (heterogametic, two different sex chromosomes) and males are ZZ (homogametic). This is exactly what the Reason states, so R correctly explains A.
- Assertion (A): Female birds are heterogametic and male birds are homogametic — TRUE. A heterogametic sex produces two kinds of gametes with respect to sex chromosomes; a homogametic sex produces only one kind.
- Reason (R): The sex chromosomes of female birds are ZW and of male birds are ZZ — TRUE. …
- CBSE 2024Set A11 markMCQQ.In human males XY type of sex determination is(a) Homogametic(b) Heterogametic(c) Both a) and b)(d) Isogametic
›Reveal solutionSolution
Human males are XY and produce two kinds of gametes (X and Y), so they are heterogametic.
In the XY type of sex determination, human males have one X and one Y chromosome. They produce two different types of sperm — 50% carrying the X chromosome and 50% carrying the Y chromosome — hence …
- CBSE 2024Set ZOOLOGY1 markMCQQ.Which of the following sex-determination mechanisms is seen in birds?(a) XX female and XY male(b) ZZ female and ZW male(c) ZW female and ZZ male(d) XX female and XO male
›Reveal solutionSolution
Bird sex is determined by the ZW system: males are ZZ (homogametic) and females are ZW (heterogametic).
Sex-determination mechanisms vary across the animal kingdom. In humans and most mammals, the male is heterogametic (XY) and the female is homogametic (XX). In birds (and also in some reptiles, fish and butterflies), the pattern is reversed: the male has two similar Z chromosomes (ZZ) and is therefore homogametic, while the female has one Z and one W chromosome (ZW) and is …
- CBSE 2024Set ANNUAL1 markQ.Give one example of female heterogamety.
›Reveal solutionSolution
In the ZW sex-determination system used by birds, the female is heterogametic (ZW) and the male is homogametic (ZZ) - the reverse of the human XY system.
In humans (XY system), males are heterogametic (XY, producing X- and Y-bearing sperm) and females are homogametic (XX). Female heterogamety is the opposite arrangement: the female produces two types of gametes (in this case Z-bearing and W-bearing eggs) while the male is homogametic (ZZ). Birds are the classic te …
- CBSE 2024Set ANNUAL1 markQ.Why is Y chromosome referred to as genetically inert?
›Reveal solutionSolution
The human Y chromosome carries only a small number of functional genes and largely does not undergo crossing over with the X chromosome, so it is described as genetically inert relative to the X.
In humans, the X chromosome is large and gene-rich (carrying genes for many traits besides sex, e.g., colour blindness, haemophilia), while the Y chromosome is much smaller and carries very few genes, most involved in maleness/testis-determination (e.g., the SRY gene) and spermatogenesis. Except for a small pseudoautosomal region at the tips that pairs and recombines with the X during meiosis …
- CBSE 2023Set 57/1/11 markMCQQ.Assertion (A) : Birds like pigeon have heterogametic females whereas the males are homogametic. Reason (R) : In pigeons, females have Z and W sex chromosomes whereas males have ZZ sex chromosomes.(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).(c) Assertion (A) is true, but Reason (R) is false.(d) Assertion (A) is false, but Reason (R) is true.
›Reveal solutionSolution
In birds, females are heterogametic (ZW) and males are homogametic (ZZ). Both Assertion and Reason are true, and the Reason correctly explains the Assertion. The correct option is (a).
The key here is understanding what "heterogametic" and "homogametic" mean — and how sex determination works in birds versus mammals. In mammals (including humans), females are XX (homogametic) and males are XY (heterogametic). But in birds, the system is reversed: it's the female that produces two different types of gametes with respect to sex chromosomes.
Let's break it down.
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What the terms mean.
- Homogametic: produces only one type of gamete regarding sex chromosomes.
- Heterogametic: produces two different types of gametes regarding sex chromosomes. In pigeons (and most birds), males have two identical sex chromosomes (ZZ), so all their sperm carry a Z chromosome — they are homogametic. Females have one Z and one W chromosome (ZW), so their eggs are either Z-bearing or W-bearing — they are heterogametic.
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Checking the Assertion (A).
"Birds like pigeon have heterogametic females whereas the males are homogametic."
This is exactly what we just described. The female pigeon is ZW (heterogametic), the male is ZZ (homogametic). So Assertion (A) is true.
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Checking the Reason (R).
"In pigeons, females have Z and W sex chromosomes whereas males have ZZ sex chromosomes."
This is a factual statement about the chromosomal makeup. It is also true.
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Does the Reason correctly explain the Assertion? …
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- CBSE 2023Set CG1 markQ.How many pairs of chromosomes are found in human beings?
›Reveal solutionSolution
Humans have 23 pairs of chromosomes (2n = 46).
Concept: In each somatic (body) cell of a human being there are 46 chromosomes, present as 23 homologous pairs:
- 22 pairs of autosomes, and
- 1 pair of sex chromosomes (XX in females, XY in males). …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following is an example of homogametic?(a) Human(b) Papaya(c) Cladophora(d) Amoeba
›Reveal solutionSolution
A homogametic (isogamous) organism forms homogametes/isogametes — gametes that look alike and are not differentiated into male and female. Of the options given, only Cladophora does this.
Gametes are classified by their morphology:
- Homogametes (isogametes): the two fusing gametes are identical in size and structure, so they cannot be labelled male or female. Cladophora, a filamentous green alga, is a standard example of an organism producing homogametes/isogametes. …
- CBSE 2023Set ANNUAL1 markMCQQ.In human beings, sex of the child is determined by –(a) both mother and father(b) mother only(c) father using his 'Y' chromosome(d) father by his 'X' chromosome
›Reveal solutionSolution
In humans, sex is determined by the XY system: the mother always contributes an X, so it is the father's sperm — carrying either an X or a Y chromosome — that decides whether the child is female or male.
Humans follow the XY type of sex determination. Every ovum produced by the mother carries a single X chromosome (since the mother's genotype is XX), so the mother's contribution is always X. The father, being heterogametic (XY), produces two types of sperm in equal numbers: 50% carrying an X chromosome and 50% carrying a Y chromosome.
- If an X-bearing sperm fertilises the ovum → XX zygote → develops into a female. …
- CBSE 2021Set D1 markMCQQ.Number of chromosomes in human being (male) is(a) 44 + XX(b) 44 + XY(c) 46 + XY(d) 46 + XX
›Reveal solutionSolution
A normal human male has 46 chromosomes = 44 autosomes + XY, so the answer is (B) 44 + XY.
Human somatic cells contain 46 chromosomes arranged as 23 pairs. Of these, 22 pairs (44 chromosomes) are autosomes, and one pair is the sex chromosomes.
…
- CBSE 2019Set HE1 markQ.Write the answer in one word/sentence: By which chromosome does sex determination take place in Human beings?
›Reveal solutionSolution
In humans, sex is determined by the Y chromosome carried by the sperm (the XY sex-determination system).
Humans have 23 pairs of chromosomes, of which 22 pairs are autosomes and one pair is the sex chromosomes. Females have two X chromosomes (XX, the homogametic sex), while males have one X and one Y chromosome (XY, the heterogametic sex). Since all eggs produced by the mother carry an X chromosome, the sex of the child is determined by the sperm from the father: a sperm carrying an X chromosome produces a female (XX) child, while a s …
- CBSE 2019Set ANNUAL1 markMCQQ.How many pairs of autosomal chromosomes are present in human?(a) 44(b) 46(c) 22(d) 23
›Reveal solutionSolution
Humans have 23 pairs of chromosomes in total; 22 of these pairs are autosomes and 1 pair is the sex chromosomes.
A human somatic cell has 46 chromosomes, arranged as 23 homologous pairs. Of these, 22 pairs are autosomes (chromosome pairs 1-22), which are identical in both males and females and carry genes for general body …
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