Q.How is sex determined in birds? Explain the ZW system and state how it differs from the XX-XY system found in humans in terms of which sex is homogametic and which is heterogametic.

Showing the 12 most recent of 14 on this concept.

1. CBSE 2026Set SEM31 markMCQ
   Q.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.

   Because the female is ZW (produces Z and W gametes → heterogametic) and the male is ZZ (produces only Z gametes → homogametic), R is precisely the reason A is true. This ZW–ZZ system in birds is the opposite of the XX–XY system in humans.

   A standard NCERT/CBSE Class 12 Biology assertion–reason question on sex determination.

   ✓Final answer

   (a) Both (A) and (R) are true and (R) is the correct explanation of (A).

2. CBSE 2024Set A11 markMCQ
   Q.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 they are heterogametic. Females are XX and produce only X-bearing eggs, so they are homogametic.

   ✓Final answer

   (b) Heterogametic

3. CBSE 2024Set ZOOLOGY1 markMCQ
   Q.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 heterogametic. It is thus the egg (from the mother) that determines the sex of the offspring in birds, unlike in mammals where the sperm (from the father) determines it.

   ✓Final answer

   (c) ZW female and ZZ male.

4. CBSE 2024Set ANNUAL1 mark
   Q.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 textbook example of the ZW/female-heterogametic system; some butterflies, moths, snakes and certain fish also show female heterogamety.

   ✓Final answer

   Birds (ZW female, ZZ male - the female is heterogametic).

5. CBSE 2024Set ANNUAL1 mark
   Q.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, the bulk of the Y chromosome does not undergo recombination with the X, so it is passed largely unchanged from father to son.

   ✓Final answer

   Because it contains very few functional genes and does not recombine with the X chromosome over most of its length.

6. CBSE 2023Set 57/1/11 markMCQ
   Q.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.

   1. 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.

   2. 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.

   3. 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.

   4. Does the Reason correctly explain the Assertion?

      The Assertion says females are heterogametic and males are homogametic. The Reason gives the chromosomal basis: females are ZW (two different chromosomes → heterogametic), males are ZZ (two identical chromosomes → homogametic). So the Reason directly explains why the Assertion is true. Therefore, (R) is the correct explanation of (A).

   Watch out

   A common mistake is to confuse the bird system with the mammalian system. In mammals, females are XX (homogametic) and males are XY (heterogametic). In birds, it's the opposite. Always check which organism is being discussed before deciding who is heterogametic.

   Tip

   A quick mnemonic: In birds, think Z for the male (ZZ — both same), and W for the female (ZW — different). The letter 'W' looks like a 'V' for "variable" — the female is the variable one.

   ✓Final answer

   The correct option is (a) — Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

7. CBSE 2023Set CG1 mark
   Q.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).

   This is the diploid number, 2n = 46. Gametes (sperm/egg) are haploid and carry 23 single chromosomes (n = 23).

   ✓Final answer

   23 pairs (46 chromosomes in total).

8. CBSE 2023Set ANNUAL1 markMCQ
   Q.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.
   • Heterogametes (anisogametes): the two gametes differ — typically a large non-motile female gamete (egg) and a small motile male gamete (sperm). Human beings and papaya form heterogametes.
   • Amoeba reproduces asexually (chiefly by binary fission) and does not normally form such gametes at all.

   Therefore the organism that is 'homogametic' is Cladophora.

   [!ANSWER]

   (c) Cladophora.

9. CBSE 2023Set ANNUAL1 markMCQ
   Q.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.
   • If a Y-bearing sperm fertilises the ovum → XY zygote → develops into a male.

   Since the mother's contribution is fixed (always X), it is the type of sperm from the father that determines the child's sex — specifically, fertilisation by a Y-bearing sperm produces a male child.

   ✓Final answer

   (c) Father, using his Y chromosome.

10. CBSE 2021Set D1 markMCQ
    Q.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.

    In a male the sex chromosomes are X and Y, so the karyotype is 44 + XY. A female has 44 + XX. Options (C) 46 + XY and (D) 46 + XX would give a total of 48 chromosomes, which is incorrect. Hence the human male chromosome complement is 44 + XY.

    ✓Final answer

    (B) 44 + XY.

11. CBSE 2019Set HE1 mark
    Q.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 sperm carrying a Y chromosome produces a male (XY) child. Thus, in humans, sex determination is chromosomally controlled by the Y chromosome (specifically the SRY gene on it).

    ✓Final answer

    Y chromosome (XY sex-determination system).

12. CBSE 2019Set ANNUAL1 markMCQ
    Q.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 traits. The remaining 1 pair is the sex chromosomes (XX in females, XY in males), which determine sex.

    ✓Final answer

    (c) 22 pairs of autosomes.