Skip to content

Given below is a… (Class 12 Biology SQP 2025-26, Q23)

questionbuddy.in/q/cbse-12-biology-2025-26-sqp-q23-showing-transport-ovum-fertilisation-growing

BiologyCBSE Class 12 · Sample paper, session 2025-26 · 3 marks · Short answer

CBSE Class 12 Biology sample paper 2025-26: question 23

From the official CBSE sample question paper for 2025-26, section C, question 23, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q23 · 3 marks · Short answerCompetency-basedHuman Reproduction

Given below is a figure showing transport of ovum, fertilisation and passage of growing embryo through fallopian tube in a human female. Answer the questions that follow:
(i) What will be the ploidy of cells shown in (a) and (c) stage in the figure given below?
(ii) What will happen if component L as shown in the figure (g) given below does not attach properly to the endometrium?
(iii) In a pregnant mother (case X), during early pregnancy, the fertilised egg splits into two embryos at stage C shown in the figure given below, resulting in the formation of twins. Will the genome of cells of these two embryos exhibit variation? Justify your answer.

(For Visually impaired students: Explain the events and journey of an ovum in the fallopian tube of a mother from fertilisation stage to implantation.)

Figure in the paper: Figure of human female reproductive tract showing ovum (a), fertilisation by sperms (b), cleavage stages (c), (d), morula (e), movement through fallopian tube with points A-H, blastocyst implantation in uterus, and blastocyst (g) with labelled parts L (outer layer), M and N (inner cell mass region); (f) is an early stage. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

Open one hint at a time and try again before the next.

Hint 1

The ovum is a gamete, while cleavage cells come from the zygote.

Hint 2

The outer layer of the blastocyst is the one that attaches to the uterus.

Hint 3

Twins formed by splitting of one zygote are called identical twins.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i) (a) The ovum is haploid (n). (c) The blastomeres formed by cleavage of the zygote are diploid (2n).

    1 mark

  2. (ii) L is the trophoblast. If it does not attach properly to the endometrium, implantation fails. The embryo cannot get nourishment and the placenta does not form, leading to pregnancy loss or other complications.

    1 mark

  3. (iii) No variation. Both embryos come from the same zygote by mitotic division, so they have identical genomes. They become identical (monozygotic) twins.

    1 mark

  4. (For visually impaired students)

    • Fertilisation occurs in the ampullary region. The zygote then undergoes mitotic cleavage while moving through the isthmus towards the uterus, forming 2, 4, 8 and 16 blastomeres. The 8–16 cell embryo is the morula.
    • The morula becomes the blastocyst, with an outer trophoblast and an inner cell mass.
    • The trophoblast attaches to the endometrium and the uterine cells cover the blastocyst, so it becomes embedded (implantation). The inner cell mass then differentiates into the embryo.

Answer: (i) (a) n, (c) 2n; (ii) Implantation fails – pregnancy loss; (iii) No, identical genome as both arise from the same zygote by mitosis.

Where the marks are

  • Ploidy: ovum n, blastomeres 2n (1 mark)
  • Failure of trophoblast attachment leads to failed implantation or pregnancy loss (1 mark)
  • No variation – both embryos from one zygote (identical twins) (1 mark)

Revise the chapter

  • Chapter: Human Reproduction (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Human Reproduction, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.