▶️ Answer/Explanation
(a)
Stage: Telophase
Reason:
- The chromosomes have reached the opposite poles of the cell.
- They appear as dense, separated clusters, indicating that nuclear membranes may start reforming soon.
- This stage marks the end of chromosome movement.
(b)
- A karyogram is an image showing the chromosomes of a cell arranged in homologous pairs and ordered by size.
- It is created by collecting fetal cells, often through amniocentesis or chorionic villus sampling.
- The chromosomes are stained, photographed, and arranged, making it easy to study their number and structure.
Uses of a karyogram in pregnancy:
Detect chromosome abnormalities, such as:
- Extra chromosomes (e.g., trisomy 21 = Down syndrome).
- Missing chromosomes (e.g., Turner syndrome).
- Structural changes like deletions or duplications.
Determine the sex of the fetus by identifying XX (female) or XY (male) chromosomes.
The Y chromosome is smaller than the X and easily visible.
Markscheme:
a. Telophase because the chromosomes/chromatids have reached the poles OR «late» anaphase as some chromosomes/chromatids are still moving/tails visible.
b.
- Is a photograph/diagram of homologous pairs of chromosomes that can be analysed.
- Information may be used to determine other chromosome abnormalities/changes in chromosome numbers/possible birth defects.
- Down syndrome/trisomy can be detected if there are three copies of a chromosome OR accept any other valid example.
- Other missing or extra pieces of chromosomes can be detected.
- Sex can be determined as the Y chromosome is shorter than the X.