Foetal sex can be determined by examining cells from the amniotic fluid by looking for

The correct option is barr bodies.

Let us understand the step-by-step biological reasoning behind this answer:

1. What is a Barr Body?
A Barr body (also known as sex chromatin) is an inactive, condensed X chromosome found in the somatic cells of females. In individuals with two or more X chromosomes (typically females who have an XX sex chromosome constitution), one of the X chromosomes is randomly and permanently inactivated during early embryonic development. This process is called lyonization or X-inactivation, and it serves as a mechanism for dosage compensation to balance the expression of X-linked genes between males (XY) and females (XX).

2. Identifying Sex via Barr Bodies:
Because normal genetic females have two X chromosomes (XX), their somatic cells show one Barr body ($2-1=1$). Normal genetic males have only one X chromosome (XY) and therefore show no Barr bodies ($1-1=0$). In general, the number of Barr bodies is equal to $N-1$, where $N$ is the total number of X chromosomes present in the cell's nucleus.

3. How the Test is Performed:
Amniocentesis is a prenatal procedure where a small amount of amniotic fluid, which contains fetal cells, is extracted. These fetal cells are cultured, stained, and examined microscopically. If the nuclei of these cells show the presence of Barr bodies, the fetus is determined to be female (XX). If Barr bodies are absent, the fetus is determined to be male (XY).

4. Why the other options are incorrect:
• Kinetochore: This is a protein structure on chromatids where the spindle fibers attach during cell division. It is present in all dividing cells regardless of sex.
• Chiasmata: These are contact points between homologous chromosomes during crossing over in meiosis, and they do not serve as a diagnostic feature for determining sex in somatic cells.
• Autosomes: Autosomes are non-sex chromosomes. Both males and females have the same number of autosomes (22 pairs in humans), so examining them generally does not distinguish sex unless karyotyping is done to look at the sex chromosomes themselves.