In humans, which group does NOT have a perfectly matching homologous pair of sex chromosomes in each body cell?

Introduction / Context:
Human beings have 23 pairs of chromosomes in each body cell, including one pair of sex chromosomes that determine biological sex. A pair of chromosomes is called homologous when the two members of the pair are similar in size, shape, and gene content. The sex chromosomes differ between typical males and females. This question asks you to identify which group does not have a perfectly matched homologous pair of sex chromosomes.

Given Data / Assumptions:

• Humans have sex chromosomes represented as XX in typical females and XY in typical males.

• Homologous chromosomes are similar in structure and carry corresponding genes.

• The options consider men, women, both, neither, or only children before puberty.

• We assume typical chromosomal patterns, ignoring rare chromosomal disorders for this general knowledge question.

Concept / Approach:
In typical human females, the sex chromosomes are XX. The two X chromosomes are similar in length and gene content and therefore form a homologous pair. In typical human males, the sex chromosomes are XY. The X chromosome is large and carries many genes, while the Y chromosome is smaller and contains fewer genes. Because of the size difference and differences in gene content, X and Y are not a fully homologous pair. Thus, men do not have a perfectly matching pair of sex chromosomes, while women do.

Step-by-Step Solution:
Step 1: Recall that the sex chromosomes in typical females are XX. Step 2: Recognise that both X chromosomes are similar in shape and size and carry corresponding genetic information, making them a homologous pair. Step 3: Recall that the sex chromosomes in typical males are XY. Step 4: Note that the X chromosome is large and the Y chromosome is much smaller and carries different genes. Step 5: Because of these differences, X and Y are not a perfectly homologous pair, even though they still pair during meiosis at short homologous regions. Step 6: Therefore, men do not have a perfect pair of homologous sex chromosomes, whereas women do. Step 7: Check the options and select the one that states that only men lack a perfect homologous pair of sex chromosomes.

Verification / Alternative check:
Cytogenetic diagrams of human karyotypes show 22 pairs of autosomes plus one pair of sex chromosomes. In female karyotypes, the sex chromosomes are labelled XX and appear similar, while in male karyotypes they are labelled XY and appear different in size. Genetics textbooks also state that females have two homologous X chromosomes, but males have one X and one Y, which are described as heteromorphic or non homologous. This confirms that only men lack a perfectly matching homologous pair of sex chromosomes.

Why Other Options Are Wrong:

Only women: Incorrect because women have two X chromosomes that form a homologous pair.

Both men and women: Incorrect, since typical women do have a homologous XX pair.

Neither men nor women: Incorrect, because women do have a homologous pair while men do not.

Only children before puberty: Puberty does not change the chromosomal composition, so this statement is irrelevant and incorrect.

Common Pitfalls:
Students sometimes think that since both men and women have two sex chromosomes, they must both have homologous pairs. Another confusion arises from focusing only on the symbol pairs (XX and XY) without considering chromosome size and gene content. Remember that homologous means structurally similar and gene matching, which applies to XX in women but not to XY in men.

Final Answer:
The group that does not have a perfectly homologous pair of sex chromosomes is Only men.