25. A normal man was found to have 45 chromosomes (44+X), the probable reason is-
(1) Deletion                (2) Translocation
(3) Ecentric Fission  (4) Point Mutation

Core concept

A typical human karyotype is 46,XY.
If a phenotypically normal man has 45 chromosomes (44 + X), one whole chromosome seems “missing,” yet development is normal. That situation is best explained by a balanced Robertsonian translocation between two acrocentric autosomes (e.g., 14;21). Two long arms fuse to form one chromosome, so the count drops from 46 to 45, but almost no genetic material is lost.

Because the sex chromosomes are listed as just X here, the question is using older exam shorthand (44 + X for autosomes + sex chromosome) and focusing on the autosomal count; the key idea is that total autosome number is reduced by one due to translocation, not true loss of a whole autosome.

Option-wise explanation

1. Deletion

• Loss of part or all of a chromosome.

• A full autosomal deletion sufficient to reduce the count to 45 would remove many genes and almost always cause severe abnormalities or lethality, not a “normal man.”

2. Translocation – correct

• In a Robertsonian translocation two acrocentric chromosomes fuse at or near their centromeres; short arms are usually lost, and long arms form a single chromosome.

• Chromosome count decreases by one (45), but gene content is largely preserved, so the carrier can be phenotypically normal.

3. Ecentric fission (centric fission)

• Splitting of one chromosome into two, which would increase chromosome number (e.g., 46 → 47), the opposite of what is observed.

4. Point mutation

• Change in a single base pair; does not alter chromosome number.

• Cannot explain 45 chromosomes.

Thus, the most probable reason a phenotypically normal man has 45 chromosomes (44 + X) is a balanced translocation (option 2).