Match Column I with Column II: Column IColumn II A. Down’s syndromei.…

The correct answer is Option (2) → A-iv, B-i, C-ii, D-iii

(A) Down’s syndrome — this matches (iv) Gain of an extra copy of chromosome 21. It is an aneuploidy: a chromosome fails to separate during cell division, so the person has three copies of chromosome 21 (47 chromosomes in all).

(B) Linkage — this matches (i) Physical association of genes on a chromosome. Morgan found that genes lying on the same chromosome tend to be inherited together, and he named this physical association linkage.

(C) Sex chromosomes — this matches (ii) X and Y. In humans, 22 pairs are autosomes. The one remaining pair, XX in females and XY in males, decides sex.

(D) Increase in a whole set of chromosomes — this matches (iii) Polyploidy. When cytokinesis fails after telophase, the cell keeps an extra full set of chromosomes. This is common in plants.

NCERT Extract:
Chromosomal Disorders – “… Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy. For example, Down’s syndrome results in the gain of extra copy of chromosome 21. Similarly, Turner’s syndrome results due to loss of an X chromosome in human females. Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and, this phenomenon is known as polyploidy.”
Linkage and Recombination – “… Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations (Figure 4.11).”
Sex Determination in Humans – “… A pair of X-chromosomes are present in the female, whereas the presence of an X and Y chromosome are determinant of the male characteristic.”