Down's syndrome is due to

Explanation

The correct option is d) Non-disjunction of chromosomes. Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. Non-disjunction refers to the failure of chromosomes to separate properly during cell division, resulting in an abnormal distribution of chromosomes in the resulting cells. In the case of Down's syndrome, non-disjunction occurs during the formation of the reproductive cells (eggs or sperm) or during early embryonic development. Now, let's examine why the other options are incorrect: a) Crossing over: Crossing over is a normal genetic process that occurs during meiosis (cell division in reproductive cells) where genetic material is exchanged between homologous chromosomes. While crossing over can lead to genetic recombination and variation, it is not the cause of Down's syndrome. b) Linkage: Linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together. While linkage can influence the inheritance patterns of certain traits, it is not directly related to the occurrence of Down's syndrome. c) Sex-linked inheritance: Sex-linked inheritance refers to the inheritance of genes located on the sex chromosomes (X and Y chromosomes). Down's syndrome is not caused by a gene on the sex chromosomes, so sex-linked inheritance is not relevant to this condition. In summary, the correct cause of Down's syndrome is non-disjunction of chromosomes, leading to the presence of an extra copy of chromosome 21.