Sister chromatids are identical forms of chromatids of a chromosomes. They are mostly formed by semi-conservative replication of DNA molecule of a single chromosome.Thus they are like 'photocopies' of original parent chromosomes; joined together at the Centromere.

They are exactly similar in all ramification; with the same gene and allele compositions..

However; slight differences arise between the two identical sisters due to mutation from errors at replication;and also in the length of telomere repeats.

Non-sister chromatids are dissimilar forms of chromatids of a chromosomes formed when each half of a chromosome at fertilisation from separate haploid sex-cells, of each parent. fused.They contain different genetic composition;because they are not on the same homologous chromosomes.Therefore crossing -over ensure variation.

However, they are genetically similar in composition; if they are contained in homologous chromosomes. This is because Synapsis of bivalent of these chromosomes allow genetic material to be shared by chromosomal crossing-over between the non-sister chromatids on the chromosomes ; therefore identical genetic characteristics are shared .

Explanation:

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