What is the difference between sister chromatids and homologous chromosomes?

This might be a bit tricky sometimes, but there is a main difference between those two that is crucial and not so hard to remember. Sister chromatids are formed during DNA replication therefore they are identical, that is, they are literally copies of each other - the same genes AND the same alleles. Homologous chromosomes are paired up after fertilisation, so one is from you mum and one is from your dad, therefore even though they carry the same genes (that's why they pair up) they have different variations of them - alleles. You might find it helpful to think about sisters chromatids as twin siblings and homologous chromosomes as parents. Twins are identical but can you say that about your parents? I can also tell you that sisters chromatids are connected together (!) by a centromere. How would you fit that in our 'family picture'?