A chromatid is a condensed DNA subunit of a chromosome. The two chromatids of a duplicated chromosome are held together at a region of DNA called the centromere (see figure below). Centromeres are the attachment points for microtubules, which are responsible for the guiding the movement of chromosomes during mitosis and meiosis.
replicated chromosomes
Most eukaryotic cells contain two sets of chromosomes, with one set originating from the father and the other from the mother. For example, every human cell has 23 pairs of chromosomes: one chromosome from each pair is inherited from the father (via the sperm), and the other is inherited from the mother (via the egg).
The figure below shows a cell that contains four chromosomes (found as two pairs); the pink chromosomes were inherited from the mother and the blue chromosomes were inherited from the father. Each chromosome contributed by the father has a corresponding chromosome that was contributed by the mother. These corresponding chromosomes, which are alike in structure and size, constitute a homologous pair (also referred to as bivalents). The DNA sequences of homologous chromosomes are usually not exactly identical.