someone asked me to explain to them what a holiday junction is. during cell division to produce gametes (meiosis), the DNA strands cross over one another and re-combine with the parallel, sister strand. this leads to formation of two new double helices (of chromatin) that are hybrids of paternal and maternal sources. this is why we get genetic variation in progeny, leading to them usually having 'halfway' phenotypes between mom and dad.
this youtube video shows what happens very well on a molecular basis: