Basic principles of genetic inheritance

In this chapter we will discuss inheritance within a family and address questions about the parental origin of specific sites in a genome. First we need to understand a few basic elements of genetics. All human cells contain two copies of each chromosome, one of parental and one of maternal origin. Exceptions are the reproductive cells or gamete s, i.e. eggs in females and sperms in males, which have only one copy of each chromosome. The gametes are formed in the process of meiosis (Figs. 20.1 and 20.2). Meiosis starts out with a cell having two copies of each chromosome (one from the father and one from the mother; two such chromosomes are said to be homologous). The DNA is first copied in order to generate two copies each of the paternal and maternal chromosomes. Through crossing-over, which occurs by homologous recombination, segments are occasionally interchanged between parental and maternal chromosomes (Fig. 20.2). Such recombination occurs about 50–100 times during meiosis, and there is evidence that in humans the recombination in maternal meiosis is about 1.7 times more frequent than in paternal meiosis (Petkov et al., 2007; Roach et al., 2010).