A coin has two sides--heads or tails. One could flip a coin to represent the possibility of producing a female or male offspring. Females are XX and their egg cell contains one X chromosome, due to meiosis. Males are XY and they can either produce a sperm cell with an X chromosome, or a Y chromosome. There is a fifty-fifty chance of either an X sperm or a Y sperm joining with an egg at fertilization. If an X sperm joins an X egg, a female is produced. If a Y sperm joins an X egg, a male is produced. The coin flip can represent the chances of either an X or Y sperm fertilizing an egg. As the number of times the coin is flipped increases, the resulting ratio will approach the predicted 50:50 ratio. This is just one example. If an individual is heterozygous for a particular allele, for example--if a person has a gene for pigment(A) and a gene for albinism(a), there is a fifty-fifty chance for their gametes to have one or the other allele, every time a sex cell is produced by meiosis.
A coin can represent an allele that is either dominant or recessive (one that follows standard inheritance) because a coin, like the alleles, are either or. A dominant allele (A) or a recessive allele (a) is passed on just as a coin is either heads or tails. By using a coin it shows that the chance of the dominant or recessive being passed on when the parent is heterozygous (Aa) is random. Every coin flip represents which form of the allele the parent will be passing on to the child.