When we talk about our DNA, we usually refer to 46 chromosomes, divided into pairs, placed inside every cell of our body. Well no, not every cell. There is one type, the gametes, that carry just half of this genetic material: one chromosome per pair. Why? Because they are the sexual cells (spermatozoid and ovule) that during the fecundation are reunited to create a new cell with 46 chromosomes. The cell with this new genetic information begins to divide to create a unique being – with its eyes, its face shape, its predisposition to diseases and personality traits.
Each gamete can bring different combinations of its half of genetic material since it includes only one chromosome per pair. For example, for a pair of chromosomes you can get one from your mother, while your brother or sister can inherit the other one. In total, there are 2 possibilities for 23 pairs: 2^23. This results in 8,388,608 (!) variations for a gamete. The number is already enormous but it increases if we consider another mechanism that complicates the inheritance of the genes: the crossing-over. This mechanism only happens during the creation of gametes to increase the variability of the genetic material – in fact, it’s an exchange of DNA pieces between pairs of chromosomes and each final chromosome becomes a puzzle of pieces from maternal and paternal genetic material. Surely we cannot be surprised that siblings often do not look alike and we can consider ourselves a unique mix!
Another source of complication: genes may have different versions due to variations of its sequence (alleles). A pair of chromosomes comes with the same set of genes, what happens if the two bring different alleles? It would be like having two editions of the same book: what happens if there are differences? How do they integrate?
In many cases it is resolved straightforward: one of the two copies has more value. This allele is called dominant because it hides the message from the other recessive copy. This is especially true for monogenic traits (determined by a single gene) and in these cases parent / child inheritance is translated in a very simple scheme (Mendelian) which allows to accurately calculate the probability of having children with a specific phenotype. Its most relevant applications in medicine are monogenic diseases. For example, if the disease is determined by a recessive allele, a person will be ill only with two equal copies of this “non-healthy” allele, one per each parent. This doesn’t imply that parents have also the disease. There are healthy individuals who do not suspect to have a copy of the “not-healthy” allele, as it is masked by the other “healthy” copy. This is the healthy carriers that can transmit the sick allele to their children and can be recognized only through genetic analysis.