Ann Swain (co-convenor of the DNA Special Interest Group of the QFHS) outlined some of the basic science of genetics and described how it can inform our family histories.
Although the science of genetics began long before the discovery of chromosomes, we now know that how information passes from parents to child depends upon these tiny structures in cells and the chemical of which they are made, DNA.
The relationship between phenotype (how an individual appears) and genotype (the genes they inherited) depends upon competition between forms of the one gene (called alleles) that have different effects. When both are present (summarised Mm), a dominant allele will override the effect of its recessive partner. We (usually) only see the effect of the recessive allele when the cell contains two copies (mm).
Ann explained that when the phenotype involves a disease (such as sickle cell anaemia) or a disability (such as short-sightedness), doctors or family historians can draw up a genogram from which the probable genotypes of individuals can be deduced. This may allow a couple to decide not to have children (in very serious cases) or to ensure that the family has frequent medical examinations to detect potential problems early.
A very famous genogram displays the spread of the (often fatal) condition haemophilia through the royal houses of Europe via the children of Queen Victoria.