2.2.4 Longer-term considerations
Something else to ponder is the effect that screening might have on the longer-term incidence of disease and (not the same thing) on the incidence of gene variants linked to disease. Sometimes, the impact on a disease can be dramatic. Take thalassaemia, a haemoglobin disorder similar to sickle cell disease, in which premature destruction of haemoglobin-containing red blood cells leads to anaemia. It is relatively common in some Mediterranean countries. Like sickle cell disease, it is understood in great detail at the molecular level, but this has produced little in the way of effective treatment. So screening may be offered to adults to identify carriers. In Cyprus, where there was such a high incidence that the Church endorsed screening before marriage in what are typically close-knit communities, thalassaemia has all but disappeared. In the UK, by contrast, medical services find it harder to reach those at risk, who are scattered among a community of largely low-risk families. As a result, the incidence of thalassemia has only fallen by around 40 per cent.
However, even if marriage avoidance or pre-natal diagnosis and abortion were more common, what effect would this have on the frequency of a disease allele, as opposed to the incidence of the disease? If the disorder results from a dominant gene, as in Huntington's disease (HD), then testing and termination tend to eliminate the allele from the population. But then there is no point in mass screening for dominant disorders, such as HD, as families involved already know that the disease is in the family – that is what dominance means. An affected person has an affected parent who in turn had an affected parent. But most of the simple genetic diseases are due to inheritance of two recessive alleles.
In recessive disorders, what effect might pre-natal diagnosis and selective abortion have on the frequency of the allele in the population?
Not much, since only fetuses with two mutated alleles will be eliminated. Heterozygotes will be spared and, as adults, may pass the mutant allele on to their children.
In fact, the availability of pre-natal testing may encourage some couples to have more children than they would have had previously. Similarly, preventing PKU by altering the diet may increase the frequency of the allele concerned, as many of the children who used to be damaged would not have reproduced.