In the main text the increased risk of inheriting three specific genetic conditions, as a result of inbreeding, was examined. However, a question often asked is: What is the increased risk of inheriting any harmful condition caused by homozygous recessive genes, and what other adverse effects can be expected?
Crow and Kimura (1970) surveyed the evidence for the overall extent of genetic damage caused by first cousin marriages. A study in Sweden showed that 16% of children from a first cousin marriage suffered from a specific genetic disease compared with only 4% in the general public. When the survey was widened to include other harmful conditions of doubtful origin, the incidence rose to 28% compared with only 6% in the control population. Crow and Kimura concluded from these results that the average person carries the equivalent of about 6 harmful recessive genes, which if made homozygous would cause a detectable genetic illness.
An earlier study in rural areas in France in 1958 found that the children of cousin marriages died before adulthood 25% of the time compared with a mortality of 12% when the parents were unrelated. In Japan, where major differences between the results of several studies were reported, Crow and Kimura concluded this was due to the variation in local environments which allowed the children of cousin marriages to survive better in some areas than in others. They also suggested that in countries where the standard of living is rising, it is to be expected that the death rates in both inbred and non-inbred children will continue to decrease.
More recently, Jones (1996) made a general statement that the children from first cousin marriages have a 4% lower survival rate than the average for the population. This supports the previous suggestion that the survival rates are improving. He also mentioned that the children of uncle/niece marriages (F = 1/8) have an increased mortality of 10% compared with marriages of unrelated people.
Returning to specific harmful genes, the relative risk of inheriting a particular condition depends not only on the level of inbreeding, but also on the frequency of the harmful gene in the population. The risk increases far more for genes which are rare than for those which are common. Table 3 illustrates this principle:
|Gene Frequency||Proportion of 'Affected'||Ratio|
|Under Random Mating||From Cousin Marriages|
|q2 + Fq(1 - q)|
|0.25||0.266||1.06 : 1|
|0.01||0.016||1.6 : 1|
|0.0025||0.0055||2.2 : 1|
|0.0001||0.00072||7.2 : 1|
|0.000025||0.000335||13.4 : 1|
|0.000001||0.000063||63.0 : 1|
e.g. Comparing the risk of inheriting the two metabolic disorders, phenylketonuria and alkaptonuria, whose gene frequencies are .01 and .001 respectively. A first cousin marriage increases the risk of inheriting phenylketonuria 7.2 times, but with alkaptonuria the risk increases 63 times. However, because the risk is much lower in the first place, an increase from 1 in a million to 63 in a million (i.e. approximately 1 in 16,000) is not too serious.
A further important aspect of cousin marriage is that if a single relative, particularly an ancestor, is affected by a harmful genetic condition, this dramatically increases the probability that any future children will also be affected. e.g. If a certain person's grandfather is an albino, what is the probability that the first child of that person will also be an albino? The gene frequency for albinism, q = 1/141 = approx. 0.0071, and p = 1 - q = 140/141 = 0.9929. The following table shows how the risk increases when a family member is an albino:
|Grandfather's Phenotype||Mating System|| Probability that the First Great-|
grandchild will be an Albino *
|Grandfather normal |
|Random mating (F = 0)||q2||1/20,000 (approx)|
|First cousin marriage (F = 1/16)||q2 + Fq(1 - q)||1/2000 (approx)|
|Grandfather albino |
|Random mating (F = 0)||0.25q||1/560 (approx)|
|First cousin marriage (F = 1/16)||F||1/16|
* The first three of these probabilities have been rounded off to show the order of magnitude more clearly. These probabilities are equivalent to the expected proportions of albinos among all the great-grandchildren.