Why are few genetic disorders controlled by dominant alleles?
The answer could be approached from the opposite end, i.e. why abnormal recessive alleles are more common in gene pool rather than an abnormal dominant allele?
It is true that genetic disorders are mostly controlled by recessive alleles rather than domonant ones.
Please note that double dose of recessive alleles cause the corresponding genetic disorder, but a single dose of dominant abnormal allele is enough to cause a disease.
- Chances of appearance of an affected individual with double recessive condition is definitely less (maximum 25% when two carriers sexually reproduce), though the recessive allele can be passed on from generation to generation without being expressed, hence detected by nature!
- Let us take example of Ashkenazi jews: number of carriers of an allele responsible for TAY SACHS disease is unusually high in their population: 1 in every 27; but number of children born with Tay Sachs in this community is about 1 in every 3600 births.
- An affected child suffers due to destruction of neurons in CNS and generally die before completing five years of life. With death of the child, alleles get rejected by nature: but carriers can still silently pass on the wrong form of gene through generations (because natural selection works on normal phenotype, not on genotype where abnormal alleles could be deceitfully loaded).
- Such a chance of rejection by nature is higher and multi-fold when the wrong version of gene is dominant. This is because dominant gene in an individual always expresses itself. Expression of abnormal phenotype means natural rejection (through death/sterility) will occur. Thus an abnormal dominant allele may not stay in genepool for long.
- It would be interesting to note that abnormal dominant allele (causing a genetic disease) may still be retained in gene pool if the allele expresses itself late in life, after the individual carrying such an allele has already successfully reproduced. This way the damage causing dominant allele may still be passed on to the next generation.
- Dominant allele for inherited disease HUNTINGTON'S CHOREA is one such example. Onset of the disease typically occurs between 35 and 44.