What are lethal recessives?
Particular recessive alleles may be lethal if they are not masked by a dominant allele; these are called lethal recessives, as they cause the death of the embryo/foetus by a miscarriage or spontaneous abortion, or the early death of the child.
Lethal recessive mutations obviously cause changes in the composition of a gene pool; individuals who inherit two of these lethal recessives would die before their alleles could be passed on to the next generation, and so the proportion of lethla recessive alleles in the gene pool would be gradually reduced.
Case Study: Tay-Sachs Disease (TSD)
Tay-Sachs Disease (TSD) is a genetic disorder affecting lipid (fat) metabolism, and is inherited through autosomes. TSD is a lethal recessive condition caused by a mutation in the HEXA gene, coding for the enzyme beta-hexosaminidase.
Beta-hexosaminidase is responsible for the breakdown of toic substances, notably GM2 ganglioside; a fatty substance located in the brain and spinal cord.
A lack of this enzyme results in an accumulation of GM2 ganglioside in the nervous system, leading to the destruction of neurons.
Babies with two recessive alleles for TSD develops normally for a few months and then inevitable deterioration causing physical and intellectual disabilities begins; death often occurs in early childhood.