Between 1856 and 1863, Gregor Mendel, an Austrian friar, created the discipline of genetics through the identification of trait heredity in pea plants. Mendel made an inspired choice in the use of pea plants for his original experiments; peas offer explicit advantages for the study of heredity, including short generation time, the ability to both self- and cross-pollinate, and easily identifiable traits coded for by single genes, such as seed colour and stem length. By looking at probabilities in his pea plants, Mendel discovered that “pods” - or alleles - were inherited by offspring from the parent generation. These alleles could be dominant or recessive, and the combination of alleles given by the parents would determine, barring mutation, the offspring phenotype. Human genetics is generally much more complicated then the simple Mendelian model discovered in peas. However, we do have certain traits - such as whether or not we have attached earlobes, or a widow’s peak - that can be directly determined by two alleles, one from each of our parents.
Mendelian genetics becomes especially interesting and relevant today as we begin to understand and explore the genetics of human disease. At first, the persistence of highly lethal genetic diseases seems a bit counter-intuitive, especially if they display typical homozygous recessive inheritance; if sufferers cannot survive to reproduce, how can the disease maintain itself in the population? There are many ways in which genetic diseases can survive; many people, for example, will not be symptomatic for a disease at all, but will still be a carrier of the recessive allele. By far one of the most interesting, however, is the concept of heterozygote advantage.
It has long been known that in areas of the world where malaria is endemic, sickle-cell anaemia is as well. Both diseases are lethal if untreated, and certainly not diseases that could be expected to be naturally selected as favourable. However, the biochemistry of sickle-cell anaemia is such that heterozygotes have many fewer “sickle cells” then those that are homozygous recessive for the disease (this is due to some of the proteins that cause the sickle cells to fold normally, resulting in a higher concentration of normal erythrocytes). Because so many fewer erythrocytes have the classic sickle shape (see above, right), the symptoms of the disease are far less severe. The mutation that causes sickle-cell anaemia also prevents severe malaria, as it stymies the malaria parasite’s attempt to hijack necessary erythrocyte machinery (see above, left). Thus, in places such as Africa, Asia, and parts of South America, sickle-cell anaemia persists because it is an advantage to be a heterozygote: On a population level, having the recessive allele present in the gene pool actually does more good then harm.
A similar phenomenon is seen with cystic fibrosis and areas of the world where tuberculosis is endemic.