In general, one gene affects a single character. But many genes are known to affect more than one character. Such genes are called pleiotropic genes and the condition is called Pleiotropy. An example of a gene in human beings is the recessive gene s ( sometimes denoted as Hbs) which produces sickle cell anemia in the ss homozygotes. More than 50% of the individuals homozygous for this gene (ss) die before the age of 20 years. The primary effect of this gene is the substitution of a valine molecule for a glutamic acid molecule at position 6 of the β-chain of hemoglobin. This mutant (sickle-cell) hemoglobin remains in solution as long as the oxygen concentration is high. However, at low oxygen concentrations, the filamentous aggregates of the sickle cell hemoglobin precipitate causing the RBC to assume the characteristic sickle shape. It is from this phenomenon the disease derives its name. In addition to the primary effect, a number of other characters of the individuals homozygous for this gene also are affected. These effects include, among other things, ‘tower skull’, dilation of heart and heart failure, poor physical development, impaired mental function, pneumonia due to lung damage, rheumatism, paralysis, kidney damage and failure etc.
A number of other recessive genes produce marked and, often, detrimental pleiotropic effects. Consequently, they are referred to as syndromes, e.g., Hunters’ syndrome, Huntington’s chorea, cystic fibrosis etc. Pleiotropic genes are known
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Modified dihybrid ratios with codominant and lethal alleles