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In this article we will discuss about the definition of pleiotropy. Also learn about some examples of pleiotropy.
The ability of a gene to influence more than one phenotypic trait is known as pleiotropy. However, of the many effects, one effect is more pronounced than the other and is called the major effect, while the other effects may be less evident and are called secondary effects. The genes with this ability are called pleiotropic genes and the phenomenon is known as pleiotropism.
Examples of Pleiotropism:
a. In garden pea, the gene which controls flower colour also controls colour of seed coat and presence of red spots in the leaf axils.
b. In Drosophila, the recessive gene for vestigial wings, control other traits such as the nature of the tiny wing like structure behind the wings, bristles, reproductive organs, fertility and reduced longevity.
c. In man, the gene that causes phenylketonuria is pleiotropic and produces many abnormal phenotypic traits such as short stature, mental retardation, pigmentation in the skin, excessive sweating etc. Another example of pleiotropism in man is seen in the condition of sickle cell anaemia. Sickle cell anaemia is characterised by sickle shaped red blood cells and anaemia.
The red blood cell of the patients is not efficient in transporting oxygen and gets destroyed. The spleen becomes enlarged because of the accumulated red blood cells. The cells become jammed in the arteries and capillaries, which causes pain in the limbs. The patient becomes weak and has poor physical development.
Linus Pauling in 1949 showed that haemoglobin in sickle cell anaemia sufferers is different from that of normal adult haemoglobin. Haemoglobin is made of four polypeptide chains, two alpha chains which have 141 amino acids and two beta chains, which are 146 amino acids long. When the sixth amino acid, glutamic acid is replaced by valine normal haemoglobin becomes abnormal in the beta chain.
The normal gene is HbA and the changed or mutated gene is HbS. The two genes are co- dominant. Normal individuals have a genotype, HbAHbA, while the sickle anaemia sufferer has a genotype which can be represented as HbSHbS. Heterozygous individuals have a genotype, HbA HbS. These people are carriers of the sickle cell trait. A marriage between two carriers will produce progeny as shown in Fig. 8.
It was observed that carriers are less susceptible to malaria since the malarial parasite multiplies inside normal red blood cells. In malarial prone areas, normal individuals, HbAHbA, will suffer from malaria, while sickle anaemia sufferer, HbSHbS will die of sickle cell anaemia. But the heterozygote will have a selective advantage over the others and are more likely to survive and pass on their genes to the next generation.