The alternative fates of the gene duplicates are demonstrated in the history of evolution of human globin genes. The development of family tree from fish ancestors to terrestrial amniotes that laid eggs to placental mammals possesses a sequence of improvement in tissue oxygenation. It also involves the development of extra globin genes with original patterns of regulation and the advancement of haemoglobin proteins with separate oxygen binding properties.
Haemoglobin of the adult is a tetramer having two a-polypeptide chains and two ß-chains, both with its attached heme molecule. Chromosome 16 contains the gene encoding for adult a-chain, whereas the gene encoding the ß-chain is on the chromosome 11. The both chains are similar about 49% in their amino acid sequences and this resemblance displays their common basis from an ancestral globin gene deep in evolutionary period. On chromosome 16, the a-chain gene exists in a group of five associated genes (a and ?), and on the chromosome 11, the ß-chain be located in a group of six associated genes (e, ß, d, and ?). Every group has a pseudogene ?a, ?ß respectively.
In the last seven months of fetal development ? – genes are expressed to produce fetal haemoglobin that contains two a-chains and two ? chains. The fetal haemoglobin has a better attraction for oxygen than does adult haemoglobin, which permits the fetus to extract oxygen from mother’s blood circulation through placental barrier. Up to 95% of haemoglobin is the fetal type at birth. After that, expression of the adult ß form substitutes ? and a minute amount of ?-globin is also formed. The position of expression of globin chains throughout the development is organized by a complicated series of cis-acting regulatory sequences and extremely follows the arrangement of genes on every chromosome.
The ?-genes are limited to placental mammals only. Their different developmental regulation and protein products signify that the duplicates have evolved variations in function that have donated to the development placental existence. The regulatory alternatives of the genes are identified that initiate the expression of the fetal haemoglobin to continue into childhood and adulthood.