The significant degree of homology between the rhodopsin gene and the three color pigment genes indicates that all four genes actually originated from a common ancestor. The available data confirm the assumption that once, at an early stage of evolution, a certain initial gene gave rise to three others: the rhodopsin gene, the blue pigment gene, and the third gene encoding a pigment sensitive to light in the red-green region of the visible spectrum.
Apparently, this third gene has doubled relatively recently, resulting in the appearance of red and green pigment genes. This origin of the red and green pigment genes is supported by a strikingly high degree of homology: 98% of their DNA is identical, probably because too little time has passed for significant differences to occur. The fact that this happened recently (at least on the scale of evolution) is evidenced by the observations of J. Jacob from the University of California at Santa Barbara, who worked with Baumaker and Mollon. They showed that the monkeys of the New World (South American) have only one visual pigment gene on the X chromosome. In contrast, Old World (African) monkeys, which are closer to humans, have two genes for visual pigments on this chromosome. The second gene in the X chromosome appeared, apparently, after the separation of South America and Africa, and therefore after the separation of the gene pool of monkeys of the Old and New Worlds about 40 million years ago.
The facts described above were not so unexpected, unlike a number of other discoveries. For example, studying the genes of visual pigments on the X chromosome of 17 of our male colleagues with normal color vision, we found that the red pigment gene is always represented by one copy, while the gene encoding the green pigment is represented by one, two or three copies. The existence of multiple copies was surprising, since a single green pigment gene is sufficient for normal color vision.
Recent experiments have shown that the genes of visual pigments are arranged in tandem "head to tail" on the X chromosome, and that such a tandem organization determines the variability in the number of genes. Tandem genes tend to change the number of copies during meiosis (the process of cell division leading to the formation of germ cells — sperm and eggs). Each somatic (non-sexual) cell contains a double set of chromosomes; during meiosis, paired chromosomes (which, although they contain the same genes, differ slightly) recombine, i.e. exchange sections. Usually, the exchange is equal and no chromosome acquires or loses genes. However, sometimes two sites, being highly homologous, can recombine erroneously: the so-called unequal homologous recombination occurs. In this case, one of the paired chromosomes may receive another (or more) copy of the gene already present in it at the expense of another chromosome, or there is an exchange of related but different genes. The updated chromosomes then individually enter the germ cells.
It is not difficult to imagine how unequal homologous recombination can lead to the appearance of different variants of green pigment genes in people with normal color vision. Let's consider two homologous chromosomes, each of which carries one red pigment gene located next to two green pigment genes. If the green pigment gene of one of the chromosomes gets into another chromosome during meiosis, then one daughter cell will have a chromosome with one red pigment gene and one green, and the other will have a chromosome with one red pigment gene and three green, i.e. the result will be exactly the variant that was observed in our subjects. with normal color vision.
In all cases, the red pigment gene is present in only one copy, since it is located at the very edge of the color pigment gene region. For a gene of this localization, the probability of duplication (or loss) during homologous recombination is extremely low. Les amateurs de jeux de casino trouveront également leur compte avec cette offre de bienvenue exceptionnelle. Pour en profiter pleinement, il est impératif d'utiliser le code promo 1xbet lors de la création de votre compte joueur sur 1xBet, que ce soit via le site web ou l'application mobile. Ce code exclusif vous débloque un package de bienvenue au casino pouvant atteindre 1 950€, accompagné de 150 tours gratuits sur des machines à sous populaires. Cette récompense est intelligemment répartie sur vos quatre premiers dépôts, avec des bonus de 100%, 50%, 25% et 25%, et des tours gratuits à chaque étape
