Ohno's law
Encyclopedia
Ohno's law was proposed by a Japanese biologist Susumu Ohno
, saying that the gene
content of the mammal
ian species
has been conserved over species not only in the DNA
content but also in the genes themselves. That is, nearly all mammalian species have conserved the X chromosome
from their primodial X chromosome of a common ancestor.
evidence, in first, mammalian X chromosomes in various species, including human
and mouse
, have nearly the same size, with the content of about 5% of the genome. Second, for individual gene loci
, a number of X-linked genes are common through mammalian species. Examples are found in glucose-6-phosphate dehydrogenase
(G6PD), a gene for polypeptide of antihemophilic globulin (AHG or Factor VIII
) in hemophilia A
and B
, and plasma thromboplastin component gene (PTC or Factor IX
). Moreover, no instances were found where an X-linked
gene in one species was located on an autosome
in the other species.
mechanism would have been established in their earlier stages of evolution
, polyploidy
would have not occurred due to its incompatibility with the sex-determining mechanism. Moreover, X-autosome translocation
would have been prohibited because it might have resulted in detrimental effects for survival to the organism. Thus in mammals, the content of X chromosomes has been conserved after typical 2 round duplication events at early ancestral stages of evolution, at the fish or amphibia (2R hypothesis
).
s and marsupial
s were not considered to be ancestors of true mammals, but they have diverged very early from the main line of mammals. Chloride channel
gene (CLCN4) was mapped to the human X but on chromosome 7 of C57BL/6 mice, species of Mus musculus
, though the gene is located on X of Mus spretus
and rat
.
Susumu Ohno
was an Asian American geneticist and evolutionary biologist, and seminal researcher in the field of molecular evolution.- Biography :Susumu Ohno was born of Japanese parents in Seoul, Korea, on February 1, 1928. The second of five children, he was the son of the minister of education of the...
, saying that the gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
content of the mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
has been conserved over species not only in the DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
content but also in the genes themselves. That is, nearly all mammalian species have conserved the X chromosome
X chromosome
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...
from their primodial X chromosome of a common ancestor.
Evidence
As a cytologicalCell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
evidence, in first, mammalian X chromosomes in various species, including human
Human
Humans are the only living species in the Homo genus...
and mouse
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
, have nearly the same size, with the content of about 5% of the genome. Second, for individual gene loci
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
, a number of X-linked genes are common through mammalian species. Examples are found in glucose-6-phosphate dehydrogenase
Glucose-6-phosphate dehydrogenase
Glucose-6-phosphate dehydrogenase is a cytosolic enzyme in the pentose phosphate pathway , a metabolic pathway that supplies reducing energy to cells by maintaining the level of the co-enzyme nicotinamide adenine dinucleotide phosphate...
(G6PD), a gene for polypeptide of antihemophilic globulin (AHG or Factor VIII
Factor VIII
Factor VIII is an essential blood clotting factor also known as anti-hemophilic factor . In humans, Factor VIII is encoded by the F8 gene...
) in hemophilia A
Haemophilia A
Haemophilia A is a deficiency in clotting factor VIII.Haemophilia A is inherited as an X-linked recessive trait, and thus occurs in males and in homozygous females. However, mild haemophilia A has been described in heterozygous females, presumably due to extremely unfavourable lyonization...
and B
Haemophilia B
Haemophilia B is a blood clotting disorder caused by a mutation of the Factor IX gene, leading to a deficiency of Factor IX. It is the second most common form of haemophilia, rarer than haemophilia A. It is sometimes called Christmas disease after Stephen Christmas, the first patient described...
, and plasma thromboplastin component gene (PTC or Factor IX
Factor IX
Factor IX is one of the serine proteases of the coagulation system; it belongs to peptidase family S1. Deficiency of this protein causes hemophilia B. It was discovered in 1952 after a young boy named Stephen Christmas was found to be lacking this exact factor, leading to...
). Moreover, no instances were found where an X-linked
Sex linkage
Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance...
gene in one species was located on an autosome
Autosome
An autosome is a chromosome that is not a sex chromosome, or allosome; that is to say, there is an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes. In addition to autosomes, there are sex chromosomes, to be specific: X and Y...
in the other species.
Conservation mechanisms
The content of a chromosome would be changed mainly by mutation after duplication of the chromosome and translocation with other chromosomes. However, in mammals, since the chromosomal sex-determinationSex-determination system
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual...
mechanism would have been established in their earlier stages of evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
, polyploidy
Polyploidy
Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...
would have not occurred due to its incompatibility with the sex-determining mechanism. Moreover, X-autosome translocation
Chromosomal translocation
In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on...
would have been prohibited because it might have resulted in detrimental effects for survival to the organism. Thus in mammals, the content of X chromosomes has been conserved after typical 2 round duplication events at early ancestral stages of evolution, at the fish or amphibia (2R hypothesis
2R hypothesis
The 2R hypothesis or Ohno's hypothesis, first proposed by Susumu Ohno in 1970, is a contested hypothesis in genomics and molecular evolution suggesting that the genomes of the early vertebrate lineage underwent one or more complete genome duplications, and thus modern vertebrate genomes reflect...
).
Contradicting and supportive evidence
Genes on the long arm of the human X are contained in the monotreme X and genes on the short arm of the human X are distributed on the autosomes of marsupials. Ohno commented to the result that monotremeMonotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s and marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s were not considered to be ancestors of true mammals, but they have diverged very early from the main line of mammals. Chloride channel
Chloride channel
Chloride channels are a superfamily of poorly understood ion channels consisting of approximately 13 members.Chloride channels display a variety of important physiological and cellular roles that include regulation of pH, volume homeostasis, organic solute transport, cell migration, cell...
gene (CLCN4) was mapped to the human X but on chromosome 7 of C57BL/6 mice, species of Mus musculus
House mouse
The house mouse is a small rodent, a mouse, one of the most numerous species of the genus Mus.As a wild animal the house mouse mainly lives associated with humans, causing damage to crops and stored food....
, though the gene is located on X of Mus spretus
Algerian Mouse
The Algerian mouse, or western Mediterranean mouse, is a wild species of mouse closely related to the house mouse, native to open habitats around the western Mediterranean.-Description:...
and rat
Rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...
.
Dictionaries having an article's name of ‘Ohno ‘s law’
- Rédei GP (2003). "Ohno's law." Encyclopedic dictionary of genetics, genomics, and proteomics, 2nd ed. New York: Wiley-Liss, p. 870. ISBN 0471268216.