Imprinted Genes In Mammals
That imprinting might be a feature of mammalian development was suggested in breeding experiments in mice carrying reciprocal chromosomal translocations. Nucleus transplantation experiments in mouse zygotes in the early 1980s confirmed that normal development requires the contribution of both the maternal and paternal genomes. The vast majority of mouse embryos derived from parthenogenesis and androgenesis die at or before the blastocyst/implantation stage. In the rare instances that they develop to postimplantation stages, gynogenetic embryos show better embryonic development relative to placental development, while for androgenones, the reverse is true. Nevertheless, for the latter, only a few have been described .
No naturally occurring cases of parthenogenesis exist in mammals because of imprinted genes. However, in 2004, experimental manipulation by Japanese researchers of a paternal methylation imprint controlling the Igf2 gene led to the birth of a mouse ” rel=”nofollow”> Kaguya) with two maternal sets of chromosomes, though it is not a true parthenogenone since cells from two different female mice were used. The researchers were able to succeed by using one egg from an immature parent, thus reducing maternal imprinting, and modifying it to express the gene Igf2, which is normally only expressed by the paternal copy of the gene.
Gene Expression In Imprinted And Nonimprinted Genes
Chromosomes, and the genes they contain, are inherited in pairs, with one copy of each supplied from each parent. For most genes, both members of the pair, called the maternal and paternal alleles, are used equally. Both are expressed in roughly equal amounts.
In contrast, for most imprinted genes, only one allele is expressed, while the other copy is silenced by imprinting. For some genes it is the maternal copy, for others it is the paternal copy. This is an exception to the Mendelian assumption that the two parents contribute equally to the phenotype controlled by autosomal genes. For some genes, both alleles are expressed, but one copy is expressed much more than the other. For some genes, the silencing occurs in some tissues but not others.
Imprinted genes should not be confused with sex-linked genes, which are carried on the X or Y chromosome. Most imprinted alleles are located on autosomes , but are “stamped” with the sex of the parent that contributed them.
Imprinting is thought to be responsible for many cases of incomplete penetrance, an inheritance pattern in which a dominant gene is not expressed in some individuals despite being present. Imprinting offers a mechanism by which a particular allele can be turned on or turned off as it is passed down through successive generations.
How Animal Imprinting Works
Imagine yourself as a young bird just cracking your way out of an egg. You’ve worked hard to poke and wiggle your way out of the shell, and now you take in the bright, new world all around you. It’s an overwhelming experience, but instinctively, you know that there must be someone around who will take care of you. Finally, your blurry eyes make out some movement this must be the protector!
Luckily, you are a precocial bird who can walk immediately after hatching . You waddle your way closer to the movement, and you notice a distinct scent getting stronger. Suddenly, that moving being emits an oddly familiar noise. You finally make your way over. The protector feels warm and soft. So, after your first hard day of work as a hatchling, you’ve done your job and found your parent, so you curl up and go to sleep.
This young precocial bird has just imprinted on its mother. In a broad sense, animal imprinting concerns how some species of animals learn during a short and sensitive period immediately after birth. In its more narrow definition, the phenomenon is exclusive to certain species of birds. When hatching, these birds don’t innately know who their parents are. Rather, they use environmental clues to both identify and attach themselves to their protector.
But first, let’s explore early studies of animal imprinting in more detail.
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Genomic Imprinting And Cancer
Genomic imprinting is a modification of DNA that leads to a different expression of the gene depending on the parent of origin. This may be passed from generation to generation. However, the specific expression of these modifications may increase, decrease, or remain stable from generation to generation . These epigenetic alterations appear to occur commonly in human malignancies and because of their unique properties may lead to novel forms of therapy.
Assumptions are made in Mendelian genetics, which we apply to human disease, that the maternal and paternal alleles are equivalent. We also assume that both copies of the gene are necessary for normal function. This is not true with respect to genomic imprinting. An example of imprinting is the differences noted between the mule and the hinny. The mule is the cross between a maternal horse and a paternal donkey. It is much larger than the hinny, which is a cross between a maternal mule and a paternal horse. Although these two have the same genomic equivalent, they are distinctly different, indicating differential expression of maternal and paternal genes.
Examples of genomic imprinting and cancer are the hydatidiform mole and the teratoma. The hydatidiform mole is composed of paternal chromosomes, and the teratoma is composed of only maternal chromosomes. These examples demonstrate that not only does it take 46 chromosomes to make a human being, but there must also be a balance between maternal and paternal elements.
Imprinting In The Wild
The conditions under which imprinting is studied in the laboratory are necessarily impoverished and artificial. The results can give a seriously misleading view of what happens in the wild. Chicks and ducklings spend most of the daylight hours on the first day after hatching being brooded by their mothers. The little birds hardly seemed to pay her any attention. Their activity around the hen does increase substantially on the second day after hatching, or even later if the ambient temperature is low.
Although the development of new preferences is initially prevented by escape from novelty or by the low level of social responsiveness to unfamiliar things, enforced contact may wear down these behavioural constraints to the point where the bird does develop a new preference. This flexibility could be of some functional importance in colonial nesting species such as gulls. In the absence of parents, for which the young bird forms its strongest preference, the bird may still be able to survive by responding socially to other adults and inducing them to feed it.
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Imprinted Genes In Plants
A similar imprinting phenomenon has also been described in flowering plants . During fertilization of the egg cell, a second, separate fertilization event gives rise to the endosperm, an extraembryonic structure that nourishes the embryo in a manner analogous to the mammalian placenta. Unlike the embryo, the endosperm is often formed from the fusion of two maternal cells with a male gamete. This results in a triploid genome. The 2:1 ratio of maternal to paternal genomes appears to be critical for seed development. Some genes are found to be expressed from both maternal genomes while others are expressed exclusively from the lone paternal copy. It has been suggested that these imprinted genes are responsible for the triploid block effect in flowering plants that prevents hybridization between diploids and autotetraploids. Several computational methods to detect imprinting genes in plants from reciprocal crosses have been proposed.
Neural Changes Localized To Specific Brain Regions
Imprinting leads to changes in the incorporation of radioactive uracil into RNA in a restricted brain region, the intermediate and medial part of the hyperstriatum ventrale , a sheet of cells in the cerebral hemispheres . Further evidence that the region is crucially involved in learning is the following: destruction of IMHV before training prevents imprinting if the region is destroyed immediately after training, chicks do not prefer the training object, though for chicks with lesions of IMHV an imprinting object still elicits approach behavior but the chicks appear incapable of learning its characteristics it is possible to bias chicks’ preferences by delivering trains of short pulses of electric current to IMHV through electrodes that have been implanted into theregion. At the end of the period of electrical stimulation, the chicks were shown two lights, one flashing at the rate of 4.5 per second and the other at 1.5 per second. If the IMHV region had been stimulated at 4.5 trains per second, the chicks preferred the light flashing at this frequency. In contrast, chicks that had received electrical stimulation of IMHV at the rate of 1.5 trains per second preferred the light flashing at this rate. Electrical stimulation of two visual receiving areas of the forebrain did not influence the chicks’ preferences. Taken together, these results stronglysuggest that the IMHV region is involved in the recognition memory of imprinting, probably storing information.
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Genomic Imprinting In Mammals
Imprinted Genes Bypass Epigenetic Reprogramming
Soon after egg and sperm meet, most of the epigenetic tags that activate and silence genes are stripped from the DNA. However, in mammals, imprinted genes keep their epigenetic tags. Imprinted genes begin the process of development with epigenetic tags in place.
Imprinted genes are not the only genes that bypass epigenetic reprogramming in the early embryo. Studying imprinting may help researchers understand how other genes make it through reprogramming without losing their epigenetic tags.
Imprinting is unique to mammals and flowering plants. In mammals, about 1% of genes are imprinted.
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Neural Control Of Filial Imprinting
Johnson and Bolhuis identified two independent neural systems that control filial imprinting in precocial birds. Newly hatched chicks will follow almost anything that has eyes and moves. After the chick follows something, another part of the brain, analogous to the frontal cortex, recognizes and imprints on the individual being followed. These mechanisms are independent. There is an instinct for chicks to follow, and then they learn what they are following.
It might seem odd that being able to identify and follow a mother does not have a genetic mechanism. Yet with a neural rather than genetic mechanism, the chick gains flexibility that might help in survival. If a chick’s mother dies, the chick can then be adopted by another family member or conspecific. If the chick’s recognition of its mother were genetic, the chick would not follow its adoptive parent, and would die. Further, detailing the recognition of a specific individual is far too complicated to achieve genetically. An individual may be able to tell what relationship he has with others, but cannot be genetically programmed to recognize a certain individual, particularly because environment plays a large part in appearance. The chick’s neural imprinting system allows more adaptive flexibility and hence is an advantage.
Whos Your Mama The Science Of Imprinting
When naturalist Joe Hutto became mother to a flock of wild turkeys, it gave him a unique opportunity to immerse himself in their lives and see the world through their eyes. He was not a stranger intruding, but rather the heart of the flock. He was able to do this by taking advantage of a biological phenomenon known as imprinting.
Imprinting refers to a critical period of time early in an animals life when it forms attachments and develops a concept of its own identity. Birds and mammals are born with a pre-programmed drive to imprint onto their mother. Imprinting provides animals with information about who they are and determines who they will find attractive when they reach adulthood.
Imprinting has been used by mankind for centuries in domesticating animals and poultry. In Rome, in the first century B.C., agriculturalist Lucius Moderatus Columella wrote a treatise on agrarian practices and suggested that anyone wishing to establish a place for rearing ducks should collect wildfowl eggs in the marches and set them under farmyard hens, for when they are reared in this way they lay aside their wild nature. For centuries in rural China, rice farmers have imprinted newly hatched ducklings to a special stick, which they then use to bring the ducks out to their rice paddies to control the snail population.
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Normal Genetic Imprinting Process
A gene is made up of long sequences of DNA . When DNA is changed into RNA and then into protein, the processes involved are known as transcription and translation. For a gene to exert an effect on the individual’s system, it has to be transcribed and translated. Some genes are constitutively transcribed. Others are only transcribed when their products are needed.
Genetic imprinting is a normal process that occurs in several dozen mammalian genes. It is thought to play a role in the transmission of nutrients from the mother to the fetus and to the newborn. Imprinted genes tend to impact fetal growth and the behavior of the newborn infant. Abnormalities involving imprinting patterns may result in many different diseases.
Disorders Associated With Imprinting
Imprinting may cause problems in cloning, with clones having DNA that is not methylated in the correct positions. It is possible that this is due to a lack of time for reprogramming to be completely achieved. When a nucleus is added to an egg during somatic cell nuclear transfer, the egg starts dividing in minutes, as compared to the days or months it takes for reprogramming during embryonic development. If time is the responsible factor, it may be possible to delay cell division in clones, giving time for proper reprogramming to occur.
An allele of the “callipyge” , or CLPG, gene in sheep produces large buttocks consisting of muscle with very little fat. The large-buttocked phenotype only occurs when the allele is present on the copy of chromosome 18 inherited from a sheep’s father and is not on the copy of chromosome 18 inherited from that sheep’s mother.
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Examples Of Imprinting In A Sentence
imprintingWiredimprintingSmithsonian MagazineimprintingThe AtlanticimprintingWSJimprinting Washington Postimprinting STATimprinting baltimoresun.com/maryland/carrollimprintingal
These example sentences are selected automatically from various online news sources to reflect current usage of the word ‘imprinting.’ Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.
Imprinting Is Required For Normal Development
An individual normally has one active copy of an imprinted gene. Improper imprinting can result in an individual having two active copies or two inactive copies. This can lead to severe developmental abnormalities, cancer, and other problems.
Prader-Willi and Angelman syndrome are two very different disorders, but they are both linked to the same imprinted region of chromosome 15. Some of the genes in this region are silenced in the egg, and at least one gene is silenced in the sperm. So someone who inherits a defect on chromosome 15 is missing different active genes, depending on whether the chromosome came from mom or dad.
Prader-Willi syndrome
Symptoms include learning difficulties, speech problems, seizures, jerky movements, and an unusually happy disposition. Individuals are missing gene activity that normally comes from mom. Happens when mom’s copy is defective or missing, or when there are two paternal copies.
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Impact Of Genomic Imprinting
In most cases genomic imprinting is a normal process and has no affect on the normal individual . However, imprinted genes are involved in the development of some genetic disorders and in cancer .
Imprinted genes are involved in the development of some cancers. The imprinted fetal growth factor gene, IGF2, is commonly expressed in cancers such as Wilms tumor of the kidney, and cancers of the breast, lung, liver, and colon. In these cancers the maternal IGF2 imprint has been lost and both gene alleles are expressed , this is termed “relaxation of imprinting.”
There are many theories for why genomic imprinting exists. One of the most favored , proposed by David Haig , suggests that imprinting is a form of genetic reproductive conflict between the sexes each vying for a different reproductive outcome. Males desire large offspring males, so they over-express growth factors such as the paternally expressed fetal growth factor IGF2. However, females needing to limit fetal growth to ensure their successful birth have repressed growth factor expression by imprinting the gene.
Neural Mechanisms Of Filial Imprinting
The neural basis of the recognition memory underlying filial imprinting has been studied most extensively in the domestic chick . When dark-reared chicks are trained by exposing them to an imprinting object for approximately one to two hours, metabolic changes occur in the dorsal part of the cerebral hemispheres. Specifically, there is an increase in the incorporation of radioactively tagged uracil into RNA in this brain region of trained chicks compared with control chicks . Several reasons support the idea that the biochemical changes are related to learning rather than to various side effects of training : when visual input is restricted to one hemisphere, incorporation of radioactive uracil into RNA is higher in the trained than in the untrained hemisphere the amount incorporated is related to how much the chicks learn and not to various other measures of behavior the increase is not related to short-term effects of sensory stimulation.
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Reversibility And Sensitive Periods
Imprinting was thought to be irreversible and to occur during a sensitive period . Numerous studies have demonstrated that filial preferences can in fact be reversed when the original object is removed and the animal is exposed to a novel object. Evidence suggests that there is a difference between the memory of the first stimulus and that of subsequent stimuli to which the animal is exposed. Under certain circumstances the preference for the first object may return . The ability to form filial attachments has been shown to depend on both developmental age and time since hatching. The ability to imprint is related to the development of the animal’s sensorimotor abilities. The sensitive period is brought to an end by the learning experience itself: Once the bird has formed a preference for a particular object, it avoids novel objects. Consequently it tends not to be exposed to them for long and so may learn little about them. When the bird is left in its cage, it may form an attachment to features of its rearing environment. Rearing the bird in a visually impoverished environment extends the period during which it forms an attachment to a conspicuous object . The sensitive period pertains to filial attachment and may relate to the link formed between the representation of the imprinted object and approach behavior. Although the formation of this link may have a sensitive period, there is no reason to suppose that the learning and recognition processes have one.