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The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific gene depends on the number of copies of each chromosome found in that species, also referred to as ploidy. In diploid species like humans, two full sets of chromosomes are present, meaning each individual has two alleles for any given gene. If both alleles are the same, the genotype is referred to as homozygous. If the alleles are different, the genotype is referred to as heterozygous.
Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic information of their parents. Through heredity, variations between individuals can accumulate and cause species to evolve by natural selection. The study of heredity in biology is genetics.
Genomic imprinting is an epigenetic phenomenon that causes genes to be expressed or not, depending on whether they are inherited from the mother or the father. Genes can also be partially imprinted. Partial imprinting occurs when alleles from both parents are differently expressed rather than complete expression and complete suppression of one parent's allele. Forms of genomic imprinting have been demonstrated in fungi, plants and animals. In 2014, there were about 150 imprinted genes known in mice and about half that in humans. As of 2019, 260 imprinted genes have been reported in mice and 228 in humans.
Mendelian inheritance is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularized by William Bateson. These principles were initially controversial. When Mendel's theories were integrated with the Boveri–Sutton chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics. Ronald Fisher combined these ideas with the theory of natural selection in his 1930 book The Genetical Theory of Natural Selection, putting evolution onto a mathematical footing and forming the basis for population genetics within the modern evolutionary synthesis.
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and the second recessive. This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child. Since there is only one copy of the Y chromosome, Y-linked traits cannot be dominant or recessive. Additionally, there are other forms of dominance such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, and co-dominance, in which different variants on each chromosome both show their associated traits.
X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males and in females who are homozygous for the gene mutation, see zygosity. Females with one copy of the mutated gene are carriers.
A horn is a permanent pointed projection on the head of various animals that consists of a covering of keratin and other proteins surrounding a core of live bone. Horns are distinct from antlers, which are not permanent. In mammals, true horns are found mainly among the ruminant artiodactyls, in the families Antilocapridae (pronghorn) and Bovidae. Cattle horns arise from subcutaneous connective tissue and later fuse to the underlying frontal bone.
Sex linked describes the sex-specific reading patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). In humans, these are termed X-linked recessive, X-linked dominant and Y-linked. The inheritance and presentation of all three differ depending on the sex of both the parent and the child. This makes them characteristically different from autosomal dominance and recessiveness.
Non-Mendelian inheritance is any pattern in which traits do not segregate in accordance with Mendel's laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. In Mendelian inheritance, each parent contributes one of two possible alleles for a trait. If the genotypes of both parents in a genetic cross are known, Mendel's laws can be used to determine the distribution of phenotypes expected for the population of offspring. There are several situations in which the proportions of phenotypes observed in the progeny do not match the predicted values.
Human genetics is the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics, and genetic counseling.
The history of genetics dates from the classical era with contributions by Pythagoras, Hippocrates, Aristotle, Epicurus, and others. Modern genetics began with the work of the Augustinian friar Gregor Johann Mendel. His work on pea plants, published in 1866, provided the initial evidence that, on its rediscovery in 1900, helped to establish the theory of Mendelian inheritance.
The Jacob is a British breed of domestic sheep. It combines two characteristics unusual in sheep: it is piebald—dark-coloured with areas of white wool—and it is often polycerate or multi-horned. It most commonly has four horns. The origin of the breed is not known; broken-coloured polycerate sheep were present in England by the middle of the seventeenth century, and were widespread a century later. A breed society was formed in 1969, and a flock book was published from 1972.
The Icelandic is the Icelandic breed of domestic sheep. It belongs to the Northern European Short-tailed group of sheep, and is larger than most breeds in that group. It is thought that it was introduced to Iceland by Vikings in the late ninth or early tenth century.
Polled livestock are livestock without horns in species which are normally horned. The term refers to both breeds and strains that are naturally polled through selective breeding and also to naturally horned animals that have been disbudded. Natural polling occurs in cattle, yaks, water buffalo, and goats, and in these animals it affects both sexes equally; in sheep, by contrast, both sexes may be horned, both polled, or only the females polled. The history of breeding polled livestock starts about 6000 years BC.
Dehorning is the process of removing the horns of livestock. Cattle, sheep, and goats are sometimes dehorned for economic and safety reasons. Disbudding is a different process with similar results; it cauterizes and thus destroys horn buds before they have grown into horns. Disbudding is commonly performed early in an animal's life, as are other procedures such as docking and castration. In some cases, it can be unnecessary.
Sex-limited genes are genes that are present in both sexes of sexually reproducing species but are expressed in only one sex and have no penetrance, or are simply 'turned off' in the other. In other words, sex-limited genes cause the two sexes to show different traits or phenotypes, despite having the same genotype. This term is restricted to autosomal traits, and should not be confused with sex-linked characteristics, which have to do with genetic differences on the sex chromosomes. Sex-limited genes are also distinguished from sex-influenced genes, where the same gene will show differential expression in each sex. Sex-influenced genes commonly show a dominant/recessive relationship, where the same gene will have a dominant effect in one sex and a recessive effect in the other. However, the resulting phenotypes caused by sex-limited genes are present in only one sex and can be seen prominently in various species that typically show high sexual dimorphism.
The genetic basis of coat colour in the Labrador Retriever has been found to depend on several distinct genes. The interplay among these genes is used as an example of epistasis.
The following outline is provided as an overview of and topical guide to genetics:
Classical genetics is the branch of genetics based solely on visible results of reproductive acts. It is the oldest discipline in the field of genetics, going back to the experiments on Mendelian inheritance by Gregor Mendel who made it possible to identify the basic mechanisms of heredity. Subsequently, these mechanisms have been studied and explained at the molecular level.
Polled Holsteins are cattle born without horns but only occur in a small portion of Holstein cattle. The Holstein breed can go through selective breeding to produce polled calves. Polled is a natural trait for Holsteins but have not been bred for specifically. That is why a very small percentage of Holsteins are naturally polled. Bulls and cows can both carry the polled trait and pass it on to the progeny. Previous testing for polledness were not completely accurate because it was not looking for the gene directly until later discovered. Polled, also known as ‘hornless’ can result in the growth of scurs which are small loose horn growths that do not develop.
References
- ↑ Dove W. (1935) The physiology of horn growth: a study of the morphogenesis, the interaction of tissues, and the evolutionary processes of a Mendelian recessive character by means of transplantation of tissues. Journal of Experimental Zoology 69, 347– 405.
- ↑ Johnston SE, Beraldi D, McRae AF, Pemberton JM, Slate J. 2009. Horn type and horn length genes map to the same chromosomal region in Soay sheep. Heredity (Edinb). 104(2):196-205. doi: 10.1038/hdy.2009.109
- ↑ Asai, M, T. G. Berryere, Schmutz, S. M. 2004. The scurs locus in cattle maps to bovine chromosome 19. Animal Genetics 35:34-39
- ↑ Georges, M., R. Drinkwater, T. King, A. Mishra, S.S. Moore, D. Nielsen, L.S. Sargeant, A. Sorensen, M.R. Steele, X. Zhao, J.E. Womack and J. Hetzel. 1993. Microsatellite mapping of a gene affecting horn development in Bos taurus. Nat. Genet. 3: 206-210.
- ↑ Long, C. R. and K. E. Gregory. 1978. Inheritance of the horned, scurred, and polled condition in cattle. J. Heredity. 69:395-400.
- ↑ Allison, B.C. "Inheritance of Polledness, Horns and Scurs in Beef Cattle". North Carolina State University. Retrieved 30 December 2013.