For example, in the XY system that is found in most mammals - including human beings - males have one X chromosome and one Y chromosome XY and females have two X chromosomes XX. The paired chromosomes that are not involved in sex determination are called autosomes , to distinguish them from the sex chromosomes. Human beings have 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes X and Y. The different forms of a gene that are found at a specific point or locus along a given chromosome are known as alleles.
Diploid organisms have two alleles for each autosomal gene - one inherited from the mother, one inherited from the father. Within a population, there may be a number of alleles for a given gene. Individuals that have two copies of the same allele are referred to as homozygous for that allele; individuals that have copies of different alleles are known as heterozygous for that allele.
The inheritance patterns observed will depend on whether the allele is found on an autosomal chromosome or a sex chromosome, and on whether the allele is dominant or recessive. If the phenotype associated with a given version of a gene is observed when an individual has only one copy, the allele is said to be autosomal dominant.
The phenotype will be observed whether the individual has one copy of the allele is heterozygous or has two copies of the allele is homozygous. If the phenotype associated with a given version of a gene is observed only when an individual has two copies, the allele is said to be autosomal recessive.
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The phenotype will be observed only when the individual is homozygous for the allele concerned. An individual with only one copy of the allele will not show the phenotype, but will be able to pass the allele on to subsequent generations. As a result, an individual heterozygous for an autosomal recessive allele is known as a carrier. In many organisms, the determination of sex involves a pair of chromosomes that differ in length and genetic content - for example, the XY system used in human beings and other mammals.
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The X chromosome carries hundreds of genes, and many of these are not connected with the determination of sex. The smaller Y chromosome contains a number of genes responsible for the initiation and maintenance of maleness, but it lacks copies of most of the genes that are found on the X chromosome. As a result, the genes located on the X chromosome display a characteristic pattern of inheritance referred to as sex-linkage or X-linkage.
Females XX have two copies of each gene on the X chromosome, so they can be heterozygous or homozygous for a given allele. However, males XY will express all the alleles present on the single X chromosome that they receive from their mother, and concepts such as 'dominant' or 'recessive' are irrelevant.
A number of medical conditions in humans are associated with genes on the X chromosome, including haemophilia, muscular dystrophy and some forms of colour blindness. Some traits or characteristics display continuous variation , a range of phenotypes that cannot be easily divided into clear categories.
Red eye color is wild-type and is dominant to white eye color. Eye color in Drosophila was one of the first X-linked traits to be identified. Thomas Hunt Morgan mapped this trait to the X chromosome in In flies, the wild-type eye color is red X W and it is dominant to white eye color X w Figure 3. Because of the location of the eye-color gene, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous , because they have only one allele for any X-linked characteristic.
Hemizygosity makes the descriptions of dominance and recessiveness irrelevant for XY males. In an X-linked cross, the genotypes of F 1 and F 2 offspring depend on whether the recessive trait was expressed by the male or the female in the P 0 generation. Now, consider a cross between a homozygous white-eyed female and a male with red eyes Figure 4. Figure 4. Punnett square analysis is used to determine the ratio of offspring from a cross between a red-eyed male fruit fly and a white-eyed female fruit fly.
What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color? Discoveries in fruit fly genetics can be applied to human genetics. When a female parent is homozygous for a recessive X-linked trait, she will pass the trait on to percent of her offspring. In humans, the alleles for certain conditions some forms of color blindness, hemophilia, and muscular dystrophy are X-linked. Females who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects. These females will pass the disease to half of their sons and will pass carrier status to half of their daughters; therefore, recessive X-linked traits appear more frequently in males than females.
In some groups of organisms with sex chromosomes, the gender with the non-homologous sex chromosomes is the female rather than the male. This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in which they are hemizygous. This practice activity will help you remember the difference between types of non-Mendelian inheritance and remember just how they work. Click here for a text-only version of the activity.
Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. We now know that this is an oversimplification. Although individual humans and all diploid organisms can only have two alleles for a given gene, multiple alleles may exist at the population level such that many combinations of two alleles are observed.
All other phenotypes or genotypes are considered variants of this standard, meaning that they deviate from the wild type.
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The variant may be recessive or dominant to the wild-type allele. An example of multiple alleles is coat color in rabbits Figure 5. The chinchilla phenotype, c ch c ch , is expressed as black-tipped white fur. The Himalayan phenotype, c h c h , has black fur on the extremities and white fur elsewhere.
In cases of multiple alleles, dominance hierarchies can exist. In this case, the wild-type allele is dominant over all the others, chinchilla is incompletely dominant over Himalayan and albino, and Himalayan is dominant over albino. This hierarchy, or allelic series, was revealed by observing the phenotypes of each possible heterozygote offspring. Figure 5. Four different alleles exist for the rabbit coat color C gene. Figure 6.
As seen in comparing the wild-type Drosophila left and the Antennapedia mutant right , the Antennapedia mutant has legs on its head in place of antennae. It is now evident from molecular genetics that all gene loci are involved in complex interactions with many other genes e. This formula applies to a gene with exactly two alleles and relates the frequencies of those alleles in a large population to the frequencies of their three genotypes in that population. For example, if p is the frequency of allele A , and q is the frequency of allele a then the terms p 2 , 2 pq , and q 2 are the frequencies of the genotypes AA , Aa and aa respectively.
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Now, if A is completely dominant to a then the frequency of the carrier genotype Aa cannot be directly observed since it has the same traits as the homozygous genotype AA , however it can be estimated from the frequency of the recessive trait in the population, since this is the same as that of the homozygous genotype aa. This formula relies on a number of assumptions and an accurate estimate of the frequency of the recessive trait.
In general, any real-world situation will deviate from these assumptions to some degree, introducing corresponding inaccuracies into the estimate. If the recessive trait is rare, then it will be hard to estimate its frequency accurately, as a very large sample size will be needed. The property of "dominant" is sometimes confused with the concept of advantageous and the property of "recessive" is sometimes confused with the concept of deleterious, but the phenomena are distinct. Dominance describes the phenotype of heterozygotes with regard to the phenotypes of the homozygotes and without respect to the degree to which different phenotypes may be beneficial or deleterious.
Since many genetic disease alleles are recessive and because the word dominance has a positive connotation, the assumption that the dominant phenotype is superior with respect to fitness is often made. This is not assured however; as discussed below while most genetic disease alleles are deleterious and recessive, not all genetic diseases are recessive. Nevertheless, this confusion has been pervasive throughout the history of genetics and persists to this day. Addressing this confusion was one of the prime motivations for the publication of the Hardy-Weinberg principle. The molecular basis of dominance was unknown to Mendel.
It is now understood that a gene locus includes a long series hundreds to thousands of bases or nucleotides of deoxyribonucleic acid DNA at a particular point on a chromosome. In this process, different alleles at a locus may or may not be transcribed, and if transcribed may be translated to slightly different versions of the same protein called isoforms.
Proteins often function as enzymes that catalyze chemical reactions in the cell, which directly or indirectly produce phenotypes. In any diploid organism, the DNA sequences of the two alleles present at any gene locus may be identical homozygous or different heterozygous. Even if the gene locus is heterozygous at the level of the DNA sequence, the proteins made by each allele may be identical.
In the absence of any difference between the protein products, neither allele can be said to be dominant see co-dominance , above. Even if the two protein products are slightly different allozymes , it is likely that they produce the same phenotype with respect to enzyme action, and again neither allele can be said to be dominant.
Dominance typically occurs when one of the two alleles is non-functional at the molecular level, that is, it is not transcribed or else does not produce a functional protein product. This can be the result of a mutation that alters the DNA sequence of the allele. For example, in humans and other organisms, the unpigmented skin of the albino phenotype  results when an individual is homozygous for an allele that encodes a non-functional version of an enzyme needed to produce the skin pigment melanin.
It is important to understand that it is not the lack of function that allows the allele to be described as recessive: this is the interaction with the alternative allele in the heterozygote. Three general types of interaction are possible:. Many proteins are normally active in the form of a multimer, an aggregate of multiple copies of the same protein, otherwise known as a homomultimeric protein or homooligomeric protein.
A mutation that leads to a mutant protein that disrupts the activity of the wild-type protein in the multimer is a dominant-negative mutation. A dominant-negative mutation may arise in a human somatic cell and provide a proliferative advantage to the mutant cell, leading to its clonal expansion.
For instance, a dominant-negative mutation in a gene necessary for the normal process of programmed cell death Apoptosis in response to DNA damage can make the cell resistant to apoptosis. This will allow proliferation of the clone even when excessive DNA damage is present.
Such dominant-negative mutations occur in the tumor suppressor gene p Dominant-negative p53 mutations occur in a number of different types of cancer and pre-cancerous lesions e. Dominant-negative mutations also occur in other tumor suppressor genes. For instance two dominant-negative germ line mutations were identified in the Ataxia telangiectasia mutated ATM gene which increases susceptibility to breast cancer. Dominant-negative mutations have also been described in organisms other than humans.
In fact, the first study reporting a mutant protein inhibiting the normal function of a wild-type protein in a mixed multimer was with the bacteriophage T4 tail fiber protein GP In humans, many genetic traits or diseases are classified simply as "dominant" or "recessive". Especially with so-called recessive diseases, which are indeed a factor of recessive genes, but can oversimplify the underlying molecular basis and lead to misunderstanding of the nature of dominance. To illustrate these nuances, the genotypes and phenotypic consequences of interactions among three hypothetical PAH alleles are shown in the following table: .
Thus, the A allele is dominant to the B allele with respect to PKU, but the B allele is incompletely dominant to the A allele with respect to its molecular effect, determination of PAH activity level 0. Note once more that it is irrelevant to the question of dominance that the recessive allele produces a more extreme [Phe] phenotype. For a third allele C , a CC homozygote produces a very small amount of PAH enzyme, which results in a somewhat elevated level of [ Phe ] in the blood, a condition called hyperphenylalaninemia , which does not result in intellectual disability.
That is, the dominance relationships of any two alleles may vary according to which aspect of the phenotype is under consideration. It is typically more useful to talk about the phenotypic consequences of the allelic interactions involved in any genotype, rather than to try to force them into dominant and recessive categories. From Wikipedia, the free encyclopedia.
This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. See also: Introduction to genetics. See also: Ploidy and Zygosity. This section is about gene notations that identify dominance.
For modern formal nomenclature, see Gene nomenclature. Main article: Sex linkage. Main article: Epistasis. Main article: Hardy—Weinberg principle. Oxford Dictionaries Online. Oxford University Press. Retrieved 14 May Modern Genetic Analysis. New York: W. Genome: The Autobiography of a Species in 23 Chapters. Harper Collins.
A Dictionary of Genetics 7th ed. Dominance [refers] to alleles that fully manifest their phenotype when present in the heterozygous Veterinary Genetics Laboratory, University of California. Retrieved April Animal Genetics. Journal of Heredity. Memorial University of Newfoundland. January
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