Biological Sciences
Multiple Alleles
Multiple alleles refer to the existence of more than two forms of a gene within a population. Each individual still carries only two alleles, but there are more than two possible versions of the gene in the population as a whole. This can lead to a wide range of phenotypic variations and is important in understanding genetic diversity and inheritance patterns.
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6 Key excerpts on "Multiple Alleles"
eBook - PDF- Tomar, Arun Kumar(Authors)
- 2021(Publication Date)
- Daya Publishing House(Publisher)
This change is called mutation which is very rare. The mutation may occur several times in a gene and so several forms of the same gene may be produced. A large numbers of alleles at a locus are thus expected to be present but in This ebook is exclusively for this university only. Cannot be resold/distributed. different individuals of the population. Only two alleles can exist in a single individual because there are only two locations on a pair of homologous chromosomes. Therefore, the genes may exist in different allelomorphic states having two or more forms (alleles) but more than two forms of a gene (alleles) will be present in different individuals of the population. This situation of existing more than two allelic form of a gene at a given locus in the population is called a multiple allelic system or series and the alleles are called as the Multiple Alleles . When more that two alleles control a character, it is said that the character is controlled by Multiple Alleles. Symbols for Multiple Alleles: The mutant trait that deviates from the ancestral type is usually chosen the basis for the symbol of gene. The multiple allelic system /series is indicated (written) by the dominance hierarchy of alleles. The dominant allele to all others in the series is distinguished by capital letter, the recessive to all others in the same series is designated by the small letter and the intermediate alleles between these two extremes will be designated by some superscript with a small letter. Thus, the multiple allele will be written in dominance hierarchy as C > C ch >C h >c. 3.3 CHARACTERISTICS OF MULTIPLE ALLELE The Multiple Alleles of a series occupy the same locus on the homologous chromosome. In diploid individuals only two alleles of the multiple series are present in the cell of an individual. The crossing over does not occur within the alleles of a same multiple allele series because only two forms of a gene are present at homologous locus in an individual.
eBook - PDFStochastic Processes In Genetics And Evolution: Computer Experiments In The Quantification Of Mutation And Selection
Computer Experiments in the Quantification of Mutation and Selection
- Charles J Mode, Candace K Sleeman(Authors)
- 2012(Publication Date)
- World Scientific(Publisher)
(2007) be consulted. 2.2 Some Thoughts on Constructing Databases of DNA Markers From Sequenced Genomes of Relatives When constructing mathematical models within the framework of Mendelian genetics, the terms genes, locus on a chromosome with multiple Linkage and Recombination at Multiple Loci 57 alleles at that locus have useful operational meanings, and, when linkage models concerning more than one locus are considered, one speaks of models with many loci with Multiple Alleles at each locus. However, when mod-els using these terms are considered at the molecular level, such terms as multiple loci with many alleles have less clear operational meanings than in the abstractions of mathematical Mendelian genetics. It is beyond the scope of this chapter to attempt to give an account of the meaning of such terms at the molecular level, but it is recommended that books, such as Strachan and Read (2004), be consulted. The reading of such books can be a formidable task for non-experts in molecular genetics, but even a perusal of the table on contents and exploratory reading of selected sections of a book can be helpful in gleaning meanings for the terms locus or multiple loci with many alleles at the molecular level. As a start in a search for examples for meanings of such terms at the molecular level, it is helpful to enter the title, Major Histocompatibility Complex (MHC), into a search engine for the world wide web. MHC is a large genomic or gene family found in most vertebrates. In humans, for example, the MHC region is a 3.6 Mb (3,600,000 base pairs) region on chromosome 6 and contains an estimated 140 genes between the flanking markers with the labels MOG and COL11A2, see Wikipedia, the free ency-clopedia. Within the MHC region there are the human leukocyte antigen (HLA) genes. Among the so-called classical HLA genes that have been studied are the HLA-A, HLA-B, and HLA-C loci that have Multiple Alleles.
eBook - PDF- Peter Donaldson, Ann Daly, Luca Ermini, Debra Bevitt(Authors)
- 2015(Publication Date)
- Garland Science(Publisher)
for the gene with two alleles A and a above, there are two alleles, but there are three genotypes. By using alleles, popula-tion diversity can be described in fewer terms than by using genotypes. Finally, by using allele frequencies in case control population studies rather than genotype frequencies, no assumptions about the impact of homozygosity or of heterozygote advantage are being made. This is especially important in the context of complex disease where in the absence of a clear pattern of inheritance it would not be appropriate to make any such assumption, at least in the initial stages of analysis. Allele frequency refers to the numbers of alleles present in a population The number of copies of an allele at a locus is divided by the total number of all alleles in the sample: Frequency of an allele number of copies of the allele number of copie = s of all alleles at the locus If we consider a gene with only two alleles A and a and we suppose the frequencies are p for allele A and q for allele a ; then p and q can be calculated as: p f A n n N AA Aa = ( ) = + 2 2 q f a n n N aa Aa = ( ) = + 2 2 In this equation n AA , n Aa , and n aa represent the numbers of AA , Aa , and aa individuals, and N represents the total number of individuals in the sample it is necessary to divide by 2N because being diploid means each individual has two alleles for each gene (one from the maternal locus and one from the paternal locus). The sum of the allele frequencies is always 1 (100%) ( p + q = 1); therefore where there are only two alleles, q can be determined by simple subtraction after p has been calculated: q = 1 − p 21 Calculating Genetic Diversity: Determining Population Variability These calculations apply only where there are two alleles. In cases where there are several different alleles at a locus the calculation used is based on the same principle, but is more complicated.
eBook - PDF- Robert Jurmain, Lynn Kilgore, Wenda Trevathan, Russell Ciochon(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
Human Polymorphisms Traits (or the DNA sequences that code for them) that differ in expression between populations and individuals are called polymorphisms , and they’re the main focus of human variation studies. A genetic trait is polymorphic if the locus that governs it has two or more alleles. (See Chapter 4 for a discussion of the ABO blood group system, which is governed by three alleles at one locus.) A locus can consist of hun-dreds of nucleotides or just one nucleotide. Understanding polymorphisms requires evolutionary explanations, and geneti-cists use polymorphisms as a principal tool to understand evolutionary processes in modern populations. By using these polymorphisms to compare gene frequen-cies between different populations, we can begin to reconstruct the evolutionary events that link human populations with one another. The ABO system is interesting from an anthropological perspective because the frequencies of the A , B , and O alleles vary tremendously among humans. In most groups, A and B are rarely found in frequencies greater than 50 percent, and usually their frequencies are much lower. Still, most human groups are polymorphic for all three alleles, but there are excep-tions. For example, in native South American Indians, frequencies of the O allele reach 100 percent. Exceptionally high frequencies of O are also found in polymorphisms Loci with more than one allele. Polymorphisms can be expressed in the phenotype as the result of gene action (as in ABO), or they can exist solely at the DNA level within noncoding regions. a SuperStock/Purestock/Alamy Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-300 CHAPTER 14 | MODERN HUMAN BIOLOGY: PATTERNS OF VARIATION 420 northern Australia, and some islands off the Australian coast show frequencies exceeding 90 percent.
eBook - PDF- D. Peter Snustad, Michael J. Simmons(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
In the preface to his book, Bateson remarked that “The study of heredity thus becomes an organized branch of physiological science, already abundant in results, and in promise unsurpassed.” 1 1 Bateson, W. 1909. Mendel’s Principles of Heredity. University Press, Cambridge, England. 62 Diverse species of plants growing in a garden. Experiments with many different plants extended Mendel’s Principles of Dominance, Segregation, and Independent Assortment. Brian Maudsley/Shutterstock. Allelic Variation and Gene Function 63 The diverse kinds of alleles of genes affect phenotypes in different ways. Allelic Variation and Gene Function Mendel’s experiments established that genes can exist in alternate forms. For each of the seven traits that he studied— seed color, seed texture, plant height, flower color, flower position, pod shape, and pod color—Mendel identified two alleles, one dominant, the other recessive. This discovery suggested a simple functional dichotomy between alleles, as if one allele did nothing and the other did everything to determine the phe- notype. However, research early in the twentieth century demonstrated this to be an oversimplification. Genes can exist in more than two allelic states, and each allele can have a different effect on the phenotype. INCOMPLETE DOMINANCE AND CODOMINANCE An allele is dominant if it has the same phenotypic effect in heterozygotes as in homozygotes—that is, the genotypes Aa and AA are phenotypically indistinguishable. Sometimes, however, a heterozygote has a phenotype different from that of either of its associated homozygotes. Flower color in the snapdragon, Antirrhinum majus, is an example. White and red varieties are homozygous for different alleles of a color- determining gene; when crossed, they produce heterozygotes that have pink flowers. The allele for red color (W ) is therefore said to be incompletely, or partially, dominant over the allele for white color (w).
eBook - PDFIntroducing Genetics
From Mendel to Molecules
- Alison Thomas(Author)
- 2014(Publication Date)
- Garland Science(Publisher)
The rediscovery of Mendel’s work, 19 years after Darwin’s death, and the subsequent develop-ment of the concept of the gene and alleles, established an understanding of how the mechanism of inheritance could lead to variation. During the early 1900s, the British mathematician Godfrey Hardy (1877–1947) and the German physician Wilhelm Weinberg (1862–1937) each independently produced a set of equations enabling the variation present in populations to be expressed in terms of allele and genotype frequencies. This chapter: • Shows how the variation present in a population can be analyzed in terms of allele and genotype frequencies • Discusses the impact upon the observed variation of crucial features of the population such as its size, mating patterns, or selective forces 10.1 Population genetics: some definitions A population can be defined as a local group of a single species within which mating is actually or potentially occurring. The interbreeding individuals share a common set of genes referred to as the gene pool . It is the characteristics of the gene pool that are studied in population genetics (e.g. frequencies of dif-ferent alleles and resulting genotypes in the sample group). Remember that a frequency refers to a proportion, and always ranges in value between 0 and 1. For example, if 19% of people in a group are left-handed, then the frequency of left-handedness in this group is 0.19. The first step in studying the variation pres-ent in a population is to measure the frequency of different alleles for individual genes. Population geneticists are often interested in knowing how many loci are polymorphic (i.e. possess two or more different alleles with the frequency of the rarest exceeding 0.1%). The more loci that are polymorphic, the more variation the population shows. 157 The Hardy–Weinberg law 10.2 Calculating allele frequencies Allele frequencies may be calculated in one of two ways.
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