Alleles

4 Key excerpts on "Alleles"

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  eBook - PDF

  Principles of Genetics

  • 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).

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  Animal Genetics and Breeding

  • 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.

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  eBook - ePub

  Comparative Psychology

  Evolution and Development of Brain and Behavior, 3rd Edition

  • Mauricio R Papini, Mauricio Papini(Authors)
  • 2020(Publication Date)
  • Routledge

    (Publisher)

  genes, which are nothing more than sections of the DNA molecule contained in the chromosomes. The fact that genes maintain their identity across generations is one major mechanism for the preservation of genetic variation.

  Germ cells (sperm and egg cells) contain a single copy of each chromosome and, therefore, a single copy of each gene (a condition known as haploid). Two copies of each chromosome and gene are produced after fertilization (a condition known as diploid). Chromosomes that contain the same (or very similar) sequence of DNA bases are said to be homologous. Different versions of homologous genes (same locus in homologous chromosomes) are called Alleles. Alleles are produced by mutations that alter the sequence of bases in the DNA molecule. In any given organism, there might be up to two Alleles for each gene, but in a population, it is possible to find more than two Alleles for a particular gene.

  Mendelian genetics can be illustrated with a simple example. Mendel collected seeds from garden peas that had consistently produced either round (R) or wrinkled (r) peas. R and r represent alternative phenotypes that, because of the consistency with which they are produced, can be assumed to be each under the control of a pair of identical genes. This condition, known as homozygous, is usually represented as RR or rr. Two homozygous plants in the parental population, P, can be crossed by fertilizing R eggs with r pollen. Mendel showed that such a cross yields plants that produce only R peas. This first filial generation is labeled F1. However, if now F1 plants are crossed with each other, the peas produced by plants from the F2 generation will be a mixture of R and r, with a precise distribution: 3R:1r

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  Genetics of Complex Disease

  • 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.

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