Molecular Basis of Abnormal Phenotype

  • Pedro J. Chedrese
  • Christine Meaden

With the exception of gametes, genes occur in homologous pairs located on chromosomes, which also occur in homologous pairs within the cell nucleus. Each gene of the homologous pair is positioned at corresponding fixed locations called the locus on each of the matching chromosomes. Natural random modifications within the DNA nucleotide sequences of genes create one or more alternative forms called alleles. The alleles constitute the genetic make-up or genotype for each gene and determine normal variability essential for genetic diversity. The genotype provides the primary code for the phenotype, which refers to observable structure, function, and behavior that result from gene expression. Identical alleles located at the same locus on each of the matching chromosomes are considered homozygous for a trait, while two different alleles are heterozygous. The allele expressed by phenotype is said to be dominant, while the other allele, which is masked by the dominant gene, is described as recessive.


Down Syndrome Angelman Syndrome Balance Translocation Homologous Pair Genome Mutation 
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Copyright information

© Springer Science+Business Media, LLC 2009

Authors and Affiliations

  1. 1.Department of BiologyUniversity of Saskatchewan College of Arts and ScienceSaskatoonCanada
  2. 2.Saskatchewan HealthSaskatoonCanada

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