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Human Genetic Variation
Seile Yohannes, H3ABioNet/H3Africa College of Natural & Computational Science, Jigjiga University, Ethiopia
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Jigjiga University
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Learning Objectives Define Human Genetic Variation
Understand the basis of Human Genetic Variation- DNA Sequence Variation Origins of DNA Sequence Variation Mutation- the ultimate source of variation Others- Recombination, Gene flow, Genetic Drift Types of DNA Sequence Variation Single Nucleotide Variations (SNVs) Copy Number Variations (CNVs) Structural Variations
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Introductory Points Chromosomes Historical Aspects
Structural & functional aspects
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What are Chromosomes? Chromosomes: Historical Aspects
“The blueprint” of life- house the genetic material (DNA) Their discovery and functional exploration coincides with the era of “modern history of genetics”
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Chromosomes… First noticed by C. von Nägeli in 1842
Pollen cell experiments Importance of the nucleus
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Chromosomes… Mendel’s experiment ( )- his “traits” indicated that inheritance is linked to transmission of “alleles”- implying involvement of cellular components
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Chromosomes… Walther Flemming’s experiments (1879-1885)-
with the aid of new staining techniques, described the chromosomes Way they moved during division (mitosis) Separation & movement of sister chromatids to opposite poles Defined the processes of “Mitosis” and “Meiosis”
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Chromosomes… W. Waldeyer (1888)
Coined the term “chromosomes” to describe the structures studied by Flemming Greek word- “Chroma”- colored; “Soma”- body = “Colored body”- to describe the deeply staining properties of these structures during cytological analysis
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Chromosomes… In 1903-1911, Walter Sutton & Theodore Boveri
Studied meiosis (gamete formation) Hypothesized that the behavior of chromosomes during meiosis explained Mendel’s rules of inheritance Discovery that genes are located on chromosomes- “chromosomal theory of inheritance”…………
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Chromosomes: from gamete formation to fertilization
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Gamete formation- Meiosis
Reductional division- reduces the number of chromosomes by half- from diploid (2n) to haploid (n)
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Restoration of diploid state- Fertilization
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Genetic Diversity & Human Genetic Variation
Heritable variations both within and between populations Imply that there are genetic polymorphisms- Multiple variants of any given gene in the human population Manifested as polymorphisms of sequences of the 4 different bases that make up the gene Vis-à-vis the DNA Nucleotide
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Genetic Variation Describes differences between DNA sequences of individual genomes. As each individual has two nuclear genomes (a paternal genome and a maternal genome), genetic variation occurs within as well as between individuals.
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Genetic Variation Broadly categorized into 2 classes based on time of origin of the variation: Constitutional Genetic Variation:- caused by changes occurring in parental genomes and/or during formation of offspring and thus inherited and present within the genome of the organism during birth Post-Zygotic or Somatic Genetic Variation- caused by changes occurring after birth throughout the life of he individual, and causing minor polymorphisms among the different cells of the same individual.
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Genetic Variation: Cause
The ultimate cause of genetic variation is DNA sequence change DNA sequence variation is caused by: Mutation Others- Recombination, Gene Flow, Genetic drift We will only highlight on the “other” causes briefly
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Mutation Structural change of a gene that leads to a variant form caused by the alteration of single base units in DNA, or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes Occur spontaneously as a result of errors in DNA replication or induced by exposure to radiation or chemicals (Mutagens)
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Mutations- classification
2 broad classes based on pattern of occurrence in the genome Gene Mutations Chromosomal Mutations 2 distinctions based on the type of cell they occur Somatic Mutations Germ-line Mutations
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Gene Mutations Mutations that only affect a single gene 3 types:
Insertion –one or more nucleotides are added within a DNA sequence for a gene Deletion –one or more nucleotides are removed from a DNA sequence for a gene Substitution (point mutation) –a nucleotide is replaced with a different nucleotide
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Type Definition Example
Insertion A new nucleotide is added …TAGCCAGATA… …TAGCGCAGATA… Deletion A nucleotide is removed …TAGCAGATA… Substitution (point mutation) A nucleotide is replaced with a different nucleotide …TAGCCAGTTA…
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Chromosomal Mutations
Mutations that affect multiple genes Structural mutations/abnormaities Chromosomal Mutations Deletions Translocations Inversions Duplications
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Small or large & in one or more genes
Deletions Loss of chromosomal material Small or large & in one or more genes Example: Cri du chat syndrome Deletion of short arm of chromosome 5 Affects motor and mental function Specific chromosomal break points are associated with specific phenotypic changes
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Translocations Exchange of chromosomal segments between nonhomologous chromosomes Two major types Reciprocal translocation Non-reciprocal translocation
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Types of Translocations
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Types of Translocations
Robertsonian Translocation Special form of reciprocal translocation Centromeres from two non-homologous chromosomes fuse and chromosomal material from the short arms is lost 5% of Down syndrome cases involve a Robertsonian translocation between chromosomes 21 and 14
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Robertsonian Translocation
A translocation between chromosome 14 and 21 may produce: Translocation carrier Normal phenotype Translocation Down syndrome Lethal monosomy 21 Lethal trisomy 14 Lethal monosomy 14 Fig. 6.27 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
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Inversions Inversion of chromosomal segment 2 types:
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Chapter 6 Human Heredity by Michael Cummings ©2006 Brooks/Cole-Thomson Learning
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Duplication Duplication /multi-copy formation/ of chromosomal segment
3 forms: Tandem Reverse Tandem Terminal Tandem
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Somatic Mutations Occur in cells of the body that do not form gametes
Occur during mitosis Are not transmitted to future generations
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Germ-line Mutations Occur in cells of the body Producing gametes
Occurs during meiosis Are heritable changes that are transmitted to future generations
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Types of DNA Sequence Variation
Single Nucleotide Variations (SNVs) Variations occurring at a single nucleotide in somatic cells When their frequency in a population is delineated, then SNVs are referred to as single nucleotide polymorphisms (SNPs) SNPs- Most common type of sequence variation (account for 90% of all sequence variation) Other sequence variations are single base exchanges, deletions and insertions (discussed in previous sections)
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Other Sources of Genetic Variation
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Recombination (Crossing-over) during gamete formation
A process that occurs when two homologous chromosomes align during meiosis and exchange a segment of genetic material. A natural process by which diversity is increased via gametic variation
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Here, the alleles for gene C were exchanged
Here, the alleles for gene C were exchanged. The result is two recombinant and two non-recombinant chromosomes.
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Other population dynamics factors contributing to variation…
Gene Flow… Genetic Drift…
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Thank You…
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