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Genetics Notes Week 5

by: Joseph Notetaker

Genetics Notes Week 5 BIO 235

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About this Document

This section begins to cover chromosomal abnormalities
Lazlo Kovacs
Class Notes
Biology, Genetics
25 ?




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This 3 page Class Notes was uploaded by Joseph Notetaker on Friday September 23, 2016. The Class Notes belongs to BIO 235 at Missouri State University taught by Lazlo Kovacs in Summer 2015. Since its upload, it has received 6 views. For similar materials see Genetics in Biology at Missouri State University.


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Date Created: 09/23/16
Genetics Notes Week 5 Mutations: o Deletion  The loss of a chromosomal segment o Duplication  The repetition of a chromosomal segment compared to the normal parent chromosome o Inversion  A change in the direction of part of the genetic material along a single chromosome (flips) o Translocation  A segment of one chromosome becomes attached to a different chromosome  Simple translocation  One-way transfer  A piece of a chromosome is attached to another chromosome  Reciprocal translocations  Two-way transfer  Two different types of chromosomes exchange pieces, producing two abnormal chromosomes with translocations Deletions: o Deletions are lethal in the homozygous state o In the heterozygous state, deletions cause an altered gene dosage which may have severe consequences  Cri-du-chat syndrome is caused by a heterozygous deletion in chromosome 5 Duplication: o Ectopic recombination between repeated elements among homologous chromosomes: duplication o A chromosomal duplication is usually caused by abnormal events during recombination  Repetitive sequences can cause misalignment between homologous chromosomes  If a crossover occurs, nonallelic homologous recombination results Inversions: o Inversions are genetic rearrangements in which the order of genes in a chromosome segment is reversed  This is caused by entopic recombination within the same chromosome between two of the same DNA segments o Inversions do not alter the genetic content o In an inversion heterozygote chromosomes twist into a loop in the region in which the gene order is inverted o Parycentric Inversion: Do include the centromere with a break on each arm o Paracentric Inversion: Do not include the centromere, both of the breaks are on one arm o Dicentric Chromosomes: An abnormal chromosome with two centromeres Translocation: o Familial Down-syndrome  Familial Down-syndrome is caused by translocation  In this example, the q arm of chromosome 21 is translocated to chromosome 10 creating an extra chromosome.  A phenotypically healthy individual has a 50% chance of passing the q+ homologue of chromosome 10 to her progeny  Because this type of down syndrome runs in families, it is named familial down syndrome Trinucleotide: o Trinucleotide repeat in people with fragile X syndrome  When present in moderate numbers, these repeats have no influence, but when excessively repeated, they cause disorders. Most commonly known examples are Huntington’s disease and fragile X syndrome o Trinucleotide instability causes mutations  FMR-1 genes in unaffected people have fewer than 50 CGG repeats  Unstable permutation alleles have between 55 and 230 repeats  Disease causing alleles have> 230 CGG repeats Dynamic mutations and Diseases o Other genetic diseases associated with dynamic mutation include: o The neurological disorders myotonic dystrophy (with an unstable repeat of CTG) o Kennedy disease (AGC) o Friedreich ataxia (AAG) o Spinocerebellar ataxia type 1 (AGC) o Huntington disease (AGC) Variation in chromosome number: o Chromosome numbers can vary in two main ways o Polyploidy  Variation in the number of complete sets of a chromosome  Gene dosage is balanced o Aneuploidy (not euploidy as in not normal ploidy)  Variation in the number of particular chromosomes within a set  Unbalanced set of chromosomes = relative gene dosage is upset o Polyploidy occurs occasionally in animals and frequently in plants o Aneuploid variations, on the other hand, are regarded as abnormal conditions o Aneuploidly  Nullisomy: loss of both members of a homologous pair of chromosomes. 2n-2  Monosomy: loss of a single chromosome. 2n-1  (double monosomy: 2n-1-1)  Trisomy: gain of a single chromosome number. 2n+1  (double trisomy: 2n+1+1)  Tetrasomy: gain of two homologous chromosomes. 2n+2 o Primary down syndrome:  Down syndrome is caused by the failure of chromosome 21 to segregate properly  This nondisjunction most commonly occurs during meiosis I in the oocyte


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