Ch 8: Chromosome Mutations Wheeler High School The Center for Advanced Studies in Science, Math & Technology Fragile X Ch 8: Chromosome Mutations Post-AP DNA/Genetics – Ms. Kelavkar

Variations in Chromosome Number Diploid species normally contain precisely two haploid chromosome sets, there are many known variations: a change in the total number of chromosomes, the deletion or duplication of genes or segments of a chromosome, and rearrangements of the genetic material either within or among chromosomes. Duplication Event Post-AP DNA/Genetics – Ms. elavkar

Aneuploidy (abnormal # of chromosomes) Variations in chromosome number are known as aneuploidy. This term describes when an organism has gained or lost one or more chromosomes but NOT a complete set. Trisomy & Monosomy as two examples of aneuploidy. Post-AP DNA/Genetics – Ms. Kelavkar

Euploidy & Polyploidy In euploidy, complete haploid sets of chromosomes are present. Polyploidy occurs when more than two sets of chromosomes are present. Euploidy Polyploid Post-AP DNA/Genetics – Ms. elavkar

Table 8-1 Copyright © 2006 Pearson Prentice Hall, Inc. Table 8.1 Terminology for Variation in Chromosome Numbers Table 8-1 Copyright © 2006 Pearson Prentice Hall, Inc.

Nondisjunction This is when chromosomes or chromatids fail to disjoin and move to opposite poles during meiosis I or II. Post-AP DNA/Genetics – Ms. Kelavkar

Figure 8-1 Copyright © 2006 Pearson Prentice Hall, Inc. Figure 8-1 Nondisjunction during the first and second meiotic divisions. In both cases, some of the gametes formed either contain two members of a specific chromosome or lack that chromosome. Following fertilization by a gamete with a normal haploid content, monosomic, disomic (normal), or trisomic zygotes are produced. Figure 8-1 Copyright © 2006 Pearson Prentice Hall, Inc.

Monosomy Monosomy: the Loss of a Single Chromosome, May Have Severe Phenotypic Effects Partial Monosomy in Humans: The Cri-du-Chat Syndrome (missing part of C5) Post-AP DNA/Genetics – Ms. Kelavkar

Figure 8-2 Copyright © 2006 Pearson Prentice Hall, Inc. A representative karyotype and photograph of a child exhibiting cri-du-chat syndrome. In the karyotype, the arrow identifies the absence of a small part of the short arm of one member of the chromosome 5 homologs. Figure 8-2 A representative karyotype and photograph of a child exhibiting cri-du-chat syndrome (46,5p). In the karyotype, the arrow identifies the absence of a small part of the short arm of one member of the chromosome 5 homologs. Figure 8-2 Copyright © 2006 Pearson Prentice Hall, Inc.

Total Monosomy Although monosomy for the X chromosome occurs in humans, monosomy for any of the autosomes is usually not tolerated in humans and other animals. However, partial monosomy (segmental deletion) only a section of a chromosome is lost and is generally tolerated and non-lethal. Post-AP DNA/Genetics – Ms. Kelavkar

Trisomy Trisomy Involves the Addition of a Chromosome to a Diploid Genome The karyotype and a photograph of a child with Down syndrome. In the karyotype, three members of the G-group chromosome 21 are present, creating the 47, condition.

Figure 8-6 Copyright © 2006 Pearson Prentice Hall, Inc. Figure 8-6 Incidence of Down syndrome births contrasted with maternal age. Figure 8-6 Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 8-7 Copyright © 2006 Pearson Prentice Hall, Inc. Trisomy The karyotype and potential phenotypic characteristics associated with Patau syndrome, where three members of the D group chromosome 13 are present, creating the 47, condition. AKA: Trisomy 13 Figure 8-7 The karyotype and potential phenotypic characteristics associated with Patau syndrome, where three members of the D group chromosome 13 are present, creating the 47, condition. Figure 8-7 Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 8-8 Copyright © 2006 Pearson Prentice Hall, Inc. Trisomy The karyotype and potential phenotypic characteristics associated with Edwards syndrome. Three members of the E group chromosome 18 are present, creating the 47, condition. Figure 8-8 The karyotype and potential phenotypic characteristics associated with Edwards syndrome. Three members of the E group chromosome 18 are present, creating the 47, condition. Figure 8-8 Copyright © 2006 Pearson Prentice Hall, Inc.

Trisomies vs. Monosomies Trisomies are often found in spontaneously aborted fetuses, but monosomies are not, which suggests that monosomic gametes may be very functionally impaired

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Wheeler High School The Center for Advanced Studies in Science, Math & Technology Ch 8: Genetic Mutations Post-AP DNA/Genetics – Ms. Kelavkar

Polyploidy Recall from last class; polyploidy describes instances in which more than 2 haploid chromosome sets are found. The naming of polyploids is based on the number of sets of chromosomes found: a triploid has 3n chromosomes; a tetraploid has 4n; a pentaploid, 5n; and so forth

How Does Polyploidy Occur Two Ways: Autopolyploidy – addition of one or more extra sets of chromosomes (same species) Allopolyploidy – hybridization (different species)

Autopolyploidy and Allopolyploidy Polyploidy can originate by the addition of one or more sets of chromosomes identical to the haploid complement of the same species (autopolyploidy) or by the combination of chromosome sets from different species as a consequence of interspecific matings (allopolyploidy)

Allotetraploid An allotetraploid arises from hybridization of two closely related species.

Figure 8-11 Copyright © 2006 Pearson Prentice Hall, Inc. The origin and propagation of an amphidiploid. Species 1 contains genome A consisting of three distinct chromosomes, and Species 2 contains genome B consisting of two distinct chromosomes, and Following fertilization between members of the two species and chromosome doubling, a fertile amphidiploid containing two complete diploid genomes (AABB) is formed. Figure 8-11 The origin and propagation of an amphidiploid. Species 1 contains genome A consisting of three distinct chromosomes, and Species 2 contains genome B consisting of two distinct chromosomes, and Following fertilization between members of the two species and chromosome doubling, a fertile amphidiploid containing two complete diploid genomes (AABB) is formed. Figure 8-11 Copyright © 2006 Pearson Prentice Hall, Inc.

Amphidiploid plants can be produced by somatic cell hybridization. Figure 8-13 Application of the somatic cell hybridization technique in the production of an amphidiploid. Cells from the leaves of two species of plants are removed and cultured. The cell walls are digested away and the resultant protoplasts are induced to undergo cell fusion. The hybrid cell is selected and stimulated to divide and differentiate, as illustrated in the photograph. An amphidiploid has a complete set of chromosomes from each parental cell type and displays phenotypic characteristics of each. Two pairs of chromosomes from each species are depicted. Figure 8-13 Copyright © 2006 Pearson Prentice Hall, Inc.

Variation Occurs in the Structure and Arrangement of Chromosomes Rearrangements of chromosome segments include deletions, duplications, inversions, nonreciprocal translocations, and reciprocal translocations.

Figure 8-14 Copyright © 2006 Pearson Prentice Hall, Inc. Figure 8-14 Overview of the five different types of rearrangement of chromosome segments. Figure 8-14 Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 8-14a Copyright © 2006 Pearson Prentice Hall, Inc. Figure 8-14a Overview of the five different types of rearrangement of chromosome segments. Figure 8-14a Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 8-14b Copyright © 2006 Pearson Prentice Hall, Inc. Figure 8-14b Overview of the five different types of rearrangement of chromosome segments. Figure 8-14b Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 8-14c Copyright © 2006 Pearson Prentice Hall, Inc. Figure 8-14c Overview of the five different types of rearrangement of chromosome segments. Figure 8-14c Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 8-14d Copyright © 2006 Pearson Prentice Hall, Inc. Figure 8-14d Overview of the five different types of rearrangement of chromosome segments. Figure 8-14d Copyright © 2006 Pearson Prentice Hall, Inc.

Figure 8-14e Copyright © 2006 Pearson Prentice Hall, Inc. Figure 8-14e Overview of the five different types of rearrangement of chromosome segments. Figure 8-14e Copyright © 2006 Pearson Prentice Hall, Inc.

A Deletion Is a Missing Region of a Chromosome When a chromosome breaks in one or more places and a portion of it is lost, the missing piece is referred to as a deletion (or a deficiency). The deletion can occur near one end (terminal deletion) or from the interior of the chromosome (intercalary deletion)

Figure 8-15 Copyright © 2006 Pearson Prentice Hall, Inc. Terminal deletion Interclay Deletion Pairing occurs between a normal chromosome and one with an interclay deletion by looping out the undeleted portion to form a deletion (compensation) loop. Figure 8-15 Origins of (a) a terminal and (b) intercalary deletion. In part (c), pairing occurs between a normal chromosome and one with an intercalary deletion by looping out the undeleted portion to form a deletion (or a compensation) loop. Figure 8-15 Copyright © 2006 Pearson Prentice Hall, Inc.

Duplication Events A Duplication Is a Repeated Segment of the Genetic Material Duplications arise as the result of unequal crossing over during meiosis or through a replication error prior to meiosis. Evolution anyone?

Figure 8-17 Copyright © 2006 Pearson Prentice Hall, Inc. The origin of duplicated and deficient regions of chromosomes as a result of unequal crossing over. The tetrad at the left is mispaired during synapsis. A single crossover between chromatids 2 and 3 results in deficient and duplicated chromosomal regions. (See chromosomes 2 and 3, respectively, on the right.) The two chromosomes uninvolved in the crossover event remain normal in their gene sequence and content. Figure 8-17 The origin of duplicated and deficient regions of chromosomes as a result of unequal crossing over. The tetrad at the left is mispaired during synapsis. A single crossover between chromatids 2 and 3 results in deficient and duplicated chromosomal regions. (See chromosomes 2 and 3, respectively, on the right.) The two chromosomes uninvolved in the crossover event remain normal in their gene sequence and content. Figure 8-17 Copyright © 2006 Pearson Prentice Hall, Inc.

Inversions An inversion involves a rearrangement of the linear gene sequence rather than the loss of genetic information. In an inversion, a segment of a chromosome is turned around 180° within a chromosome. An inversion may arise from chromosomal looping.

Figure 8-19 Copyright © 2006 Pearson Prentice Hall, Inc. Figure 8-19 One possible origin of a pericentric inversion. Figure 8-19 Copyright © 2006 Pearson Prentice Hall, Inc.

Paracentric and Pericentric Inversions A paracentric inversion does not change the relative lengths of the two arms of a chromosome, whereas a pericentric inversion does.

Figure 8-20 A comparison of the arm ratios of a submetacentric chromosome before and after the occurrence of a paracentric and pericentric inversion. Only the pericentric inversion results in an alteration of the original ratio. A comparison of the arm ratios of a submetacentric chromosome before and after the occurrence of a paracentric and pericentric inversion. Only the pericentric inversion results in an alteration of the original ratio.

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