28.1 Specific Terminology Describes Variations in Chromosome Number AneuploidyEuploidyPolyploidy
3NondisjunctionFigure 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
48.2 Monosomy, the Loss of a Single Chromosome, May Have Severe Phenotypic Effects
5Section 8.38.3 Trisomy Involves the Addition of a Chromosome to a Diploid GenomeTrisomy (2n + 1 chromosomes) for the sex chromosomes has a less dramatic phenotype than trisomies for autosomes, which are often lethal.
6Jimson weed: Datura stramonium Figure 8-2 Drawings of capsule phenotypes of the fruits of the Jimson weed Datura stramonium. In comparison with wild type, each phenotype is the result of trisomy of 1 of the 12 chromosomes characteristic of the haploid genome. The photograph illustrates the plant itself.Figure 8.2
7Section 8.3Down syndrome results from trisomy of chromosome 21. (Figure 8.4)
8Figure 8-6 Incidence of Down syndrome births contrasted with maternal age.
9Patau SyndromeFigure 8-6 The karyotype and potential phenotypic characteristics associated with Patau syndrome, where three members of the D group chromosome 13 are present, creating the 47,13+ condition.Figure 8.6
10Edwards SyndromeFigure 8-7 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 8.7
11Section 8.3Trisomies are often found in spontaneously aborted fetuses, but monosomies are not.This suggests that monosomic gametes may be functionally impaired.
12Section 8.48.4 Polyploidy, in Which More Than Two Haploid Sets of Chromosomes Are Present, Is Prevalent in PlantsThe naming of polyploids is based on the number of sets of chromosomes found:a triploid has 3n chromosomesa tetraploid has 4n chromosomesa pentaploid, 5n chromosomesand so forth
13Section 8.4 Polyploidy can originate by: the addition of one or more sets of chromosomes identical to the haploid complement of the same species (autopolyploidy)or the combination of chromosome sets from different species as a consequence of interspecific matings (allopolyploidy) (Figure 8.8)
14Figure 8-9 Contrasting chromosome origins of an autopolyploid with an allopolyploid karyotype.
15Section 8.58.5 Variation Occurs in the Internal Composition and Arrangement of ChromosomesRearrangements of chromosome segments include:deletionsduplicationsinversionsnonreciprocal translocationsreciprocal translocations (Figure 8.13)
16Figure 8-14 Overview of the five different types of rearrangement of chromosome segments.
178.6 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 8.14)
18Figure 8-15 Origins of (a) a terminal and (b) intercalary deletion 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 8.14
19Section 8.6Cri-du-chat results from a segmental deletion of a small terminal portion of the short arm of chromosome 5 (Figure 8.15).
20Section 8.6The type of Notch phenotypic expression of recessive genes in association with a deletion in Drosophila is an example of pseudodominance.
218.7 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 (Figure 8.17).
22Section 8.7Organisms have multiple copies of the ribosomal RNA genes (rDNA).This is an example of gene redundancy.Gene amplification is another mechanism to increase the rRNA.
23The Bar-eye phenotype in Drosophila results from duplication (Figure 8
24Section 8.8 8.8 Inversions Rearrange the Linear Gene Sequence 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.
25Figure 8-19 One possible origin of a pericentric inversion.
26Figure 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.Figure 8.20
27Section 8.8Synapsis of inverted chromosomes requires an inversion loop (Figure 8.21).
28Section 8.8Figure 8.22 shows the effects of a single crossover within an inversion loop for both paracentric and pericentric inversion heterozygotes.For a paracentric inversion crossover:one recombinant chromatid is dicentric (two centromeres)one is acentric (lacking a centromere)
29Figure 8-22 The effects of a single crossover within an inversion loop in cases involving (a) a paracentric inversion and (b) a pericentric inversion. In (a), two altered chromosomes are produced, one that is acentric and one that is dicentric. Both chromosomes also contain duplicated and deficient regions. In (b), two altered chromosomes are produced, both with duplicated and deficient regions. Ultimately, the number of viable gametes resulting from these events is reduced by about 50 percent because the abnormal outcomes are likely to be lethal.Figure 8.22
308.9 Translocations Alter the Location of Chromosomal Segments in the Genome Translocation - movement of a chromosomal segment to a new location in the genome.Reciprocal Translocation:involves exchange of segments between 2 nonhomologous chromosomeshas an unusual synapsis configuration during meiosis
31Figure 8-23 (a) The possible origin of a reciprocal translocation; (b) the synaptic configuration formed during meiosis in an individual that is heterozygous for the translocation. (c) Two possible segregation patterns, one of which leads to a normal and a balanced gamete (called alternate segregation) and one that leads to gametes containing duplications and deficiencies (called adjacent segregation).Figure 8.23
32Section 8.9Robertsonian translocation or centric fusion involves breaks at the extreme ends of the short arms of 2 nonhomologous acrocentric chromosomes
33Familial Down Syndrome Figure Chromosomal involvement in familial Down syndrome. The photograph illustrates the relevant chromosomes from a trisomy 21 offspring produced by a translocation carrier parent.Figure 8.25
34Section 8.108.10 Fragile Sites in Humans Are Susceptible to Chromosome BreakageFragile sites are more susceptible to chromosome breakage when cells are cultured in the absence of certain chemicals such as folic acid.
35Section 8.10Fragile X syndrome (Martin–Bell syndrome) is the most common form of inherited mental retardation, affecting about 1 in 4000 males and 1 in 8000 females, and is a dominant trait (Figure 8.26).