Variation in chromosome structure and number chapter 8

PowerPoint Presentation Materials to accompany Genetics: Analysis and Principles Robert J. Brooker Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display CHAPTER 8 VARIATION IN CHROMOSOME STRUCTURE AND NUMBER

INTRODUCTION Genetic variation refers to differences between members of the same species or those of different species Allelic variations are due to mutations in particular genes Chromosomal aberrations are substantial changes in chromosome structure or number These typically affect more than one gene They are quite common, which is surprising 8-2 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Cytogenetics -The field of genetics that involves the microscopic examination of chromosomes A cytogeneticist typically examines the chromosomal composition of a particular cell or organism This allows the detection of individuals with abnormal chromosome number or structure This also provides a way to distinguish between species 8.1 Variation in Chromosome Structure 8-3 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Cytogenetics 8-4 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cytogeneticists use three main features to identify and classify chromosomes 1. Location of the centromere 2. Size 3. Banding patterns These features are all seen in a Karyotype Figure 8.1 c

Cytogenetics 8-6 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Since different chromosomes can be the same size and have the same centromere position, chromosomes are treated with stains to produce characteristic banding patterns Example: G-banding Chromosomes are exposed to the dye Giemsa Some regions bind the dye heavily Dark bands Some regions do not bind the stain well Light bands In humans 300 G bands are seen in metaphase 2,000 G bands in prophase

8-7 Figure 8.1 Banding pattern during metaphase Banding pattern during prophase

Cytogenetics Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The banding pattern is useful in several ways: 1. It distinguishes Individual chromosomes from each other 2. It detects changes in chromosome structure 3. It reveals evolutionary relationships among the chromosomes of closely-related species 8-8

Mutations Can Alter Chromosome Structure Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display There are two primary ways in which the structure of chromosomes can be altered 1. The total amount of genetic information in the chromosome can change Deficiencies/Deletions Duplications 2. The genetic material remains the same, but is rearranged Inversions Translocations 8-9

Deficiency (or deletion ) The loss of a chromosomal segment Duplication The repetition of a chromosomal segment compared to the normal parent chromosome Inversion A change in the direction of part of the genetic material along a single chromosome Translocation A segment of one chromosome becomes attached to a different chromosome Simple translocations One way transfer Reciprocal translocations Two way transfer 8-10 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Figure 8.2 8-11 Human chromosome 1 Human chromosome 21

Deficiencies 8-13 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The phenotypic consequences of deficiencies depends on the 1. Size of the deletion 2. Chromosomal material deleted Are the lost genes vital to the organism? When deletions have a phenotypic effect, they are usually detrimental For example, the disease cri-du-chat syndrome in humans Caused by a deletion in the short arm of chromosome 5

Duplications 8-17 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A chromosomal duplication is usually caused by abnormal events during recombination Figure 8.5

Duplications 8-15 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Like deletions, the phenotypic consequences of duplications tend to be correlated to size Duplications are more likely to have phenotypic effects if they involve a large piece of the chromosome However, duplications tend to have less harmful effects than deletions of comparable size In humans, relatively few well-defined syndromes are caused by small chromosomal duplications

The genes in a duplicated region may accumulate mutations which alter their function After many generations, they may have similar but distinct functions They are now members of a gene family Two or more genes derived from a common ancestor are homologous Homologous genes within a single species are paralogs Refer to figure 8.6 Duplications can provide additional genes, forming gene families 8-16 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

8-18 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The globin genes all encode subunits of proteins that bind oxygen Over 500-600 million years, the ancestral globin gene has been duplicated and altered so there are now 14 paralogs in this gene family on three different chromosomes Different paralogs carry out similar but distinct functions All bind oxygen myoglobin stores oxygen in muscle cells different globins are in the red blood cells at different developmental stages provide different characteristics corresponding to the oxygen needs of the embryo, fetus and adult Refer to figure 8.7

8-30 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Figure 8.7 Duplication Better at binding and storing oxygen in muscle cells Better at binding and transporting oxygen via red blood cells Expressed very early in embryonic life Expressed maximally during the second and third trimesters Expressed after birth

Duplications and Gene Families 8-27 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The majority of small chromosomal duplications have no phenotypic effect However, they are vital because they provide raw material for additional genes This can ultimately lead to the formation of gene families A gene family consists of two or more genes that are similar to each other

Experiment : Comparative Genomic Hybridization to detect deletions and duplications 8-21 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Chromosomal deletions and duplications have been associated with human cancers May be difficult to detect with karyotype analysis Comparative genomic hybridization can be used Developed by Anne Kallioniemi and Daniel Pinkel in 1992 Largely used to detect changes in cancer cell chromosomes

Inversions 8-22 A chromosomal inversion is a segment that has been flipped to the opposite orientation Figure 8.9 Centromere lies within inverted region Centromere lies outside inverted region

8-23 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In an inversion, the total amount of genetic information stays the same Therefore, the great majority of inversions have no phenotypic consequences In rare cases, inversions can alter the phenotype of an individual Break point effect The breaks leading to the inversion occur in a vital gene Position effect A gene is repositioned in a way that alters its gene expression About 2% of the human population carries inversions that are detectable with a light microscope Most of these individuals are phenotypically normal However, a few an produce offspring with genetic abnormalities

Inversion Heterozygotes 8-24 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Individuals with one copy of a normal chromosome and one copy of an inverted chromosome Such individuals may be phenotypically normal They also may have a high probability of producing gametes that are abnormal in their genetic content The abnormality is due to crossing-over in the inverted segment During meiosis I, homologous chromosomes synapse with each other For the normal and inversion chromosome to synapse properly, an inversion loop must form If a cross-over occurs within the inversion loop, highly abnormal chromosomes are produced Refer to figure 8.10

Translocations 8-26 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A chromosomal translocation occurs when a segment of one chromosome becomes attached to another In reciprocal translocations two non-homologous chromosomes exchange genetic material Reciprocal translocations arise from two different mechanisms 1. Chromosomal breakage and DNA repair 2. Abnormal crossovers Refer to Figure 8.11

8-27 Figure 8.11 Telomeres prevent chromosomal DNA from sticking to each other

Translocations 8-28 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Reciprocal translocations lead to a rearrangement of the genetic material, not a change in the total amount Thus, they are also called balanced translocations Reciprocal translocations, like inversions, are usually without phenotypic consequences In a few cases, they can result in position effect

8-29 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In simple translocations the transfer of genetic material occurs in only one direction These are also called unbalanced translocations Unbalanced translocations are associated with phenotypic abnormalities or even lethality Example: Familial Down Syndrome In this condition, the majority of chromosome 21 is attached to chromosome 14 The individual would have three copies of genes found on a large segment of chromosome 21 Therefore, they exhibit the characteristics of Down syndrome

8-30 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Familial Down Syndrome is an example of Robertsonian translocation This translocation occurs as such Breaks occur at the extreme ends of the short arms of two non-homologous acrocentric chromosomes The small acentric fragments are lost The larger fragments fuse at their centromeric regions to form a single chromosome which is metacentric or submetacentric This type of translocation is the most common type of chromosomal rearrangement in humans Approximately one in 900 births

Balanced Translocations and Gamete Production 8-31 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Individuals carrying balanced translocations have a greater risk of producing gametes with unbalanced combinations of chromosomes This depends on the segregation pattern during meiosis I During meiosis I, homologous chromosomes synapse with each other For the translocated chromosome to synapse properly, a translocation cross must form Refer to Figure 8.13, slide 8-33

8-41 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Meiotic segregation can occur in one of three ways 1. Alternate segregation Chromosomes on opposite sides of the translocation cross segregate into the same cell Leads to balanced gametes Both contain a complete set of genes and are thus viable 2. Adjacent-1 segregation Adjacent non-homologous chromosomes segregate into the same cell Leads to unbalanced gametes Both have duplications and deletions and are thus inviable 3. Adjacent-2 segregation Adjacent homologous chromosomes segregate into the same cell Leads to unbalanced gametes Both have duplications and deletions and are thus inviable

8-34 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Alternate and adjacent-1 segregations are the likely outcomes when an individual carries a reciprocal translocation Indeed, these occur at about the same frequency Moreover, adjacent-2 segregation is very rare Therefore, an individual with a reciprocal translocation usually produces four types of gametes Two of which are viable and two, nonviable This condition is termed semisterility

Chromosome numbers can vary in two main ways Euploidy Variation in the number of complete sets of chromosome Aneuploidy Variation in the number of particular chromosomes within a set Euploid variations occur occasionally in animals and frequently in plants Aneuploid variations, on the other hand, are regarded as abnormal conditions 8.2 VARIATION IN CHROMOSOME NUMBER Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-35

Figure 8.14 8-36 Polyploid organisms have three or more sets of chromosomes Individual is said to be trisomic Individual is said to be monosomic

Aneuploidy 8-37 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The phenotype of every eukaryotic species is influenced by thousands of different genes The expression of these genes has to be intricately coordinated to produce a phenotypically normal individual Aneuploidy commonly causes an abnormal phenotype It leads to an imbalance in the amount of gene products Three copies will lead to 150% production A single chromosome can have hundreds or even thousands of genes Refer to Figure 8.15

8-38 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Figure 8.15 In most cases, these effects are detrimental They produce individuals that are less likely to survive than a euploid individual

Aneuploidy 8-39 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The harmful effects of aneuploidy were first discovered in the 1920s by Albert Blakeslee and his colleagues They studied the Jimson weed ( Datura stramonium ) All of its 12 possible trisomies produce capsules (dried fruit) that are phenotypically different In addition, the aneuploid plants have other morphologically distinguishable traits Including some detrimental ones Refer to Figure 8.16

8-49 Figure 8.16 Blakeslee noted that this plants is “weak and lopping with the leaves narrow and twisted.”

Aneuploidy 8-41 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Alterations in chromosome number occur frequently during gamete formation About 5-10% of embryos have an abnormal chromosome number Indeed, ~ 50% of spontaneous abortions are due to such abnormalities In some cases, an abnormality in chromosome number produces an offspring that can survive Refer to Table 8.1

8-43 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The autosomal aneuploidies compatible with survival are trisomies 13, 18 and 21 These involve chromosomes that are relatively small Aneuploidies involving sex chromosomes generally have less severe effects than those of autosomes This is explained by X inactivation All additional X chromosomes are converted into Barr bodies

8-44 Some human aneuploidies are influenced by the age of the parents Older parents more likely to produce abnormal offspring Example: Down syndrome (Trisomy 21) Incidence rises with the age of either parent, especially mothers Figure 8.17

8-45 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Down syndrome is caused by the failure of chromosome 21 to segregate properly This nondisjunction most commonly occurs during meiosis I in the oocyte The correlation between maternal age and Down symdrome could be due to the age of oocytes Human primary oocytes are produced in the ovary of the female fetus prior to birth They are however arrested in prophase I until the time of ovulation As a woman ages, her primary oocytes have been arrested in prophase I for a progressively longer period of time This added length of time may contribute to an increased frequency of nondisjunction

Euploidy 8-46 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Most species of animals are diploid In many cases, changes in euploidy are not tolerated Polyploidy in animals is generally a lethal condition Some euploidy variations are naturally occurring Female bees are diploid Male bees (drones) are monoploid Contain a single set of chromosomes A few examples of vertebrate polyploid animals have been discovered

Euploidy 8-47 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In many animals, certain body tissues display normal variations in the number of sets of chromosomes Diploid animals sometimes produce tissues that are polyploid This phenomenon is termed endopolyploidy Liver cells, for example, can be triploid, tetraploid or even octaploid (8n) Polytene chromosomes of insects provide an unusual example of natural variation in ploidy

Euploidy 8-51 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In contrast to animals, plants commonly exhibit polyploidy 30-35% of ferns and flowering plants are polyploid Many of the fruits and grain we eat come from polyploid plants In many instances, polyploid strains of plants display outstanding agricultural characteristics They are often larger in size and more robust

8-52 Polyploids having an odd number of chromosome sets are usually sterile These plants produce highly aneuploid gametes Example: In a triploid organism there is an unequal separation of homologous chromosomes (three each) during anaphase I Figure 8.21 Each cell receives one copy of some chromosomes and two copies of other chromosomes

8-53 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sterility is generally a detrimental trait However, it can be agriculturally desirable because it may result in 1. Seedless fruit Seedless watermelons and bananas Triploid varieties Asexually propagated by human via cuttings 2. Seedless flowers Marigold flowering plants Triploid varieties Developed by Burpee (Seed producers) Keep blooming since the don’t form desired end product (competitors can’t sell seeds grown from their plants)

There are three natural mechanisms by which the chromosome number of a species can vary 1. Meiotic nondisjunction 2. Mitotic abnormalities 3. Interspecies crosses 8.3 NATURAL AND EXPERIMENTAL WAYS TO PRODUCE VARIATIONS IN CHROMOSOME NUMBER Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-54

Meiotic Nondisjunction Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Nondisjunction refers to the failure of chromosomes to segregate properly during anaphase Meiotic nondisjunction can produce haploid cells that have too many or too few chromosomes If such a gamete participates in fertilization The resulting individual will have an abnormal chromosomal composition in all of its cells Refer to Figure 8.22 8-55

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8-56 Figure 8.22 All four gametes are abnormal During fertilization, these gametes produce an individual that is trisomic for the missing chromosome During fertilization, these gametes produce an individual that is monosomic for the missing chromosome

Meiotic Nondisjunction Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In rare cases, all the chromosomes can undergo nondisjunction and migrate to one daughter cell This is termed complete nondisjunction It results in a diploid cell and one without chromosomes The chromosome-less cell is nonviable The diploid cell can participate in fertilization with a normal gamete This yields a triploid individual 8-58

Mitotic Abnormalities Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Abnormalities in chromosome number often occur after fertilization In this case, the abnormality occurs in mitosis not meiosis 1. Mitotic disjunction (Figure 8.23 a ) Sister chromatids separate improperly This leads to trisomic and monosomic daughter cells 2. Chromosome loss (Figure 8.23 b ) One of the sister chromatids does not migrate to a pole This leads to normal and monosomic daughter cells 8-59

8-60 Figure 8.23 This cell will be monosomic This cell will be trisomic Will be degraded if left outside of the nucleus when nuclear envelope reforms This cell will be monosomic This cell will be normal

Mitotic Abnormalities Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Genetic abnormalities that occur after fertilization lead to mosaicism Part of the organism contains cells that are genetically different from other parts The size and location of the mosaic region depends on the timing and location of the original abnormality In the most extreme case, an abnormality could take place during the first mitotic division Refer to Figure 8.24 for a bizarre example 8-61

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Consider a fertilized Drosophila egg that is XX One of the X’s is lost during the first mitotic division This produces an XX cell and an X0 cell 8-62 The XX cell is the precursor for this side of the fly, which developed as a female The X0 cell is the precursor for this side of the fly, which developed as a male This peculiar and rare individual is termed a bilateral gynandromorph Figure 8.24

Variation in chromosome structure and number chapter 8

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Variation in chromosome structure and number chapter 8