Lecture outline 6: Variation in Chromosome Structure and Number PDF

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Champa Gopalan was the instructor for this course. ...

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AGRO/ANSC/BIO/GENE/HORT 305 Fall, 2017 Variation in Chromosome Structure and Number Chpt 8, Genetics by Brooker (5th edition), Lecture Outline # 6 Introduction - While most diploid species normally contain precisely two haploid chromosome sets, many known cases vary from this pattern. Modifications have occurred through a change in the total number of chromosomes, the deletion or duplication of genes or segments of chromosomes or rearrangement of the genetic material within or among chromosomes. Collectively, such changes are called chromosome mutation or chromosome aberration. - Changes in chromosome number is called a genome mutation. Variation in Chromosome Structure Natural Variation in Chromosome Structure - A cytogeneticist studies variations in chromosome structure and number. - The chromosomes for a given species vary in both size and shape. - Chromosomes are named according to the location of their centromere (Figure 8.1). Based on the location of the centromere, the chromosome is either called metacentric, submetacentric, acrocentric, or telocentric. All chromosomes have a long and short arm. a. The long arm of the chromosome is called q. b. The short arm of the chromosome is called p. - A karyotype is a micrograph that arranges the chromosomes with the short arm at the top, and then in descending order by size. - Cytogeneticists may use stains to further identify the chromosomes. The use of Giemsa stain produces a G banding pattern, which is used as a standard identification pattern for chromosomes. Banding patterns may also be used to identify changes in chromosome structure. Mutations and Chromosome Structure - Changes in chromosome structure may either change the total amount of genetic material within the chromosome (increase or decrease) or rearrange the genetic material within a chromosome or between two chromosomes. - Examples of changes to chromosome structure include (Figure 8.2): a. Deficiencies and deletions. This changes the total genetic content of the chromosome. b. Duplications. This changes the total genetic content of the chromosome. c. Inversions. This changes the arrangement of the chromosome. d. Translocations. These may be either simple translocations or reciprocal translocations. These typically change both the arrangement of the chromosome and the total genetic content. Deficiencies - Chromosomal deficiencies are the result of a break in a chromosome. After the break, the piece without the centromere will be lost. This is called a terminal deficiency. - If the chromosome breaks at two locations, and the end pieces rejoin, it is called an interstitial deficiency

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- Recombination may also produce deficiencies. - The effect of a deficiency depends upon the size of the deletion and whether it includes genes or portions of genes. Cri-du-chat syndrome in humans is caused by a deficiency in the short arm of chromosome 5, and Prader-Willi and Angelman syndromes (chromosome 15). Deficiencies may be detected using microscopic (cytological), genetic, or molecular methods. At the genetic level, deletions may be revealed by a phenomenon called pseudodominance, in which a recessive allele is expressed in an individual because the corresponding gene on the homologous chromosome is missing. Figure 8.3 Duplications Figure 8.5 - A duplication creates extra genetic material. Duplications are usually the result of incorrect crossing over events. - These are usually rare, spontaneous events during the evolution of the species. - The effects of a duplication on the phenotype are associated with the size of the duplication and number of genes that are duplicated. - Usually, duplications are less detrimental than deletions. - An example of a duplication in humans is Charcot-Marie-Tooth disease (peripheral neuropathy). Duplications and Gene Families - Duplications may be responsible for the creation of gene families. A gene family is two or more genes that are similar to one another. These genes gradually diverge from one another by accumulating mutations (Figure 8.6, 8.7). a. Genes that are derived from a single ancestral gene are called homologues. b. Homologous genes in a single species are called paralogues. Inversions - A rearrangement of the genetic material that includes a segment that has been flipped to the opposite orientation is called an inversion. The total amount of genetic material remains the same. - An inversion that contains the centromere is called a pericentric inversion. An inversion that does not include the centromere is called a paracentric inversion (Figure 8.10). - Inversions may not have phenotypic consequences, unless it disrupts the function of a vital gene. - An example is hemophilia (type A) which is due to an inversion on the X chromosome. - Approximately 2% of the human population carries inversions. Inversion Heterozygotes - An individual who carries one copy of an inverted chromosome is called an inversion heterozygote. These individuals are phenotypically normal, but produce abnormal gametes. - During meiosis, the homologous chromosomes form an inversion loop. - If it is a pericentric inversion, the result is two normal chromosomes and two abnormal chromosomes. One of the abnormal chromosomes has a duplication, while the other has a deletion.

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- If it is a paracentric inversion, the result is two normal chromosomes and two chromosomes that possess deletions. The chromosomes with the deletions were caused by the formation of a dicentric bridge. In addition, some genetic material is lost due to the formation of an acentric fragment. Translocations (Figure 8.12). - The ends of normal chromosomes have telomeres, which contain specialized repetitive DNA. - Telomeres identify, and protect, the ends of the chromosomes. - If the telomeres are removed, the ends of the chromosome become reactive. DNA repair mechanisms may then join the ends of reactive chromosomes together, producing a translocation - Abnormal crossover may also produce a translocation - balanced translocation - This is often called a reciprocal, or balanced, translocation. It does not result in a change in the total amount of genetic information in the cell. If a piece of one chromosome is attached to another, it is called an unbalanced translocation. - These usually produce phenotypic abnormalities or lethality. - Familial Down syndrome in humans is one example (Figure 8.13). Variation in Chromosome Number Variation in chromosome number may be the result of a change in the number of sets of chromosomes, or variation in the number of chromosomes within a set (Figure 8.15). - Organisms that are euploid have a chromosome number that is an exact multiple of the chromosome set - The term triploid indicates an organism with three multiples of the chromosome set. - The term polyploidy indicates an organism with three or more sets of chromosomes. - Aneuploidy refers to the change in the number of a specific chromosome - Trisomic (2n + 1) indicates an extra copy of one chromosome. - Monosomic (2n-1) indicates a missing copy of one chromosome. Aneuploidy and Gene Expression - Aneuploidy commonly causes an abnormal phenotype. This is usually due to the change in the production of gene product . - Studies of Jimson weed (Datura stramonium) indicate how aneuploidy can influence phenotype Aneuploidy and Humans - Approximately 5-10% of fertilized human eggs have an abnormal chromosome number, and almost 50% of spontaneous abortions are due to aneuploidy. - Trisomic conditions for the autosomes are listed in Table 8.1. - Changes in the number of X chromosomes are usually nonlethal, due to the formation of inactive Barr bodies. There may be phenotypic consequences due to the expression of genes early in embryonic development (prior to X inactivation) or from genes in pseudoautosomal regions. - Sex chromosome aneuploids in humans are listed in Table 8.1. - The age of the parents may also influence the formation of an aneuploid condition. An example is Down syndrome, in which the chromosomes do not separate correctly during anaphase (called nondisjunction). This may be due to the age of the oocyte, but other factors may also contribute.

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Variations in Euploidy in Animals Most animals are diploid, and changes in the number of chromosome sets is not tolerated. Exceptions are: a. male bees, which are monoploid. b. some vertebrate animals, such as amphibians and reptiles, that are polyploid. Variations in Euploidy in Animal Tissues 1. Tissues of the body may have normal variations in the number of chromosome sets. 2. Liver cells in humans are triploid, tetraploid, or octoploid. This is called endopolyploidy. Variations in Euploidy in Plants - Unlike animals, plants commonly exhibit euploidy. - Polyploidy plants often have outstanding agricultural characteristics, or produce large flowers. - Polyploids with an odd number of chromosome sets are usually sterile, due to the production of aneuploid gametes a. Sterility is often selected for in modern agriculture. Meiotic Nondisjunction The consequences of meiotic nondisjunction are illustrated in Figure 8.22. Mitotic Nondisjuntion - This form of nondisjunction occurs after fertilization has occurred (Figure 8.23). The result is an organism whose contains a group of cells that are genetically different from one another. This is called mosaicism. - The size and location of the mosaic region depend on when and where the event occurred. Changes in Euploidy (Figure 8.24) - An autopolyploid refers to an increase in the number of chromosome sets in a single species - Alloploidy is the result of an interspecies cross - Allodiploids have one set of chromosomes from each parent. - Allopolyploid contains a combination of both alloploidy and autopolyploidy. a. An allotetraploid has two complete sets of chromosomes from two species. Fertility and Euploidy - Allodiploid are usually sterile, unless the species are closely related. - evolutionarily related chromosomes from two different species are called homeologous. - Allotetraploids are usually fertile, because they possess two complete sets of the chromosomes (including sex chromosomes) from each species.

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