Chapter 8--Lecture 4, July 25th
Chapter 8--Lecture 4, July 25th 30156
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This 5 page Class Notes was uploaded by Hannah Kennedy on Saturday July 30, 2016. The Class Notes belongs to 30156 at Kent State University taught by Dr. Helen Piontkivska in Spring 2016. Since its upload, it has received 20 views. For similar materials see ELEMENTS OF GENETICS in Biological Sciences at Kent State University.
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Date Created: 07/30/16
7/25 Lecture 4—Ch. 8 and Ch. 7 Variation in Chromosome Structure and Number and genetic linkage and mapping in eukaryotes 1. Changes in Chromosome number a. Variations in chromosome number can be categorized in 2 ways i. Variation in the number of sets of chromosomes 1. Polyploid organism = an organism with 3 or more sets of chromosomes ii. Variation in the number of particular chromosomes within a set 1. Aneuploidy = an alteration in the number of particular chromosomes therefore the total number of chromosomes isn’t an exact multiple of a set; can have negative effect on phenotype (e.g. trisomy 21 aka Down Syndrome) b. Euploid organisms = organisms that have a chromosome number that is an exact multiple of the chromosome set 2. Variation in the number of chromosomes within a set, aka aneuploidy a. Aneuploidy causes imbalance in gene expression often detrimental to phenotype i. The level of gene expression is influenced by the number of genes per cell i.e. if a gene is carried on a chromosome that is present in 3 copies instead of 2, more of the gene product is made 1. This leads to an imbalance in either monosomic or trisomic ppl a. When a person in monosomic or trisomic, a lot of gene products are made in excessive or deficient amts b. Nondisjunction = process in which the chromosomes do not segregate properly; most often causes Down Syndrome i. Why down syndrome is associated with maternal age: age of the oocytes. Oocytes are made in the ovary and stay in prophase I and stay here until ovulation. As a woman ages, her primary oocytes have been in prophase I for longer period of time which may contribute to increased frequency of nondisjunction 3. Variation in the number of sets of chromosomes a. Variations in euploidy can occur in certain tissues within animal i. Diploid animals can produce tissues that are polyploid: e.g. liver cells contain nuclei that can be triploid, tetraploid, and octaploid ii. Endopolyploidy = the occurrence of polyploid tissues/cells that are otherwise diploid 1. Biological implication: increase in chromosome number may enhance cells’ ability to produce specific gene products needed in abundance 4. (Ch 8) Mechanisms that produce variation in chromosome number: non-disjunction a. meiotic nondisjunction = type of nondisjunction that produces cells that have too many or too few chromosomes; if this cell gives rise to gamete that fuses with normal gamete during fertilization, the resulting offspring will have abnormal chromosome number in all cells i. can occur during anaphase I or anaphase II 1. if in anaphase I: entire bivalent migrates to 1 pole and the 4 resulting haploid cells made from this are abnormal 2. if in anaphase II: 2 abnormal and 2 normal haploid cells b. mitotic nondisjunction = type of nondisjunction that occurs after fertilization in on of the somatic cells; may lead to a patch of tissue in the organism that has an altered chromosome number i. may happen bc sister chromatids separate improperly so 1 daughter cell has 3 sets of a chromosomes, and the other daughter cell has 1 ii. may also happen bc the sister chromatids separate during anaphase, but 1 of the chromosomes is improperly attached to the spindle so it doent migrate to a pole iii. mosaicism = occurs when genetic abnormalities occur after fertilization and the organism contains a subset of cells that are genetically different from those of the rest of the organism 1. size and location dependent upon the timing and location of original abnormal event c. alloploid organism = organism that contains sets of chromosomes from two or more different species d. changes in euploidy can occur via autopolyploidy, alloploidy, and allopolyploidy i. autopolyploid = an individual in which complete nondisjunction occurs due to a general defect in the spindle apparatus, producing 1 or more extra sets of chromosomes ii. alloploidy = common mechanism for change in chromosome number that is a result of interspecies crosses; has 1 set of chromosomes from 2 diff species is called an allodiploid (most likely to occur btwn species that are close evolutionary relatives) 1. allopolyploid has 2 or more sets of chromosomes from two or more species a. allotetraploid = contains 2 complete sets of chromosomes from 2 diff species for a total of 4 sets 2. determinant of success in producing fertile allodiploid is degree of similarity of diff species chromosomes 3. homologous chromosomes = evolutionary related chromosomes from 2 different species 5. Chromosomal Abnormalities a. 5 types of diff chromosomal rearrangements (small) Chromosomal Rearrangement Impact on genetic material Additional Info Deletion (2 types)—terminal Change in the total amount - occurs when a segment deletion and intercalary of DNA of chromosomal deletion material is missing so 1. terminal deletion = the chromosome is deletion in which a deficient in genetic chromosome breaks material into 2 and the piece - deletion of a region that without the centromere encodes many genes is lost and degraded can have pleiotropic or 2. interstitial deletion = variable effects deletion in which the - can cause partial chromosome breaks in monosomy (e.g. Cri-du- 2 places and the central chat syndrome) fragment is lost Duplication Change in the total amount - a section of the of DNA chromosome is repeated compared to the normal parent nd - allows the 2 copy to evolve into new functions - Ohno’s hypothesis: states that duplications are essential for evolution bc they lead to gene families (e.g. gene duplications in voltage-gated Ca2+ channels) Inversion (2 types) Chromosomal - Change in the direction 1. Pericentric = rearrangement of the genetic material inversion in which the along the chromosome centromere is within the inverted region 2. Paracentric = inversion in which the centromere is outside the inverted region Translocation Chromosomal - Section of the rearrangement chromosome becomes attached to a different chromosome or a different part of the same chromosome Simple translocation Chromosomal - A single piece of the rearrangement chromosome is attached to another chromosome Reciprocal translocation Chromosomal - 2 different chromosome rearrangement types exchange pieces to make 2 abnormal chromosomes i. deletion (2 types) 1. when: created when recombination in prophase I doesn’t happen at correct places between the homologous chromosomes therefore one homologous chromosome will have a deletion and the other will have a duplication 2. phenotypic consequence: dependent upon size and what kinds of genes are lost ii. Duplication (i.e. gene redundancy) 1. When: during abnormal recombination during prophase I (i.e. the crossover occurs at unaligned site of the homologs) a. Nonallelic homologous recombination = recombination that happens at homologous sites, but the alleles aren’t aligned right 2. Cause of misalignment: chromosome carriers 2 or more DNA sequence (i.e. repetitive sequences) 3. Phenotypic consequence: correlated with size therefore the larger pieces of a chromosome are more likely to give rise to a phenotypic consequence a. Majority of small duplications don’t have a phenotypic effect but they’re important for the addition of more genes to chromosomes i. Gene family = 2 or more genes that are similar to each other that result from evolution 1. Basis of gene fam: 2 copies of ancestral gene accumulate mutations. After generations the 2 genes diverge and are similar but not identical. a. Homologous genes = genes that are derived from a single ancestral gene b. Paralogs = homologous genes within a single species that constitute a gene family 2. Ex of gene family: globin genes encode polypeptides that are subunits of proteins that function in binding O2. Ancestral globin gene → myoglobin and hemoglobin alpha and beta → chains of hemoglobin 3. Advantage of gene family: specialized function 4. Copy number variation = CNV = type of structural variation in which a segment of DNA that is 1000 bp or more exhibits copy number differences among members of same species a. Segmental duplication = a process that a homolog undergoes to obtain 2 copies of a single gene b. 3 mechanisms that may bring CNV i. nonallelic homologous recombination bc it can cause deletion or duplication which alters the number of genes ii. proliferation of transposable elements increase the CNV iii. error in DNA replication c. phenotypic consequences: associated with specific diseases (e.g. schizophrenia) iii. inversion 1. phenotypic consequence: has one when a chromosome breaks in 2 places and the center piece flips, if the breakpoint happens in a vital gene, the gene function is disrupted a. ex: hemophilia type A: patient inherits X-linked inversion in which the breakpoint inactivated the gene for the blood-clotting protein 2. inversion heterozygote = a person carrying 1 copy of a normal chromosome and 1 copy of an inverted chromosome with high probability of producing gametes that have abnormal genetic content 3. consequence of pericentric crossover: single crossover. 2 of 4 sister chromatids are involved. After meiosis there are 2 abnormal chromosomes that each have deleted and duplicated segments 4. consequence of paracentric crossover = 1 normal chromosome, 1 w inversion, 2 w deletions iv. Translocation 1. Telomeres = chromosome pieces found at the end of normal chromosomes that prevent translocations from happening; allow cell to ID where the chromosome ends to prevent attachment of cDNA to ends 2. Mechanisms that cause translocations (2): a. Cells are exposed to agents that cause multiple chromosomes to break w loss of telomeres. Reactive ends are improperly joined by DNA repair enzymes to result in abnormal chromosomes b. Nonhomologous crossover resulting in rearrangement of genetic material 3. Balanced translocations = translocations in which the total amount of DNA isn’t altered (i.e. no phenotypic consequence) but carriers can have offspring w unbalanced translocations 4. Unbalanced translocation = significant portions of genetic material are deleted or duplication; phenotypically abnormal 5. Robertsonian translocation = the most common translocation in humans that arises from breaks near the centromeres of 2 nonhomologous acrocentric chromosomes 6. Ppl with reciprocal translocations can produce abnormal gametes due to the pairing and segregation of chromosomes, depending on the pattern during meiosis I a. Pairing of homologous regions leads to the formation of a translocation cross with 4 pairs of sister chromatids (i.e. 8 chromatids) 6. (ch 7) independent assortment vs. linkage: corn kernel (heterozygote at loci; what gametes would be formed): a. independent assortment: 2 genes on 2 diff homologous pairs of chromosomes b. linkage: 2 genes on a single pair of homologs; no exchange occurs c. linkage with recombination: 2 genes on a single pair of homologs; exchange occurs between 2 nonsister chromatids i. where crossing over occurs is important for whether we see recombination or not 1. when crossing over occurs between 2 genes we see recombinant gametes 7. Q: a homozygous groucho fly (gro, bristles clumped above the eyes) is crossed with a homozygous rough fly (ro, eye abnormality). The F1 females are testcrossed, producing the following offspring: a. 518 groucho b. 471 rough c. 6 groucho, rough d. 5 wild-type e. 1000 total i. are these 2 genes unlinked? ii.If they are linked, what is the genetic distance between these genes?
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