Genetics Notes week 6
Genetics Notes week 6 Bios 206
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This 5 page Class Notes was uploaded by Becca Sehnert on Friday February 19, 2016. The Class Notes belongs to Bios 206 at University of Nebraska Lincoln taught by Dr. Christensen in Fall 2016. Since its upload, it has received 21 views. For similar materials see Genetics in Biological Sciences at University of Nebraska Lincoln.
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Date Created: 02/19/16
WEEK 6 NOTES MONDAY A completely orange female cat is bred with a black male. One of their kittens is a tortoiseshell male, who is found to have Kleinfelters syndrome (XXY). The father contributed an XY gamete. When did the nondisjunction event occur? a. Meiosis I b. Meiosis II c. Impossible to tell Polyploidy • Names based on # of complete sets of chromosomes found” triploid has 3n chrom, etc • Odd numbers (ie 3n) result in multiple nondisjunction events in meiosis, and sterility results o Bananas and seedless watermelon are examples (variable survival) o Make new banana trees from cutting part of banana tree and planting it o Triploid seeds? Breed a diploid and tetraploid • Even numbers are sometimes fertile, depending on pairing and nondisjunction. o Tetraploids must form 2 divalent seeds. • Autopolyploidy –multiple sets of chromosomes from one species • Allopolyploidy –combination of chromosome sets from different species (errors in mitosis) o Can make it happen by adding colchicine o Happens all the time in nature o A lot of diversity in plants, multiple genome duplications o Also in yeasts, bacterias, etc. o All behave like diploids in meiosis • ALLOTETRAploid o Arises from hybridization of 2 close related species o If sterile hybrid undergoes natural chromosomal doubling, fertile amphidiploid is produced o Mules are sterile, except 1 in Champion, NE o Aka amphidiploid –originated from a haploid with gametes from o Wheat evolved by selection and has at least 2 sets of each chromosome § Allohexaploid o More DNA means bigger cells = bigger seeds in food o Can get just a branch on a plant that is different from roots o Cross radishes with cabbage and create sterile gametes that makes a hybrid • Haploid DOES NOT MEAN monoploid o Monoploid –one set of chromosomes o Haploid –half of what organism haves • Variation in chromosome structure o Deletion –deficiencies § Near one end (terminal deletion) § Interior (intercalary deletion) § Alleles on normal chrom expressed (even if recessive) called pseudodominance § Deletion has no centromere so not separated, done for o Duplication o Inversion o Nothing lost and nothing gained, but connections between, § Nonreciprocal translocation § Reciprocal translocation WEDNESDAY Deletions and meiosis • For synapsis to occur, unpaired region of normal homolog must loop out of linear structure into a deletion loop • Indels –don’t know if it’s a deletion or an insertion because you don’t know which happened first Duplications often arise as result of unequal crossing over during meiosis • Duplication happens between an inappropriate cross happens • Duplications are a lot more viable than deletions • Alike sequences might diverge and create new genetic material • Tandem duplication –bar eyed drosophila. If hetero for mutation, and eye size is smaller • Gene duplication plays role in evolution (ie globins) • Origin of gene families: duplication and divergence The horse is 2n=64, the donkey is 2n=62. IF crossed, they produce a hybrid mule. How many chromosomes are found in a mule somatic cell? This cell is _____. a. 2n=126; aneuploid b. 2n=126; euploid c. 2n=63; aneuploid d. 2n=63; euploid (one complete set of donkey and one complete set of horse, not missing anything) e. Either 2n=64 or 2n=62, depending on whether mother was the horse or donkey; euploid None are pairing. Rearranged relative to each other and bad when hetero for all mutations from horse and donkey. Aneuploid vs euploid –missing/extra chromosome from a set Inversions • Inversion involves a rearrangement of linear gene sequence rather than loss or gain of genetic info. • Fine for viability or fertility • Random events that rearrange chromosome • 2 breaks in chromosome and rejoining • can occur intrachromosomal unequal crossing over • PERaCENTRIC –does not include centromere • PERiCENTRIC –involves centromere –perimeter around centromere • And Meiosis o Synapsis of inverted chromosome require inversion loop o Effects of single crossover within inversion loop are similar but for paracentric inversion o DRAWING –don’t try and draw the circle, just draw important parts next to each other, don’t try and match up the ends that aren’t involved. § If no centromere, its done, no good § If 2 centromeres, pulled both ways and either broken or not in nuclear envelope. No bueno o Pericentric –most of the time, get massive duplications and deletions and get unviable progeny from the recombinations. § Don’t recover recombinant in progeny The following strucuture is observed during prophase i. it could be due to which of the following conditions? a. Pericentric inversion b. Paracentric inversion c. Deletion d. Insertion e. Deletion or insertion (indel) Translocations alter linkage of chromosomal segments • Nonreciprocal translocation of A-B o 2 different chromosomes (not homologous) • Reciprocal translocation of A-B and H-I-J o 2 breaks and both are recombinant • 2 possible segregation patterns o One has normal and balanced gamete (alternate segregation) o One leads to duplications and deficiencies (adjacent segregation) o Draw in square o Still independent assortment (like meiosis and Mendel) • Nothing special except recombinants FRIDAY Robertsonian translocation or centric fusion • Breaks at extreme ends of the 2 short arms of 2 nonhomologous acrocentric chromosomes • 2 telocentric chromosomes usually happens –with centromeres fused • Familial down syndrome is a possible consequence of a Robertsonian Translocation o Only see when chromosome 21 is involved (causes downs) o Families that have this have more miscarriages and more downs babies • Northern Europeans have inversions on chrom 17 and women who are hetero for this have more children. • Segregation distortion o Some things are more likely to get transmitted o Changes the frequencies quickly o Somehow only one geno shows up in sperms and the other never does 15 MUTATIONS –alterations in DNA sequence • Luria-Delbruck fluctuation test showed mutations occur randomly, are subject to selection –not adaptive • Rates of spontaneous mutations vary among loci in different organisms • Induced mutations result from influence of extraneous factor, either natural or artificial • Somatic mutations occur in any cell except germ cells and are not inheritable Mutant phenotypes • Dominant autosomal mutations will be expressed phenotypically in first generation • Recessive mutation will not be observed until a homozygote occurs • X-linked recessive mutations arising in gametes of female • Molecular change o Point mutations = base substitutions o Deletions and duplications also occur o Often cause frame shift mutations § Very bad and not helpful • Missense, nonsense, silent o Missense –change a codon for different amino acid § CAA Gln à CGA Arg o Nonsense –changes codon to stop codon and results in premature termination of translation CAA Gln à TAA stop o Silent mutation doesn’t change protein sequence CAA Gln à CAG Gln o Transitions –pyrimidine replacing pyrimidine or purine replacing a purine (MANY MORE) o Transversions –purine and pyrimidine changed o Fainting goats –similar to Thomsen’s disease in humans = myotonia § G to C (transversion) § Ser to Pro (Missense) What kind of mutation does this change represent? a. Transition, missense b. Transition, nonsense c. Transversion, missense d. Transversion, nonsense Phenotypic effects to classify mutations • Loss of function –doesn’t work at all anymore o Can go unnoticed if not dependent on amount o If dependent on amount, may show up as dominant • Gain of function –does more of what it used to do or does something different o Usually dominant mutation • Morphological –observable • Nutritional (biochemical) –affects metabolic pathways • Behavioral –phenotype that’s different is behavioral • Regulatory –expressed in wrong area or amount • Lethal –interrupt essential process and results in death • Conditional –depend on environment (Siamese cats) Affects • Neutral –no affect on fitness, most all mutations o Many in large part of genome that don’t contain genes and have no affect o Mutation of hairs in humans vs chimps o More mutations in genes we don’t use because no selection to select against mutations • Silent –usually neutral, many missense mutations are also neutral • Rate of deleterious mutation in humans is at least 1.6 per individual per generation in germ cells Spontaneous mutations –from replication errors and DNA damage • Replication errors –DNA polymerase occasionally inserts incorrect nucleotide, due to mispairing, and failure to proofread • Lead to point mutations • Slippage can lead to small insertions/deletions
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