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UT Knoxville BIOL 240 - Exam 2 Study Guide
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BIOL 240 1st EditionExam # 2 Study Guide Lectures: 8-16Lecture 8 (January 28)Not all genes sort independently. In fact, some are linked to certain chromosomes like the sex chromosomes.I. Sex-linked Genes- Genes located on the X or Y chromosome- X-linkage: patterns of inheritance are different than autosomal- Y-linkage: male-specific genes; lacks copies of genes found on the X- Sex-linked inheritance:o Reciprocal crosses (i.e. red male x white female AND white male x red female) are expected to give the same resultso Since males only carry one copy of the X linked genes (they are XY), they are hemizygous.II. Nondisjunction- When chromosomes don’t separate correctly during meiosis- Euploid=correct chromosome number- Aneuploidy=wrong chromosome number- Proves chromosome theory of inheritance- Examples:o Klinefelter’s syndrome—XXYo Turner’s syndrome—missing sex chromosomeo Down Syndrome—extra 21st chromosome; ovum source for 95% of trisomy caseso Many more……..Lecture 9 (February 2)Not all organisms determine sex like humans do. In humans, the presence of 2 X sex chromosomes indicates a female, while one X and one Y chromosome indicates a male. However, in some organisms, sex is determined differently.I. Sex DeterminationEx: Worms…. Most are XX hermaphrodites (both ovaries and testes). Nondisjunction yields XO worms, which are male.- Can evolve from the standard XX or XY system by the deterioration of the Y chromosome- Terms:o Homogametic sex—2 identical sex chromosomes (XX); produces like gameteso Heterogametic sex—2 different sex chromosomes (XY) or (XO); produces different gameteso ZZ/ZW—in chickens, females are ZW and males are ZZIn humans, males have a Y chromosome, while females do not. What is so different about the Y chromosome from the X?II. Structure of Y chromosome1. Pseudoautosomal region (PAR)—homologous to X chromosome; located at tips; only region where synapsis occurs between X and Y2. Sex-determinging region (SRY)—produces protein TDF which is expressed at 6-8 weeks gestation and causes gonads to develop as testisSo, how do males get away with only having one X chromosome? After all,autosomal chromosomes exist in like pairs…III. Barr bodies- Female cells exhibit barr bodies, which are condensed DNA masses- Ohno proposed that they are inactivated X chromosomes- All extra X chromosomes are inactivated- Lyon hypothesis: X chromosome inactivation occurs randomly and inherited clonallyLecture 10 (February 4)We have already discussed that one way there is variation in offspring is through crossing over. Crossing over, or recombination, experience may help determine where genes arelocated on a chromosome.I. Recombination- Recombination is the rearrangement of genes on a chromosome as a result ofcrossing over.- Recombination Fraction—percentage of recombinant gametes- Genes on separate chromosomes, RF=50%- Genes on same chromosome are considered linked genes, RF=0% and is called complete linkage- Exchange occurs between two nonsister chromatids- For test crosses, one individual should be homozygous recessive for all genes.- Double crosses may occurLecture 11 (February 6)I. Chromosome Mapping- The first chromosome map:The distance between 2 genes is equal to their RF (in map units)…II. Procedure for mapping crosses1. Identify the parent gametes (these are the ones that occur most frequently, that is, with no recombination).2. Identify instances of double crossover (these are the least frequent occurances)3. Identify the gene in the middle (the middle gene is the only place where double crossover can occur in a triple cross).4. Identify the single crossover gametes5. Calculate the map distances (RF values) between the genes that are beside each other by adding the single crossover and double crossover valuesLecture 12 (February 9)I. Cystic Fibrosis- Most common recessive lethal inherited syndrome (1/2500)- Carriers are very common: 1 in 30- Caused by recessive mutation on chromosome 7- Gene discovered by combining recombination mapping and DNA cloningII. Single Nucleotide Polymorphisms- Homologous chromosomes differ at a single nucleotide- Once every 500-1000 base pairs- Fairly common; make useful molecular markers for mappingIII. Chromosome Rearrangements- Deletion: when a gene is deleted completelyo Chromosome breaks are driven by radiation and certain chemicals. Inaccurate repair is the origin of many rearrangements.o “deletion loop”: to pair with a deletion bearing homolog, the normal homolog must loop out the unpaired DNAo Example: cri du chat syndrome—result of deletion of the end of chromosome 5; also Williams syndrome—deletion of 17 genes- Duplication: when a gene or genes are duplicated- Inversion: when the order of the genes gets mixed up- Nonreciprocal translocation: when a part of the gene sequence on one homolog gets placed on another homolog (nonhomologous chromosomes)- Reciprocal translocation: when a part of the gene sequence on homolog 1 is exchanged for a part of the gene sequence on homolog 2 (nonhomologous)Lecture 13 (February 11)I. Fragile X Disease- Most common inherited mental disorder- Caused by mutations in the x-linked FMR1 gene- Fragile site near the tip of the X chromosomeII. Inversions- Change the order of the genes but no effect on gene balance- Paracentric—does not include the centromere- Pericentric—includes the centromere- Chromosome breakbroken ends joinrepetitive DNA segmentscrossover- Crossover suppressors (no crossing over within inverted region)III. Robertsonian Translocation- Reciprocal translocation that fuses most of one chromosome with most of another- Most common between telocentric chromosomesLecture 14 (February 13)I. Inversions- Can help distinguish human chromosomes from syntenic chromosomes in other mammals- Large blocks of the same genes in the same order (syntenic block) are found between mouse and human chromosomes- Human chromosome #2 came to be by a Robertsonian translocation that fused two small chromosomes found in apesII. Gene Duplication and Genome Evolution- Repeated gene familieso rRNA genes—several hundred copies per genomeo histone genes—a dozen copies of all 5 major histone genes per genome- copy number variationo copy number correlates with importance of starch in dieto variation in number of large repeats is commomo salivary amylase genes vary in copy number from 2-16III. Alternative fates of duplicated genes- After duplication, genes can either become


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UT Knoxville BIOL 240 - Exam 2 Study Guide

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