Objectives for Cumulative Part of the Final Exam Genetics, Fall 2012 Meiosis: Know what happens in meiosis, how chromosomes are separated in the two divisions, when crossing over occurs. Know what homologous chromosomes are, what haploid and diploid, chromosome and chromatid mean. Be able to state how many chromosomes, chromatids, and pairs of chromosomes are present at different stages of meiosis. Most organisms are diploid—have two copies of each chromosome (2n). Homologous chromosomes carry the same genes but may have different alleles. (A B C on one and a b c on the other). Some organisms are haploid—fungi, some algae, protozoan’s (1n).Meiosis is the formation of gametes (sex cells). One diploid cell divides twice andforms 4 haploid cells. Each daughter cell gets one chromatid from each pair of homologous chromosomes. This leads to half the number of chromosomes as the parent. There are two stages of mitosis (I and II). In meiosis I, homologous chromosomes are separated into different daughter cells. Chromosome recombination (crossing over) occurs and exchanges bits of DNA and forms tetrads (prophase I). In meiosis II, the sisterchromatids are separated into different daughter cells. This second division follows the same steps as mitosis would, except the daughter cells would now have half the amount of DNA as the parent, hence gametes.Crosses and inheritance: Be able to do a dihybrid cross and crosses involving partial dominance, codominance and X linkage.Partial dominance—crossing of red and white flowers yields pinkCodominance—both alleles are expressed. Phenotype of the heterozygote is a combination of the two homozygotes. Crossing of red and white flowers yields flowers that have petals that are both red and white. Blood types are a good example of codominance.**Know that the frequency of crossing over between genes – recombination – can be used to map the positions of genes on chromosomes** Be able to make a genetic map from a three point cross.Step 1: identify the progeny with parental phenotypes, single and double crossover phenotypes. (Double crossovers have the lowest numbers, parental non-recombinant types are the most frequent.)Step 2: compare the double crosses (least frequent) with the parentals to see which gene is in the center (chromosome that is flipped)Sex chromosomes: Know the XY sex determination mechanism, the differences in the genetic makeup of the X and Y chromosomes, know the name of the gene that determines sex and where itis located on the Y.The X chromosome carries thousands of genes, whereas the Y chromosome carries very few (it’s smaller than the X). The Y chromosome is the one that determines sex. TDF is testis-determining factor, which is coded for by the STY gene located on the Y chromosome. The SRY gene is located near the pseudoautosomal region (PAR). WhenSRY is expressed, the gonads develop into testes. When it’s not expressed, the gonads develop into ovaries. In this way, the SRY represses female development (testis secrete anti-Mullerian hormone that blocks development into uterus/ovaries) and stimulates the formation of internal and external male structures (via testosterone).Be able to provide an explanation for the existence of XX males and XY females. The existence of xx males and xy females is due to translocations on the genes as well as TDF and SRY. XY females had a portion of a chromosome that was missing/deletion of part of the Y (the SRY)—because SRY was missing, they became females instead of developing into males. In XX males, part of their Y picks up the missing SRY from the Y chromosome and adds it to the X chromosome that it has.Know what X-inactivation is, what the Barr body is and what dosage compensation is. Because females have two x-chromosomes and males only have one, expression of genes on the X is unbalanced. Dosage compensation takes care of this by the development of Barr body’s and inactivating one x-chromosome in each cell in females during early development.Chromosome mutations: Know the genetic makeup of individuals with Down syndrome, Turner syndrome, and Klinefelter syndrome and know the general phenotype of these syndromes.Individuals with Down syndrome have 21 chromosomes. Turner syndrome: 45 chromosomes, X--Short stature, webbed neck, shield chest, underdeveloped breast and widely spaced nipples, rudimentary ovaries, sterileKlinefelter syndrome: 47 chromosomes, XXY—Tall stature, poor beard growth, minor breast development, female pubic hair pattern, testicular atrophy, sterileBe able to correctly use the terms aneuploidy, monosomy, trisomy, and polyploidy.Aneuploidy—changes in chromosome number due to mistakes in mitosis or meiosis. Monosomy—one copy of a chromosome. Trisomy—three copies of one chromosome. Polyploidy—multiple copies of chromosome sets (triploid, tetraploid, etc.)Know what translocations are, why they can cause miscarriages, and know the example of familial Down syndrome. A translocation is a chromosomal mutation associated with the reciprocal or nonreciprocal transfer of a chromosomal segment from one chromosome to another. Familial Down syndrome is a human translocation because of the translocation carrier14/21. The resulting progeny are Normal (46 chromosome #), monosomy—lethal (45), trisomy—down (21), translocation carrier (45)Bacterial genetics:Know that bacteria are haploid and have a circular chromosomeBacteria have no nucleus, are haploid, have no mitosis or meiosis, and have circular chromosome DNA with circular plasmids.Know the three mechanisms by which bacteria share DNA and be able to describe them(conjugation, transformation, transduction).In order for conjugation recombination to occur, the cells must be in contact withone another. Conjugation is also unidirectional. If Strain A has an F-factor plasmid F+ and is a donor and is crossed with stain B, F- recipient, strain B will pick up that plasmid.F+ x F- One strand of the F factor is nicked by an endonuclease. This strand moves acrossthe conjugation tube. As the strand proceeds, complementary DNA synthesis occurs on both strands (both the one moving across the tube and the one that has unwound in order to allow part of it to move across tube). Movement across the tube is then completed as well as DNA synthesis, and ligase closes the circles.Genetic recombination can also occur through transformation by the uptake of “free DNA.” Free DNA binds to a component bacterium and enters. The
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