MeiosisClass discussionI. Multicellular organisms like animals, plants, and ourselves inherit one setof chromosomes from each parent when a sperm fertilizes an egg.Meiosis, zygote, diploid, (haploid gametes in diploid organisms). Increases variabilityII. Meiosis is a type of cell division that produces haploid gametes in diploidorganisms.i. Diploid organisms contain a set of homologous chromosome in their somatic.ii. The diploid number, or 2n, in humans = 46.iii. Diploid organisms produce haploid gametes.1. Examples: sperm and the eggiv. The haploid number, or n, in humans =23.The haploid number is constant for a cell no matter what stage of the cell cycle it is in. If I showed you a picture of a human cell in metaphase of mitosis and asked you what n=? 23 because the haploid number is always 23III. Meiosis reduces the chromosome number from diploid to haploid.i. Diploid or Haploid?ii. What would the haploid gametes of this cell look like?iii. What is n=3?;;IV. In meiosis I HOMOLOGOUS CHROMOSOMES separate. Illustrate meiosis I below.V.Inmeiosis II SISTER CHROMATIDS separate. Illustrate meiosis II below.VI. Chromosomes orient independently at the metaphase platei. What does this mean and how do the equally probable arrangements of chromosomes contribute to genetic diversity?Multiple options give way to many possibilities for different offspring. There are lotsof different combinations of chromosomes. Humans have 8 million different combosGenetic disorders caused by an abnormal number of chromosomes VII. Errors in meiosis lead to nondisjunction of the chromosomes.i. When nondisjunction occurs in meiosis I a pair of homologous chromosomes fail to separate.ii. When nondisjuction occurs in meiosis II a pair of sister chromatidsfail to separate.Prophase II Metaphase II Anaphase II Telophase IIProphase I Metaphase I Anaphase I Telophase IDirections:Cut out the chromosomes below. BRING TO S.I.!The chromosomes represent the chromosomes from a diploid cell. (Replicates areprovided so you can have sister chromatid pairs. This is a cell in which 2n = 6. On alarge clean surface use the chromosomes to illustrate the processes of meiosis (andmitosis) for yourself.With your cut-out chromosome you should be able to answer these questions:1. Do you need all the pieces above for both mitosis and meiosis?2. What are the major differences between mitosis and meiosis?3. When does crossing over occur?4. Why is 2n = 6? 5. Pick up two chromosomes that are homologs. Pick up two chromosomes that aresister chromatids. What is the difference in definitions between sister chromatids andhomologs?6. Think about the idea of independent orientation at the metaphase I equator. Howmany different orientations are possible? For this concept, don’t think about crossingover). 7. Does independent orientation at the metaphase I equator lead to gamete variation?How?8. Can a gamete form that has alleles: A;b; H; R; d? Why or why not?9. Can a gamete form that has alleles: A; B; H; R; d? Why or why not?10. Can a gamete form that has alleles: A; A; b; b; a; a; B; B? Why or Why not?11. Why am I using cut-outs like I am an elementary student? Has Dr.Steinwand losther mind or have I learned something about this process and studying?Inherited traitsClass discussionI. Traits are passed one from one generation to the next in the form of alleles.i. What is an allele? A different or an alternative form of a gene1. When individuals have two of the SAME alleles, they are homozygous.2. When individuals have two DIFFERENT alleles they are heterozygous.3. Dominant alleles determine the organisms appearance.Even when present in one copy4. Recessive alleles have no obvious effect on the organisms appearance. 5. As diploid organisms we carry 2 alleles for each gene in our genome. Within populations however, many alleles may correspond to a single gene.For example: blood types (codominant)6. Alleles can be either incompletely dominant or codominant.ii. The Law of Equal Segregation: a sperm or egg carries only one allele for a single gene because alleles segregate away from each other during meiosis.APPLY WHAT YOU KNOW! Add alleles to the cell shown below: II. Single gene crosses: applying Mendel’s Law of Equal segregationUsing punnett squares to make a cross (example: A/a x A/a)What are the gametes that will form from each parent? A/a A/a Mom DadA or a A or aThinking like a geneticist and applying what you know: Mendels pea plantsproduced both yellow and green peas. If yellow is the dominant color for peas andI give you a plant with yellow peas, do you automatically know its genotype?How can you figure it out? Hint: geneticists use a cross! Test crossGenotypes of parentscrossedGenotypes ofoffspringA/A x a/a 4 AaA/a x A/a 1 AA, 2, Aa, 1 aaA/a x A/A 2 AA, 2 AaA/a x a/a 2 Aa, 2 aaThinking like a geneticist and applying what you know: In fruit flies, amutation leads to small wings. Is this dominant or recessive? How can you findout? The cross that you might do: A purebreeding fruit fly with small wings wascrossed to a purebreeding fruit fly with normal wings and all the progeny hadsmall wings. When the F1 generation was interbred, 400 F2 flies were counted.305 had small wings and 95 had wild type wings.Which allele is dominant? Small wingsWrite the genotypes of the F1 flies. Write the genotypes of the F2 flies.AA and aa (2)Aa, (1)AA, (1)aa3:1 ratioIn this example the mutant allele is not recessive to the wildtype allele. Thewildtype allele is NOT always the dominant allele!III. Pedigrees allow us to determine the pattern of inheritance of a familial disorder. i. Recessive Disorders: caused byrecessive alleles (mating betweentwo heterozygous individuals,tend to skip generations)ii. Dominant Disorders: caused bydominant alleles (show up inevery generation but the parentsand offspring are both effected)1. Is the genetic disorderrepresented in the pedigree to the right recessive or dominant? (Hint- Think about how we defined these terms.)Recessive (skip generations)2. How do you know?The disease skips generation (present only in 2nd and 4th generation) Parents are heterozygousWhat are the probable genotypes of various individuals in the pedigree? For example,what is the genotype of I-2? II-4?Important assumption: the disease allele is rare in the population so people marryinginto the family are assumed to not have the disease allele.IV. The rule of multiplication can be used to determine
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