Meiosis Class discussion I Multicellular organisms like animals plants and ourselves inherit one set of chromosomes from each parent when a sperm fertilizes an egg Meiosis zygote diploid haploid gametes in diploid organisms Increases variability II Meiosis is a type of cell division that produces haploid gametes in diploid organisms 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 egg iv 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 23 III 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 Prophase I V Metaphase I Anaphase I Telophase I Anaphase II Telophase II In Prophase II Metaphase II meiosis II SISTER CHROMATIDS separate Illustrate meiosis II below VI Chromosomes orient independently at the metaphase plate i 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 lots of different combinations of chromosomes Humans have 8 million different combos Genetic 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 chromatids fail to separate Directions Cut out the chromosomes below BRING TO S I The chromosomes represent the chromosomes from a diploid cell Replicates are provided so you can have sister chromatid pairs This is a cell in which 2n 6 On a large clean surface use the chromosomes to illustrate the processes of meiosis and mitosis 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 are sister chromatids What is the difference in definitions between sister chromatids and homologs 6 Think about the idea of independent orientation at the metaphase I equator How many different orientations are possible For this concept don t think about crossing over 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 lost her mind or have I learned something about this process and studying Inherited traits Class discussion I 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 gene 1 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 copy 4 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 segregation Using punnett squares to make a cross example A a x A a What are the gametes that will form from each parent A a Mom A or a A a Dad A or a Genotypes of parents crossed A A x a a Genotypes of offspring 4 Aa A a x A a 1 AA 2 Aa 1 aa A a x A A 2 AA 2 Aa A a x a a 2 Aa 2 aa Thinking like a geneticist and applying what you know Mendels pea plants produced both yellow and green peas If yellow is the dominant color for peas and I 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 cross Thinking like a geneticist and applying what you know In fruit flies a mutation leads to small wings Is this dominant or recessive How can you find out The cross that you might do A purebreeding fruit fly with small wings was crossed to a purebreeding fruit fly with normal wings and all the progeny had small 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 wings Write the genotypes of the F1 flies AA and aa Write the genotypes of the F2 flies 2 Aa 1 AA 1 aa 3 1 ratio In this example the mutant allele is not recessive to the wildtype allele The wildtype 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 by recessive alleles mating between two heterozygous individuals tend to skip generations ii Dominant Disorders caused by dominant alleles show up in every generation but the parents and offspring are both effected 1 Is the genetic disorder represented 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 heterozygous What 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 marrying into the family are assumed to not have the disease allele IV The rule of multiplication can be used to determine the probability that a child has a rare genetic disorder i Rule of multiplication the probability that two independent events will occur is the product of their individual
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