Download 11-4-15 SI Session Answers

11/4/15 SI Session: Meiosis and Genetics! YAY! Homologous Chromosomes Pairs of chromosomes with alleles of the same genes in the same location; one member of each pair came from mom, the other from dad Diploid We call cells this when there are 23 pairs of homologous chromosomes present, each with 46 individual chromosomes in total Haploid We call cells this when there are 23 individual chromosomes present, or, when they have half the number of the expected chromosome count Synapsis *Pairing of homologous chromosomes Chiasmata *Sites where crossing over of homologous chromosomes occur Recombinant Chromosomes Chromosomes with new allele combinations due to crossing over in the chiasmata are called this Alleles Different variations of the same gene; usually denoted with an uppercase letter or a lowercase letter Gene A unit of heredity that is passed down from parent to offspring Independent Assortment Chromosomes randomly line up and independently segregate from one another in a random fashion after metaphase, leading to genetic diversity. (This is an example of how you can have two cells with the same set of chromosomes, but end in genetically different daughter cells by the end of meiosis. See Fig 14.8) Meiosis I By the end of this half of meiosis, the homologous chromosomes separate. (The cells are still diploid in this half) Meiosis II By the end of this half of meiosis, the sister chromatids separate. (The cells are now considered haploid in this half) *These two event are unique to meiosis, and do not happen in mitosis Meiosis produces 4 genetically distinct haploid daughter cells. In animals, meiosis leads to the production of gametes, or to the production of spores in plants. How do sexually reproducing organisms produce offspring that are SIMILAR to their parents? Genetic similarity results from 1) the accurate replication of DNA and 2) the careful distribution of chromosomes to gametes (or spores) during meiosis How do sexually reproducing organisms produce offspring that are DIFFERENT to their parents? Variability results from 1) events during meiosis (independent assortment, crossing over), 2) from bi-­‐
parental inheritance 3) as well as mutations MEIOSIS Where does crossing over occur? Metaphase I Where are the cells still considered diploid? All of meiosis I Where are they considered haploid? All of meiosis II Genetics: In the World of Harry Potter Huntington’s disease is an autosomal dominant genetic disorder that afflicts the central nervous system and leads to an early death. It’s been rumored that this disease originated from the being of He-­‐Who-­‐Must-­‐Not-­‐Be-­‐Named when h e failed to kill Harry Potter, and has plagued the wizarding world ever since. Fast-­‐forwarding a few years, you learn than Ron and Hermione are wanting to expand their family and would like to bring a baby into the world! However, Hermione is concerned about the Weasley’s family h istory of Huntington’s disease, and seeks the family planning advice of a certified genetic counselor. What would the phenotypes of the offspring be in each of the following scenarios? If this is a dominant d isorder, what does that mean for the fate of their offspring? (Hint: be sure you identify what the genotypes are of a homozygous dominant, heterozygote, and homozygous recessive individual look like. Cross using a punnet square) 1) Ron is a carrier of Huntington’s disease, but Hermione is normal HH X HH: 50% are carriers Hh, 50% are normal hh There is a 50% chance their offspring would develop H’s disease later in life, and 50% chance that their offspring would be totally normal 2) Ron and Hermione are b oth carriers Hh X Hh: 25% are HH, 50% are carriers Hh, and 25% are normal hh There is a 25% chance that their child would not be born (the HH is lethal, most babies do not survive), a 50% chance that their offspring would develop H’s disease later in life, and 50% chance that their offspring would be normal