Exam 2 Genetics Inheritance and Sexual Life Cycles Haploid is a cell that contains one complete set of chromosomes Diploid a cell or an organism consisting of two sets of chromosomes usually one set from the mother and one set from the father Homologous Chromosomes is a set of one maternal chromosome and one parental chromosome that pair up with each other inside a cell during meiosis Sister Chromatid to either of the two identical copies formed by the replication of a single chromosome with both copies joined together by a common centromere Gamete haploid cells that are produced by meiosis Zygote initial cell formed when two gametes cells are joined by means of sexual reproduction in all eukaryotes Meiosis is a special type of cell division necessary for sexual reproduction occurs Fertilization the union of sperm and egg Gonad an organ that produces gametes a testis of ovary Sexual reproduction is a process that creates a new organism by combining the genetic material of two organism Asexual reproduction is a mode of reproduction by which offspring arise from a single organism and inherit the genes of that parent only Clone a cell cell product or organism that is genetically identical to the unit or Spore a reproductive cell capable of developing into a new individual without individual from which it was derived fusion with another reproductive cell Gametophyte is a haploid multicellular stage in the alternation of generations during the life cycle of land plants and algae Sporophyte is the diploid multicellular stage in the life cycle of a plant or alga Mitosis genetically identical daughter cells Development growth and asexual reproduction Meiosis daughter cells contain a subset of Parent cell s genes Preconditions for sexual reproduction formation of gametes Diploid Cell Pairs of homologous chromosomes diploid condition Each Chromosome 2 sister chromatids at start of meiosis mitosis SUPER IMPORTANT Homologous Chromosomes genetically similar but not necessary identical Sister chromatids of same chromosome genetically identical result of S phase of cell cycle Meiosis produces haploid cells No homologous pairs only one of each type of chromosome Producing haploid cells is necessary for sexual life cycles VERY IMPORTANT diploidy vs haploidy has NOTHING to do with sister chromosomes A sexual life cycle involves alternation between haploid cells and diploid cells MEIOSIS Stage of Meiosis 1 Prophase I Metaphase I Anaphase I Telophase I Just like in mitosis during prophase DNA condensation occurs the nuclear envelope and nucleoli disappear and the spindle starts to form The big difference is what is going on with the chromosomes themselves As DNA condensation proceeds and the chromosomes first become visible they are visible as tetrads So tetrads become visible during prophase Tetrads line up at the equator The spindle has completely formed It is during prophase I and metaphase I that genetic recombination is occurring Take a look at the genetic recombination page to find out about how that happens here Keep in mind that it only happens when there are tetrads so as soon as anaphase I gets going genetic recombination is over Tetrads pull apart and chromosomes with two chromatids move toward the poles Chromosomes with two chromatids decondense and a nuclear envelope reforms around them Each nucleus is now haploid Keep in mind that it is not the number of chromatids per chromosome that determine whether a cell is diploid or haploid but it is the number of chromosomes and whether they are paired that determines this Stages of Meiosis II Prophase II Metaphase II Anaphase II Telophase II Chromosomes with two chromatids become visible as they condense and the nuclear envelope and nucleoli disappear and the spindle is forming Chromosomes with two chromatids line up at the equator The spindle is fully formed Although genetic recombination primarily occurs during meiosis I the way the chromosomes line up during metaphase II can also help to make unique daughter cells I mention this on the genetic recombination page Chromosomes split so that a chromosome with only one chromatid heads toward each pole Chromosomes with only one chromatid decondense and get surrounded by new nuclear envelopes The four daughter cells are now all haploid and have the right amount of DNA They are ready to develop into sperm or eggs now How does the number of chromosomes affect the number of combinations N Number of chromosomes in a haploid pair N 2 Number of combinations 4 N 3 Number of combinations 8 N 10 Number of combination 32 Humans have 46 chromosomes so N 23 Number of combinations 2 23 Synapsis is the joining of chromosomes in a homologous pair Occurs in prophase I Synapsis results in crossing over The site of synapsis is a chiasma Mendelian Genetics How are Characteristics passed from parents to offspring o Offspring resemble parents BUT not exactly the same o Often Intermediate between parents Offspring between both parents o Blending inheritance Offspring is a blend of mom and dad not true or we would all look alike o Offspring are blends o the characteristic of their parents Particulate vs Blending Inheritance Black Black White White black White Black White Gray Mendel An Experimental Scientist o Did test with flowers crossed purple flowers with white flowers to find the outcome of there offspring o Both parents from a pure breeding line Purple Dominate PP White Recessive pp Purple Flower White Flower Purple Flower All of the offspring had purple Flowers Next Generation 75 Purple Flowers 25 White Flowers F1 Generation has a Pp giving it the ability to pass on Purple and White Characteristics Pure breeding line parental generation Offspring f1 Generation Final Offspring F2 Generation Law of Segregation the principle stating that during the production of gametes the two copies of each hereditary factor segregate so that offspring acquire one factor from each parent Meiosis 1 Locus physical location on a chromosome gene Allele Particular version of a gene Homozygous two copies are the same PP or pp Heterozygous two copies are not the same Pp Genotype Actual combination of alleles that are there PP Pp pp Phenotype actual characteristics purple vs white Dominant allele whose phenotype is expressed in homozygotes and heterozygotes Recessive allele whose phenotype is expresses in homozygotes only Particulate inheritance particles can keep their ability to be expressed while not always appearing in the
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