chapter 9

• Asexual reproduction -  one parent organism replicates its DNA and splits the contents of one cell into two, generates genetically identical offspring
• Single-celled organisms such as bacteria, archaea, and protists reproduce asexually, but among multicellular organisms it is not very common.​
• Sexual Reproduction - offspring comes from two parents, egg fuses with a sperm, yielding the first cell of the next generation, genetically diverse
• Sexual reproduction increases the chance that offspring survive in changing environments.
• Diploid cells - cells with two sets of chromosomes
• Human cells contain 23 homologous sets of chromosomes, allowing us to have 46 chromosomes.
• karyotype - an image of the chromosomes in a cell
• autosomes - chromosomes that are not sex chromosomes, 1-22
• sex chromosomes - chromosomes denoted by a letter, XX for female and XY for male
• Homologous pair - chromosomes of similar size and structure
• Alleles - different versions of a gene that code for the same trait
• Members of a homologous pair may carry different alleles.
• When members of a homologous pair are replicated, identical alleles are on sister chromatids.
• Gamates - sperm and egg cells, haploid sex cells
• Haploid - a cell with only one set of chromosomes
• When two haploid sex cells fuse at fertilization, one diploid zygote is formed.
• Zygote - first cell of a new organism, fusion of egg and sperm
• Germ cells - specialized diploid cells that divide by meiosis to form haploid gametes.
• meiosis - consists of two cell divisions resulting in four haploid daughter cells
• Prophase I - chromosomes condense, nuclear envelope breaks down, spindle forms, pair of homologous chromosome undergo synapsis and cross over
• Metaphase I - tetrads line up at the equator
• Anaphase I - pair of chromosomes seperate
• Telophase I - nuclear envelope reforms around haploid number of chromosomes
• Prophase II - spindle forms, nuclear envelope breaks up
• Metaphase II - chromosomes line up along the equator of the cell
• Anaphase II - sister chromatids split to opposite sides of poles
• Telophase II - nuclear envelope reassembles, chromosomes decondense, spindle dissapear, and cytokinesis occurs
• Crossing over happens in Prophase I.
• Independent Assortment happens in Metaphase I.
• Crossing over - the exchange of genetic material between homologous chromosomes
• Independent Assortment - chromosome pairs align randomly
• A germ cell with three pairs of chromosomes has four possible arrangements during metaphase I,  yielding eight possible gametes (2^3= 8)
• Fraternal twins - two sperm cells fertilize two separate egg cells, and the offspring might look very different
• Monozygotic twins - an embryo splits in two, each embryo then develops independently
• Nondisjunction - the failure of chromosomes to seperate
• Nondisjunction occurs in Anaphase I or II.
• Extra chromosomes could cause defects but less chromosomes reduces chance of survival.
• Down Syndrome - an extra copy of a chromosome (trisomy) on #21
• Triplo-X - XXX, female, tall stature, menstrual irregularities, increased risk of giving birth to triplo-X daughters or XXY sons​
• Klinefelter Syndrome - XXY, male, sexual underdevelopment, long limbs, large hands and feet, development of breast tissue​