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Unit III Chapter 13 – Meiosis and Sexual Life Cycles Explain why organisms only reproduce their own kind, and why offspring more closely resemble their parents than unrelated individuals of the same species. Parents endow their offspring with coded information in the form of hereditary units called genes. The tens of thousands of genes we inherit from our mothers and fathers constitute our genome. Our genetic link to our parents account for family resemblance. Our genes program the emergence of specific traits as we develop from fertilizes eggs to adult. Distinguish between asexual and sexual reproduction. In sexual reproduction, a single individual is the sole parents and passes copies of all its genes on to its offspring. Compared to asexual reproduction, sexual reproduction usually results in greater variation; two parents give rise to offspring that have unique combination of genes inherited from both parents. Diagram the human life cycle and indicate where in the human body that mitosis and meiosis occur; which cells are the result of meiosis and mitosis; and which cells are haploid. Figure 12.3-12.4 List the phases of meiosis I and meiosis II and describe the events characteristic of each phase. Meiosis I consists of 4 stages: prophase I, metaphase I, anaphase I, and telophase I. Prophase I begins like prophase of mitosis. The nucleolus disappears, chromatin condenses in to chromosomes, the nuclear envelope breaks down, and the spindle apparatus develops. However, once the chromosomes are condensed, homologues chromosomes pair, a process called sypnasis. These pairs of homologues chromosomes are variously referred to as tetrad or bivalents. During sypnasis, corresponding regions along nonsister chromatids form close associations called chiasmata, chiasmata are sites where genetic material is exchange between nonsister homologues chromatids, a process called crossing over. A tetrad together with chiamata and cross over events is referred to as a syptonemal complex. At metaphase I, homologues pairs of chromosomes are spread across the metaphase plate. Microtubules extending from other pole are attached to the kinetochore of one of one member of each homologues pair. Microtubules from the other pole are connected to the second member of each homologues pair. Anaphase I begins when homologues whinthin tetrads uncouple as they are pulled to opposite poles. In Telophase I, the chromosomes have readhed their respective poles, and a nuclear membrane develops around them. Each pole will form a new nucleus that will have half the number of chromosomes, but each chromosomes will contain two chroamtids.l Since daughter nucleic will have half the number of chromosomes, cells that they eventually form will be haploid. Beginning in the Telophase I, the cells of many species, cytokinesis and form cleavage furrow or cell plates. In other species, cytokinesis is delayed until after meiosis II. Also, a short interphase II may begin. In any case, no replication of chromosomes occurs during this period. Instead, part II of meiosis begins in both daughter nucleic. In prophase II, the nuclear envelope disappears and the spindle develops. There are no chiasmamta and no crossing over of genetic material as in Prophase I. In Metaphase II, the chromosomes align singly on the metaphase plate. Single alignment of chromosomes is exactly what happens in mitosis except that now there is only half the number of chromosomes. Anaphase II begins as each chromosome is pulled apart into two chromatids by the microtubules of the spindle apparatus. The chromatids (how chromosomes) migrate to their respective poles. Again, this is exactly what happens in mitosis except that now there is only half the number of chromosomes. In Telophase II, the nuclear evenlope reappears at each pole and cytokinesis occurs. The end result of meiosis is four haploid cells. Each cell conatins half the number of chromosomes and each chromosome consists of only one chromatid. Like in interphase, a second chromatid in each chromosome is replicated, but the cell will still have only half the number of chromosome. Recognize the phases of meiosis from diagrams or micrographs. Figure 12.6 Explain how independent assortment, crossing over, and random fertilization contribute to genetic variation in sexually reproducing organisms. -Independent assortment of homologous- During Metaphase I, tetradsof homologues chromosomes seperate into chromosomes that go to opposite poles. Which chromosome goes to which pole depends upon the orientation of a tetrad at the metaphase plate. This orientation and subsequent seperation is random for each tetrad. For some chromosome pairs, the chromosome that is mostly maternal maygo to one pole, but for another pair, the marternal chromosome may go to the other pole. -Crossing over- During Prophase I, nonsister chromatids of homologues chromosomes exchange pieces of genetic material. As a result, each homologue no longer entirely represents a single parents. -Random fertilization- Which sperm fertilizes which egg is to a large degree a random event. In many cases, howver, this event may be affected by the genetic composition and of a gamete. For example, some sperm may be faster swimmers and have a better chance of fertilization of the egg. Chapter 14 – Mendel and the Gene Idea State, in your own words, Mendel's law of segregation. Mendel’s law of segregation mean that there is one recessive allele and one dominant allele. Use a Punnett square to predict the results of a monohybrid cross and state the phenotypic and genotypic ratios of the F2 generation. Distinguish between gemotype and phenotype; heterozygous and homozygous; dominat and recessive. Genotype-means the ratio of the genetic make up when two alleles are crossed Phenotype- is the ratio of the outcome of the genes. Heterozygous-means that there is a combination of a dominant and recessive allele. Homozygous- means that there are either two dominants or two recessive alleles. Dominant- the alleles that controls. Recessive- the allele that will show later and its hidden trait. Explain how a testcross can be used to determine if a dominant phenotype is homozygous or heterozygous. The testcross is designed to reveal the genotype of an organism that exhibits the dominant trait. Define random event, and explain why it is significant that allele segregation during meiosis and fusion of gametes at fertilization are random events. The random event causes genetic variation and the differentiates the traits of the population. State, in their own words, Mendel’s law of assortment The law of assortment is that in a dihybrid cross each characteristics is by itself. Use a Punnett square to predict the results of a dihybrid cross and state the phenotypic and genotypic ratios of the F2 generation. Give an example of incomplete dominance and explain why it is not evidence for the blending theory of inheritance. The crossing of snapdragons. Explain how the phenotypic expression of the heterozygote is affected by complete dominance, incomplete dominance and codominance. Define and give examples of pleiotropy. Pleiotrophy is the ability of a gene to affect an organism in many way, a good example of this allele that are responsible for certain hereditary diseases in humans. Explain, in their own words, what is meant by "one gene is epistatic to another." This means that one gene alters the seconf gene at the same locus in the other chromatid. Describe the inheritance of the ABO blood system and explain why the IA and IB alleles are said to be codominant. The ABO blood system depends on the carbohydrates that is in the blood. The A and B are codominant because they form four kinds of blood types. Chapter 15 – The Chromosomal Basis Of Inheritance Define linkage and explain why linkage interferes with independent assortment. Linked genes do not assort independently because they are located on the same chromosomes and tend to move together through meiosis and fertilization Explain how crossing over can unlink genes. Describe sex determination in humans. What determines the sex is the male who carries the X and Y chromosome in the sperm. Describe the inheritance of a sex-linked gene such as color-blindness. This occurs when there is a genetic disorder. Explain why a recessive sex-linked gene is always expressed in human males. Because females carry one of the two X-chromosomes in each randomly inactivity during the early embryonic development. Distinguish among nondisjunction, aneuploidy, and polyploidy; explain how these major chromosomal changes occur and describe the consequences. Nondisjunction- when members of a pair chromosomes don’t move properly during meiosis I Aneuploidy- when the offspring has an abnormal number of chromosomes. Polypliody- when an organism has more than two complete chromosome sets. Distinguish among deletions, duplications, translocations, and inversions. See Figure 14.12