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University of Waterloo
BIOL 239
Christine Dupont

BIOL 139 Midterm Review Mendelian Genetics Artificial selection purposeful matings Phenotype = observable characteristic; genotype = genetic make up Monohybrid cross matings between individuals that differ in only one trait; Dihybrid cross matings between individuals that differ in two traits; multihybrid cross matings between individuals that differ in three of more traits Alleles are versions of a gene; a polymorphic gene is a gene that has several alleles that normally occur in a populations; a monomorphic gene is a gene that has only one allele that is normally present in a population. A maximum of two alleles can pre present in one gene of a diploid individual The law of product: the probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itself The law of sum: the probability of either of two mutually exclusive events occurring is the sum of their individual probabilities Gregor Mendel performed genetic crosses in a systematic way, using mathematics to analyze the data he obtained and to predict outcomes of other experiments. Mendels law of segregation states that alleles of genes separate during gamete formation and then come together randomly at fertilization. The Punnet square is one tool for analyzing allele behaviour in a cross. In the analysis of a genetic cross, the product rule multiplies probabilities to predict the chance of a particular fertilization event (if, then). The sum rule adds probabilities to predict the proportion of progeny that share a particular trait such as pea colour (either, or). The results of a test cross, in which an individual showing the dominant phenotype is crossed with an individual showing the recessive phenotype, indicate whether the individual with the dominant phenotype is a homozygote or heterozygote. The law of independent assortment states that the alleles of genes for different traits segregate independently of each other during gamete formations. Branched-line diagrams or direct calculations of probabilities are often more efficient methods than Punnet squares for the analysis of genetic crosses involving two or more genes. Modifications of Mendelian Ratios Incomplete dominance the F1 hybrid resembles neither pure bred parent (often an intermediate phenotype); codominance alternative traits are both visible in the F1 hybrid Allele frequency the percentage of the total number of copies of a gene in a population represented by a particular allele. Wild type allele greater than 1% in population. Mutant allele less than 1% in population Multifactorial inheritance arising from the action of two or more genes (polygenic), or from interactions between genes and the environment Complementary gene action two or more genes can work in tandem, in the same biochemical pathway to produce a particular trait (9:7) Heterogeneous trait a mutation at any one of a number of genes can give rise to the same phenotype Epistasis a gene interaction in which the effects of an allele at one gene hide the effects of alleles at another gene. Epistatic control over another gene. Recessive epistasis (9:3:4) and Dominant epistasis (12:3:1 or 13:3) Penetrance percentage of the population with a particular genotype that demonstrates the expected trait. Expressivity the degree or intensity with which a particular genotype is expressed in a phenotype within a population Sex linked traits due to genes on the x or y chromosome. Sex-limited traits affect a structure or process that is found in one sex but not the other. Sex-influenced traits show up in both sexes but their expression may differ between the two sexes Conditional lethality occurs with a particular type of allele which is lethal under only certain conditions. Permissive (allowing life) vs restrictive (restricting life) conditions In cases of incomplete dominance or codominance, mating of F hybr1ds produces an F 2 generation with a 1:2:1 phenotypic ratio. The reason is that heterozygotes have a phenotype different from that of either homozygote. Within a population, a gene may have multiple alleles, but any one individual can have at most two of these alleles. Considered in pairs, the alleles can exhibit a variety of dominance relationships. Genes and alleles can be classified according to allele frequencies. A monomorphic gene has a single common allele referred to as the wild-type allele; a polymorphic gene has several common variants. Rare or newly arisen alleles of any gene are mutant alleles. A mutant allele can disrupt many biochemical processes; as a result, mutations often have pleiotropic effects that can include lethality at various times in an organisms life cycle. F2phenotypic ratios of 9:3:3:1 or its derivatives indicate the combined action of two independently assorting genes. For heterogeneous traits caused by recessive alleles of two or more genes, a mating between affected individuals acts as a complementation test, revealing whether they carry mutations in the same gene or different genes. Because modifier genes, the environment, and chance events can affect phenotypes, the relationship of a particular genotype and its corresponding phenotype is not always absolute: An alleles penetrance can be incomplete, and its expressivity can be variable. The action of a handful of genes, combines with environmental effect, can produce an enormous range of phenotypic variation for a particular trait. Pedigree Analysis Autosomal one that is conferred by a gene residing on a chromosome not involved in sex determination. Sex-linked one that is conferred by a gene residing on the X or Y chromosome If an individual is affected by a rare dominant trait, the trait should also affect at least one of that persons parents, one of that persons grandparents, and so on. The idea that genes reside on chromosomes was verified by experiments involving sex-linked genes in Drosophila and by the analysis of pedigrees showing X-linked patterns of inheritance in humans. The chromosome theory provides a physical basis for understanding Mendels laws. Chromosomes Chromosomal theory of inheritance hereditary information is on genes and genes are located on chromosome. Egg and sperm contribute equally to the genetic endowment of offspring through their nuclei Mitosis nuclear division that results in 2 daughter cells each containing identical numbers of chromosomes to the parent cell. Meiosis nuclear division that results in each egg and sperm containing half the number of chromosomes found in other cells (somatic) Homologous chromosomes contain the dame genes, vary in alleles, pair at meiosis and are called homologs. Non-homologous chromosomes contain different genes and do not pair at meiosis Microscopic studies suggested that the nuclei of egg and sperm contribute equally to the offspring by providing a single set of n chromosomes. The zygote formed by the union of haploid gametes is diploid (2n). Karyotyping, the analysis of stained images of all the chromosomes in a cell, reveals that different species have different numbers and shapes of chromosomes. In many species, the sex of an individual correlates with a particular set of chromosomes termed the sex chromosomes. The segregation of the sex chromosomes during gamete formation and their random reunion at fertilization explains the 1:1 ratio of the two sexes. Mitosis Mitosis the process of nuclear division in cells that produces daughter cells that are genetically identical to each other (cloning cells) G1 cell grows and carries out normal metabolism and organelles duplicate. S DNA replication begins through semi-conservative replication and chromosomes duplicate. G2 cell grows and prepares for mitosis Prophase, metaphase, anaphase, telophase Centrosome microtubule organizing center, the core of which is seen as two darkly staining bodies in animal cells, known as centrioles Kinetochore site at which chromosome is attached to microtubules Cell division = cytokinesis The essence of mitosis is the arrangement of chromosomes at metaphase. The kinetochores of sister chromatids are connected to fibres from opposite spindle poles, but the sister chromatids remain held together by their connection at the centromere. After mitosis plus cytokinesis, the sister chromatids of every chromosome are separated into two daughter cells. As a result, these two cells are genetically identical to each other and to the original parent cell. Meiosis Somatic cells mitotically divide and make up the vast majority of an individuals tissues. Germ cells produce gametes (egg/sperm) and undergo meiosis Prophase stages leptotene, zygotene, pachyetene, diplotene, diakineses Centromere remains intact during meiosis Oogenesis egg formation in humans During prophase I, homologous chromosomes pairs, and recombination occurs between nonsister chromatids of the paired homologs. The essence of the fir
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