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BIOL 239
Christine Dupont

Genetics Midterm Textbook Notes Mendelian Genetics: pg. 40-48  The Principle of Dominance- in a heterozygote, one allele may conceal the presence of another  The Principle of Segregation- in a heterozygote, two different alleles segregate from each other during the formation of gametes  The Principle of Independent Assortment- The alleles of different genes segregate, or as we sometimes say, assort, independently of each other  Mendel studied the inheritance of 7 different traits in garden peas, each trait being controlled by a different gene Modifications of Mendelian Ratios: pg. 62-82  Incomplete dominance- expression of two alleles in a heterozygote that allows the heterozygote to be distinguished from either of its two parents  Codominant- alleles that produce independent effects when heterozygous  Polymorphic- the existence of two or more variants in a population of individuals, with at least two of the variants having frequencies greater than 1 percent  Amorphic- mutant allele that completely abolishes gene expression  Hypomorphic- mutant allele that has less expression than a wild-type allele but does not completely abolish expression  Genes often have multiple alleles  Mutant alleles may be dominant, recessive, incompletely dominant, or codominant  If a hybrid that inherited a recessive mutation from each of its parents has a mutant phenotype, then the recessive mutations are alleles of the same gene; if the hybrid has a wild phenotype, then the recessive mutations are alleles of different genes  Most genes are encoded polypeptides  In homozygous condition, recessive mutations often abolish or diminish polypeptide activity  Some dominant mutations produce a polypeptide that interferes with the activity of the polypeptide encoded by the wild-type allele of a gene  Epistasis- interactions between products of nonallelic genes. Genes suppressed are said to be hypostatic. Dominance is associated with members of allelic pairs, whereas epistasis results from interactions of the products of nonalleles  Pleiotropic- condition in which a single gene influences more than one trait  Gene action is affected by biological and physical factors in the environment  Two or more genes may influence a trait  A mutant allele is epistatic to a mutant allele of another gene if it has an overriding effect on the phenotype  A gene is pleiotropic if it influences many different phenotypes  Inbreeding increases the frequency of homozygotes and decreases the frequency of heterozygotes Pedigree Analysis: pg. 53-56  Pedigree- a table, chart, or diagram representing the ancestry of an individual  Albinism- absence of pigment (melanin) in skin, hair, and eyes of an animal. Absence of chlorophyll in plants  Pedigrees are used to identify dominant and recessive traits in human families  The analysis of pedigrees allows genetic counselors to asses the risk that an individual will inherit a particular trait Chromosomes: pg. 89-94, 100-103, 113, 119-123  Chromatin- the complex of DNA and proteins in eukaryotic chromosomes  Euchromatin- genetic material that is not stained so intensely by certain dyes during interphase and that comprises of many different kinds of genes  Heterochromatin- chromatin staining darkly even during interphase, often containing repetitive DNA with few genes  Sex chromosomes- chromosomes that are connected with the determination of sex  Autosomes- any chromosome that is not a sex chromosome  Individual chromosomes becomes visible during cell division; between divisions they form a diffuse network of fibers called chromatin  Diploid somatic cells have twice as many chromosomes as haploid gametes  Sex chromosomes are different between the two sexes, whereas autosomes are the same  Hemizygote- an individual that carries one copy of a chromosome or gene, as in sex linkage or as a result of deletion  Nondisjunction- failure of disjunction or separation of homologous chromosomes in mitosis or meiosis, resulting in too many chromosomes in some daughter cells and too few in others  Chromosome Theory of Heredity- the theory that chromosomes carry the genetic information and that their behaviour during meiosis provides the physical basis for the segregation and independent assortment of genes  Karyotype- the chromosome constitution of a cell or an individual; chromosomes arranged in order of length and according to position of the centromere  Aneuploidy- an organism or cell having a chromosome number that is not an exact multiple of the monoploid (n) with one genome, that is, hyperploid, higher (for example, 2n+1), or hypoploid, lower (for example, 2n-1)  Trisomy- an otherwise diploid cell or organism that has an extra chromosome of one pair (2n+1)  Down syndrome- the phenotype due to the presence of an extra chromosome 21 in humans  Klinefelter syndrome- a condition produced when two X chromosomes and one Y chromosome (XXY) are present in the human karyotype  Monosomy- a diploid cell or organism lacking one chromosome of its proper compliment (2n-1). A specific case of this condition is called a monosomy  Turner syndrome- the phenotype due to the XO genotype in humans Mitosis: pg. 23-26  Clone- all the individuals derived by vegetative propagation from a single original individual. A population of identical DNA molecules all carrying a particular DNA sequence from an organism  Fission- a mode of cell division among the prokaryotes in which the genetic material of the mother cell is first duplicated and then apportioned equally to the two daughter cells  Colony- a compact collection of cells produced by the division of a single progenitor cell  Mitosis- disjunction of duplicated chromosomes and division of the cytoplasm to produce two identically identical daughter cells  Cytokinesis- cytoplasmic division and other changes exclusive of nuclear division that are a part of mitosis or meiosis  Prokaryotic cells divide by fission; eukaryotic cells divide by mitosis and cytokinesis  Eukaryotic chromosomes duplicate when a cell’s DNA is synthesized; this event, which precedes mitosis, is characteristic of the S phase of the cell cycle  Chromatin- the complex of DNA and proteins in eukaryotic chromosomes  Interphase- the stage of the cell cycle when the cell is not dividing; the metabolic stage during which DNA replication occurs  Microtubules- hollow filaments in the cytoplasm making up a part of the locomotor apparatus of a motile cell; component of mitotic spindle  Spindle- system of microtubules that distributes duplicated chromosomes equally and exactly to each of the daughters of a dividing eukaryotic cell  Centrosomes- a barrel-shaped organelle associated with the mitotic spindle in animal cells  Centrioles- an organelle in many animal cells that appears to be involved in the formation of the spindle during mitosis  Prophase- the centriole divides and the two daughter centrioles move apart. Each sister DNA strand from interphase replication becomes coiled, and the chromosome is longitudinally double except in the region of the centromere. Each partially separated chromosome is called a chromatid  Kinetochores- a proteinaceous structure associated with the centromere of a chromosome during eukaryotic cell division; the point at which microtubules attach to move the chromosome through the division process  Metaphase- the stage of cell division in which the chromosomes are most discrete and arranged in an equatorial plate  Metaphase plate- the equatorial plane where duplicated chromosomes gather in a cell during the metaphase of mitosis  Anaphase- the stage of mitosis or meiosis during which the daughter chromosomes pass from the equatorial plate to the opposite poles of the cell (towards the end of the spindle)  Telophase- the last stage in which the chromosomes gather at the pole of the dividing cell and begin to decondense  As a cell enters mitosis, its duplicated chromosomes condense into rod-shaped bodies (prophase)  As mitosis progresses, the chromosomes migrate to the equatorial plane of the cell (metaphase)  Later in mitosis, the centromere that holds the sister chromatids of a duplicated chromosome Meiosis: pg. 27-32, 34-36, 95-99  Meiosis- the process by which the chromosome number of a reproductive cell becomes reduced to half the diploid (2n) or somatic number; results in the formation of gametes in animals or of spores in plants; important for variability  Homologous- chromosomes that occur in pairs and are generally similar in size and shape, one having come from the male parent and the other from the female parent. Contain the same array of genes  Heterologous- a chromosome that contains a different set of genes than the chromosome to which it is compared  See Meiosis sheet  Synapsis- the pairing of homologous chromosomes in the meiotic prophase  Tetrad- quadruple groups of chromatids formed by the association of duplicated homologous chromosomes during meiosis  Crossing over- a process in which chromosomes exchange genetic material through the breakage and reunion of their DNA molecules  During meiosis I, homologous chromosome pairs (synapse), exchange material (cross over), ad separate (disjoin) from each other  During meiosis II, chromatids disjoin from each other  Oogenesis- the formation of egg or ovum in animals  In mice and other mammals, one cell from female meiosis becomes the egg (other 3 polar bodies disintegrate), whereas all 4 cells from male meiosis become sperm  The disjunction of chromosomes during meiosis is responsible for the segregation and independent assortment of genes Changes in Chromosome Number: pg. 104-105, 111-112, 114-119, 124-125  Barr body- a condensed mass of chromatin found in the nuclei of placental mammals that contains one or more X chromosomes; named after Murray Barr  In Drosophila, dosage compensation for X-linked genes is achieved by hyperactivating the single X chromosome in males  In mammals, dosage compensation for X-linked genes is achieved by inactivating one of the two X chromosomes in females  Cytogenetics- area of biology concerned with chromosomes and their implications in genetics  Cytogenetic analysis usually focuses on chromosomes in dividing cells  Dyes such as quinacrine and Giemsa create banding patterns that are useful in identifying individual chromosomes within a cell  Polyploidy- an organism with more than two sets of chromosomes  Endomitosis- duplication of chromosomes without division of the nucleus, resulting in increased chromosome number within a cell. Chromosome strands separate, but the cell does not divide  Polyploids contains extra sets of chromosomes  Many polyploids are sterile because their multiple sets of chromosomes segregate irregularly in meiosis  Polyploids produced by chromosome doubling in interspecific hybrids may be fertile if their constituent genomes segregate independently  Duplication- the occurrence of a segment more than once in the same chromosome or genome Linkage and Recombination: pg. 48-52, 135-140, 150-153  Chi-square- a statistic used to test the goodness of fit of data to the predictions of a hypothesis  Critical value- the threshold value of a statistic that marks off a fraction of the statistics frequency distribution  Degrees of freedom- an index associated with the frequency distributed of a test statistic calculated from sample data  The chi-square statistic is X =  (observed number-expected number) /expected number, with the sum computed over all categories comprising the data  Each chi-square statistic is associated with an index, the degrees of freedom, which is equal to the number of data categories minus one  Linkage- the tendency of different genes to be inherited together because they are located on the same chromosome  Recombination- the production of gene combinations not found in the parents by the assortment of non-homologous chromosomes and crossing over between homologous chromosomes during meiosis (high frequencies do not yield accurate estimates)  Chiasma- visible change of partners in two of a group of four chromatids during the first meiotic prophase nd  Tetrad- the 4 cells arising from the 2 meiotic division in plants or fungi  Linkage between genes is detected as a deviation from expectations based on Mendel’s Principle of Independent Assortment  The frequency of recombination measures the intensity of linkage. In the absence of linkage, this frequency is 50%; for very tight linkage, it is close to 0  Recombination is
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