BIL 250 test 1 study guide
BIL 250 test 1 study guide BIL 250
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This 16 page Study Guide was uploaded by Annmarie Jaghab on Sunday January 17, 2016. The Study Guide belongs to BIL 250 at University of Miami taught by Dr. Alvarez in Winter 2016. Since its upload, it has received 64 views. For similar materials see Genetics in Biology at University of Miami.
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BIL 250 Test 1 Study Guide Chapter 1 -preformation theory: Homunculus present which is a sperm containing a miniature fully formed adult -theory of epigenesis: body organs not initially present in the embryo and form later -The cell theory: all cells come from pre-existing cells and all organisms are made of cells -Darwin’s Origin of Species: existing species came from ancestral species and natural selection kept favorable traits in the population. -DNAGenechromosomegenome -alleles are the source of genetic varation -genotype: set of alleles for a given trait -phenotype: the expression of the genotype which is observable -research of Avery, MacLeod, and McCarty showed that DNA is the carrier of genetic information, not protein -mitochondrial DNA contains 37 genes and is located outside of the nucleus. There are over a dozen genetic disorders associated with mtDNA -chloroplasts also have own DNA -proteins are the end product of gene expression and are responsible for phenotype -lacking a functional protein is what makes you sick (not having a mutated gene) -proteins are diverse; 20 different amino acids and numerous combinations -restriction enzymes cut viral DNA at specific sites and allowed for recombinant DNA and cloning -genetics and biotechnology are used in: 1) health care: gene therapy in prenatal diagnosis and testing for heritable disorders, gene editing, drugs and treatments such as recombinant insulin 2) industry 3) agriculture (increased herbicide, insect and viral resistance, nutritional enhancement, water use reduction) 4) court system -Genomics: studies structure, function, and evolution of genes and genomes -Proteomics: identifies a set of proteins present in cell under a given set of conditions. Studies their functions and interactions -Bioinformatics: uses hardware and software for processing nucleotide and protein data -only 1.5% of our DNA encodes protein -DNA sequences comprise the exome -genes that are known to cause disorders are in a database (OMIM) -annotation: the ongoing effort to understand what individual genes do -genomes come in different sizes and vary among species and is not related to how simple or complex an organism is -humans are 98% similar to chimpanzees -forward/classical genetics: rely on natural occurring mutations or intentionally induce them to produce a phonotype and then identify the gene involved -Reverse genetics: DNA sequence of a particular gene of interest is known, but its function is not. The gene is rendered nonfunctional to investigate the possible role (knockout). Gene knockdown reduces gene expression, but some gene activity may still remain. Genome editing is done with CRISPR-Cas9 Technology -model organisms are: easy to grow, have a short life cycle, abundant offspring, amendable genetic manipulation -zebrafish as an animal model: easy to maintain and breed in large numbers, become sexually mature after 3 months, each mating pair generates around 200 offspring per week, eggs fertilize and develop externally into transparent embryos, embryonic development is rapid and takes only a few days. Share 84% of genes with humans. Can be used to study human diseases such as cancer, regeneration and drug discovery, hereditary muscle diseases, neurological disorders, diabetes, CVD, blood disorders, and vision disorders -bioethics: address moral issues and controversies. Concerns are present that arise from new genetic technology. (stem cells, gene editing, knowledge of person’s genome in work force or heath care, cloning, selection of gene characters, social impacts of biotechnology, xenotransplantation) Chapter 2 -mitosis: results in two cells each with the same number of chromosomes as the parent cell (2n) -meiosis: reduces the amount of genetic material by half. Haploid gametes or spores (n) are made each with one member of a homologus pair of chromosomes -homologs: one of maternal and one of paternal origin. Identical in size, position of centromere, and gene loci, but NOT identical in terms of DNA sequence -sister chromatids: identical in DNA sequence due to DNA replication -production of haploid gametes occurs in anaphase I of meiosis and is the basis for Mendel’s 1 law, the law of segregation -meiosis produces genetic variation via: 1) crossing over: during prophase I, homologous chromosomes snyapsis and this is the basis for linkage maping 2) Independent assortment/Mendel’s second law: during anaphase I of meiosis unique combinations of maternal and paternal chromosomes result -Meiosis begins with a diploid cell (DNA is duplicated during interphase and chromosomes are made up of sister chromatids) -Meiosis is similar to mitotic prophase except homologous chromosomes pair up (synapsis) -meiosis I and II both have prophase, metaphase, anaphase, and telophase -DNA synthesis occurs during interphase before the beginning of meiosis I. It does not occur again before meiosis II -Meiosis occurs in two divisons: 1) Reductional division (Meiosis I): tetrads become dyads 2) Equational division (Meiosis II): dyads become monads Tetrad: the four chromatids that make up paired homologs in the prophase of the first meiotic division. In eukaryotes with a pre- dominant haploid stage (some algae and fungi), a tetrad denotes the four haploid cells produced by a single meiotic division. Dyad: The products of tetrad separation of disjunction at meiotic prophase I. Each dyad consists of two sister chromatids joined at the centromere. Monad: consists of one chromosome each. During equational division, each dyad splits into two monads. Synapse: when homologus chromosomes form pairs. Chiama (pl. chiasmata): the crossed strands of non sister chromatids seen in diplotene of the first meiotic division. Regarded as the cytological evidence for exchange of chromosomal material or crossing over. Equational division: A division stage where the number of centromeres is not reduced by half but where each chromosome is split into longitudinal halves that are distributed into two daughter nuclei. Chromosome division in mitosis and the second meiotic division are examples of equational divisions. Reductional division: the chromosome division that halves the number of centromeres and thus reduces the chromosome number by half. The first division of meiosis is a reductional division producing haploid cells. -synapsis gives rise to a tetrad (two pairs of sister chromatids) with overlapping nonsister chromatids (chiasma) -exchange of genetic material (parental/maternal chromosomes) through recombination (crossing over) -nuclear envelope and nucleolus break down and the two centromeres of the tetrad attach to the spindle fibers -Prophase I: chromatin thickens and coils into visible chomosomes. Chromosomes condense and homolgs pair -Sub-stages of prophase I: 1) Leptonema: chromomeres present. Chromosomes appear as long, single threads, unassociated with each other 2) Zygonema: bivalent form. Synapsis. Each pair of homologous chromosomes is known as bivalent 3) Pachynema: tetrad form. Each bivalent becomes shorter, thicker, and splits into two sister chromatids, called tetrads. Crossing over occurs, exchange of genetic material between nonsister chromatids 4) Diplonema: chiasma, chromatids are intertwined. Within tetrads, sister chromatids separate 5) Diakinesis: terminalization. Nucleus and nuclear envelope break down and centromeres attach to spindle fibers -Metaphase I: the alignment of chromosomes in the metaphase I plate will dictate the final outcome of meiosis (independent assortment) -Anaphase I: Cohesin is degraded between sister chromatids and homologous chromosomes separate. -Non-disjunction may occur during anaphase I. These errors lead to gametes with an incorrect number of chromosomes (+ or – 1). Ex. Down syndrome has one extra chromosome in pair 21 and is caused by a gamete wrongly carrying 24 chromosomes -Telophase I: two haploid daughter cells are formed after cytokinesis occurs -during anaphase II sister chromatids are separated to opposite poles. Each haploid daughter cell from meiosis II has one member of each pair of homologous chromosomes n -independent assortment during metaphase I results in 2 possible combinations (2 23in humans, 8 million) -ganetogenesis occurs as part of sexual reproduction -the development of gametes varies between spermatogenesis and oogenesis -male gametes are made via spermatogenesis in the testes. - Spermatogonium Primary spermatocyte (2n) undergoes meiosis I 2 secondary spermatocytes which undergo meiosis 2 4 haploid spermatidsspermatozoa/sperm cells -female gametes are produced by oogenesis in the ovary -cytosolic division in oogenesis is asymmetrical and only one daughter cell gets most of the cytoplasm. The first meiotic division starts in the embryonic ovary and is arrested in prophase I, then meiosis I resumes during puberty once a month but is halted at metaphase II. Meiosis II is completed after fertilization occurs and the second polar body is formed. Polar bodies eventually degenerate. -fungi have haploid vegetative cells that arise via meiosis and proliferate via mitotic cell division -plants have a life cycle called alternation of generations which alternates between diploid sporophyte stage and haploid gametophyte stage. Meiosis produces haploid spores that form haploid gametes. Meiosis and fertilization are vital bridge to both stages. In flowering plants, the sporophyte is the dominant stage. -two types of cells: 1) prokaryotic (bacteria, archaea): have a single circular chromosome attached to the cell membrane. Contain plasma membrane, cytoplasm, ribisomes, and DNA. Can have cell wall, pili and flagellum 2) Eukaryotic (protists, plants, fungi, animals): contain free-floating linear chromosomes within a nucleus -all cells share some common features including a plasma membrane, DNA, and ribosomes -plasma membrane: surrounds all cells, delimits cell from external environment -cell wall: present in plants (plants also have a plasma membrane). Composed of mainly cellulose, a polysaccharide -bacterial cells have peptidoglycan on their cell walls -glycocalyx: a covering on the plasma membrane of animal cells. Made of glycoproteins and polysaccharides -function of glycocalyx is biochemical identity at cell surface. Ex. ABO blood groups are carbohydrates from the glycocalyx on the surface of RBCs -nucleus: found in eukaryotes, membrane bound, houses genetic material, DNA (complex array of acidic and basic proteins into thin fibers) -nucleoid: found in prokaryotes, not membrane bound -cytoplasm: includes extranuclear cellular organelles -cytosol: colloidal material surrounding organelles -cytoskeleton: maintains shape, facilitates mobility, and anchors organelles, organizes and moves cell components, targets of many drugs such as cancer drugs. Made of an extensive system of tubules and filaments. Microtubules are made of tubulin and microfilaments are made of actin. Intermediate filaments are made of different proteins. -motile cilia move secretions such as mucus in the respiratory tract -flagellum: made of microtubules. Function is cell movement. Typically, cells possess one or two long flagella -microtubules make the spindle fibers for chromosome movement during cell division -Endoplasmic reticulum: an organelle that compartmentalizes cytoplasm. Increases surface area for biochemical synthesis. -SER: appears smooth in places due to the lack of ribosomes. Site of fatty acid and phospholipid synthesis -RER: rough ER is studded with ribosomes and is the site of protein synthesis -mitochondria: animals and plant cells have them. Sites of oxidative phases of cellular respiration which generate ATP -chloroplasts: plants, algae, and protozoans. Photosynthesis occurs there -both mitochondria and chloroplasts have their own DNA similar to prokaryotic DNA -endosymbiotic theory: mitochondria and chloroplasts were once primitive free living organisms that established a symbiotic relationship with a primitive eukaryotic cell -centrioles: found in centrosome of animal and plant cells. Organize spindle fibers (microtubules) for movement of chromosomes during meiosis and mitosis. Plants do not have centrioles but they have microtubule organizing centers (MTOCs) -karyotypes show chromosomes in homolgous pairs -centromeres: constricted regions on chromosomes and the location of the centromere establishes whether the appearance is: 1) Metacentric: middle 2) Submetacentric: between middle and end, short p arm and long q arm 3) Acrocentric: close to end, very short p arm and long q arm 4) Telocentric: end, two q arms present -sex-determining chromosomes are not truly homologous. They behave as homologs during meiosis -mitosis serves as the basis for asexual reproduction for many single- celled organisms such as protozoans and some fungi and algae -mitosis serves as the basis for asexual reproduction for many single celled organisms such as protozoans and some fungi and algae -in multicellular organisms, mitosis is responsible for wound healing, cell replacement, and growth -in mitosis the genetic material is evenly divided into two nuclei (karyokinesis) which is followed by cytoplasmic division (cytokinesis). Together, karyokinesis and cytokinesis comprise cellular division -The cell cycle: G1G0G1S (Interphase)G2M (PMAT) -the average red blood cell is in circulation only for about 6 weeks and then must be replaced -cells lining the intestines are replaced about every 3 weeks -hair follicles, too are among the most rapidly dividing cells -in cancer, mitosis is out of control -interphase includes the S phase, G1, and G2 -G0 is the point in G1 phase where cells are non-dividing but a metabolically active state -G1/S checkpoint: monitors size cell has achieved and evaluates condition of DNA -S phase is where DNA replication occurs -G2/M checkpoint: monitors if DNA replication is complete and monitors damaged DNA -M checkpoint: monitors successful formation of spindle fiber system and attachment to kinetochores. -interphase: chromosomes are extended and uncoiled forming chromatin -prophase: chromosomes coil up and condense to become visible and centrioles divide and move apart, poles are established, nuclear envelope breaks down -prometaphase: chromosomes are clearly double structures, period of chromosome movement to equatorial plane of cell. Centrioles reach poles. Spindle fibers form and equatorial plane is referred to as a metaphase plate -metaphase: centromeres begin to align on the metaphase plate. Chromosome configuration following migration -microtubules (spindle fibers) attach to kinetochores, proteins structures that assemble at the centromeres -cohesin: protein complex that holds sister chromatids together -separase: enzyme that degrades cohesin -anaphase: centromeres split and daughter chromosomes migrate to opposite poles. Period of disjunction. Sister chromatids separate and separase degrades cohesin (now the sister chromatids have turned into daughter chromosomes) -telophase: daughter chromosomes arrive at the poles and cytokinesis commences, uncoiling of chromosomes, reformation of nuclear envelope, and spindle fiber disappears -contractile ring: two sets of protein filaments that help bring about cytokinesis in animal cells -plants have the same steps in mitosis as animal cells but different cytokinesis due to the presence of a cell wall and a plasma membrane that runs roughly down the middle of the parental cell. Membrane-lined vesicles accumulate near the metaphase plate, these vesicles contain precursors to the cell wall vesicles fuse together forming a cell plate that grows toward the parent cell wallthe newly formed plasma membrane and cell wall fuse with the parent plasma membrane and cell wall forming two distinct daughter cells Chapter 3 -mendel started his research using garden peas (Pisum sativum) -genes are the hereditary units found on chromosomes -pea plants were chosen as a model organism because they are easy to grow, have true breeding strains, has controlled matings (self- fertilization or cross fertilization), grows to maturity in one season, has observable characteristics with two distinct forms, traits used were derived from “unlinked genes” -using 7 visible features, each with two contrasting traits, and true breeding strains, Mendel kept accurate, quantitative records of his experiments -the monohybrid cross reveals how one trait is transmitted from generation to generation -monohybrid crosses involve crossing a single pair of contrasting traits -the original parents are the P1 generation and their offspring are the F1 and self-fertilization of the F1 offspring results in the F2 offspring -in a monohybrid cross, the F1 generation was identical to one of the parents -in the F2 generation, ¾ of the plants exhibit the same trait as the F1 generation and ¼ exhibit the contrasting trait that disappeared in the F1 generation -in the F1 and F2 generation, the results were identical regardless of which is the pollen parent and which served as the source of the ovum (reciprocal cross) -a monohybrid cross leads to an F2 with a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio -Mendel’s postulates: 1) unit factors in pairs: genetic characters are controlled by unit factors existing in pairs in individual organisms 2) dominance: in the pair of unlike unit factors for a single characteristic in an individual, one unit factor is dominant and the other is recessive st 3) segregation (1 law): the paired unit factors segregate (separate) independently during gamete formation 4) (2ndlaw) during gamete formation, segregating pairs of unit factors assort independently of each other (at anaphase I of meiosis). All possible combinations of chromatids in the gametes will form with equal frequency (for each homolog pair there is a 50/50 chance of a maternal or paternal version) -wild type: a strain, gene, or characteristic that prevails among individuals in natural conditions, as distinct from an atypical mutant type -punnett square: a visual representation of Mendelian Genetics. Two parents can produce the offspring in the square at all equal proportions. Sperm alleles on top and egg on the side. The insides of the squares are all the possible matings of children -in the F2 of a monohybrid cross, Mendel says phenotypic ratio of 3:1 (4 parts total) dominant to recessive and genotypic ratio of 1:2:1 (4 parts total) homozygous dominant to heterozygous to homozygous recessive -testcross: a way to determine whether an individual displaying the dominant phenotype is homozygous or heterozygous for that trait. A homozygous recessive tester is used. -the dihybrid cross: Mendel studied the inheritance of two traits simultaneously. A cross involving two pairs of contrasting traits which generates a unique F2 ratio. Phenotypic ratio is 9:3:3:1 -the frequencies of all possible F2 phenotypes can be calculated by applying the product law of probabilities. When two independent events occur simultaneously the combined probability of the two outcomes is equal to the product of their individual probabilities of occurrence. -trihybrid cross: Mendel demonstrated that the identical processes of segregation and independent assortment apply to the three pairs of contrasting traits in a three factor cross. The results of the cross are easily calculated if the principles of segregation and independent assortment are followed -the forked-line method is easier than a punnett square for analysis of inheritance of a larger number of traits. This method uses the simple application of the laws of probability established for a dihybrid cross. The forked-line method can be used to solve crosses involving any number of gene pairs providing that all gene pairs assort independently of each other -genetic ratios, expressed as probabilities predict the outcome of each fertilization event (range from 0 [no occurrence] to 1 [event will occur]) -the probability of two independent events occurring at the same time can be calculated using the product law. The probability of both events occurring is the product of the probability of each individual event. -to determine how many gametes can be formed by a certain genotype, yn count the heterozygous gene pairs involved which is n and then 2 possible gametes are formed. -chromosomal theory of inheritance: genetic material in living organisms contained in chromosomes. Separation of chromosomes during meiosis served as basis for Mendel’s principles of segregation and independent assortment which leads to extensive genetic variation -unit factors exist in pairs: alleles in homologous chromosomes. First meiotic prophase -segregation of unit factors during gamete formation: separation of homologs during first meiotic anaphase -independent assortment of segregating unit factors: random placement of homologs in the metaphase plate. First meiotic metaphase -if genes are close together the 9:3:3:1 F2 ratios would change because alleles would co-segregate and not show independent assortment (linkage present) -independent assortment leads to extensive genetic diversity where h the number of different possible 23metes is 2 where h is the haploid number. So for humans h=23, 2 possible combinations and then raise that to the second power to find the possible combinations in the zygote -chi square analysis evaluates the influence of chance on genetic data. -the outcomes of independent assortment and fertilization are subject to random fluctuations from their predicted occurrences as a result of chance deviation. As sample size increases, the average deviation from the expected results decreases. A larger sample size diminishes the impact of chance deviation on the final outcome -when we assume that data will fit a given ratio we establish a null hypothesis that assumes that there is no real difference between the measured values/observed (or ratio) and the predicted values/expected (or ratio) -apparent difference is attributed purely to chance -chi square analysis is used to test how well the data fit the null hypothesis -chi square analysis involves that the degrees of freedom (df) be taken into account because the greater number of categories, the more deviation is expected as a result of chance. The degrees of freedom is equal to n-1 where n is the number of different categories into which each data point may fall -when p is less than .05 you reject the null hypothesis -p value can be viewed as a percentage which indicates that if the same experiment were repeated many times, that percent of the trials would be expected to exhibit chance deviation as great or greater than seen in the initial trial. -alleles are alternative forms of the same gene -the wild-type allele occurs more frequently and is often yet not always dominant -autosomes: all chromosomes that are not the sex chromosomes. Humans have 22 pairs -albinism: autosomal recessive. Mutation in melanin synthesis pathway (pigment in eyes, skin, and hair). Muations in the gene that encodes for the enzyme tyrosinase is the cause. -sickle cell anemia: autosomal recessive. Individual must be homozygous for the sickle cell allele to suffer from sickle cell. Wild type hemoglobin is A and mutant is S. being a carrier for hemoglobin S confers resistance to malaria. The presence of the malaria parasite causes the RBC with defective hemoglobin to rupture prematurely making the Plasmodium parasite unable to reproduce. Caused by a point mutation in the B-globin gene -Familial Hypercholesterolemia: autosomal dominant. Mutation in LDL receptor. Some LDLR mutations reduce the LDL receptors produced within cells and others disrupt the receptor’s ability to remove LDL’s from the bloodstream. As a result people have high cholesterol which is deposited abnormally in tissues -Huntington’s disease: autosomal dominant. Progressive brain disorder that causes uncontrolled movements, emotional problems, and loss of thinking ability (cognition). 50% probability of inheriting the disorder. Mutations in the HTT gene that make the protein huntingtin for nerve cells in brain -tongue rolling: autosomal dominant. -hanging earlobe: autosomal dominant. -pedigrees are family trees that show patterns of inheritance. Females are circles and males are squares and unknown sex is diamond. Generations are in roman numerals. Parents are connected by a single horizontal line and vertical lines lead to offspring. If parents are related such as first cousins they are connected by a double line and offsprings called sibs (siblings) are connected by a horizontal sibship line in order of birth left to right. Shading shows expression of phenotype. Heterozygous individuals that don’t express the trait but are known carriers have a dot. -proband is whoever seeks pedigree information -twins have diagonal lines stemming from a vertical line connected to sibship line. If identical the diagonal lines are linked by a horizontal line. Fraternal dizygotic twins lack this line. Deceased have a diagnonal line through them -poly-genic: caused by multiple genes Chapter 4 -Law of Segregation: Allele pairs separate randomly, or segregate, from each other during the production of gametes egg and sperm. Because allele pairs separate during gamete production, a sperm or egg carries only one allele for each inherited trait. –Law of Independent Assortment: each pair of alleles segregates independently of the other pairs of alleles during gamete formation. -Non-Mendelian Genetics: Explanations of ratios that did not conform to the expected Mendelian ratios. Modifications of the 9:3:3:1 -The expression of a trait can vary depending on the overall environment in which a gene, a cell, or an organism is exposed to. -Extranuclear inheritance involves DNA within organelles influencing an organism’s phenotype: mitochondria and chloroplasts -New phenotypes (due to mutations) result from changes in functional activity of the cellular product specified by that gene. –Loss-of-function mutation: Mutation may change overall enzyme shape and thus reduce/ eliminate affinity for substrate. -Null allele: Mutation may cause a complete loss of function. –Gain-of-function mutations: Some mutations may enhance allelic function. Usually increases quantity of gene product by affecting regulation of transcription of the gene. -Neutral mutation: Some mutations do not show any change in function -mutant alleles: Alleles that have been altered by mutation. These tend to be rare in natural populations. They are likely to cause a reduction in the amount or function of the encoded protein. Such mutant alleles are often inherited in a recessive fashion -In Drosophila, an initial letter is used. Ebony mutant phenotype is indicated by e. + –N+rm+l gray (wild-type) is indicated by e . –e /e : gray homozygote (wild type) –e /e: gray heterozygote (wild type) –e/e: ebony homozygote (mutant) –The names of genes are italicized and the proteins are not. -incomplete dominance: cross between parents with contrasting traits may generate offspring with an intermediate phenotype in the heterozygote. Ex. Pink flowers in snapdragons that are white and red -Tay Sach’s disease: incomplete dominance. Homozygous recessives die from fatal lipid-storage disorder, when hexosaminidase A activity is absent. Heterozygotes that have only a single copy of the mutant gene are phenotypically normal but have only 50 percent of the enzyme activity found in a homozygous normal individual. This enzyme level is adequate to achieve normal biochemical function. -Codominance: The joint expression of both alleles in a heterozygote. Two alleles at a locus produce different and detectable gene products in the heterozygote. No dominance or recessiveness. Example: MN blood group in humans ( L and L locus in chromosome 4 glycoprotein found on the surface of red blood cells) –For any specific gene, there can be more than two alleles within members of a population. Multiple alleles refers to three or more alleles of the gene. Multiple alleles can only be studied in populations, not individuals. Ex. ABO blood group -Blood type: A and B antigens are present on the surface of blood cells which are distinct from the MN antigens. A and B antigens (glycolipids) controlled by gene on chromosome 9. Precursor molecule to the sugar: H substance. The ABO system exhibits the codominant mode of inheritance. -The biochemical basis of the ABO blood groups: The wild-type FUT1 allele, present in almost all humans, directs the conversion of a precursor molecule to the H substance by adding a molecule of fucose A B to it. The I and I alleles are then able to Oirect the addition of terminal sugar residues to the H substance. The I allele is unable to direct either of these terminal additions. Failure to produce the H substance results in the Bombay phenotype, in which individuals are type O A B regardless of the presence of an I or I allele. Gal: galactose; AcGluNH: N- cetylglucosamine; AcGalNH: N-acetylgalactosamine. -bombay phenotype: A woman typed as blood type O, but… one parent B was type AB blood and she passed the I allele to her children. The H substance (the precursor to A/B antigens) was incompletely formed. No antigens were formed. Type O was expressed. Gene designated FUT1 (encoding an enzyme, fucosyl transferase). Even with I , I alleles, no A or B antigens were added to cell surface. Failure to produce the H substance results in the rare Bombay phenotype, in which individuals are type O regardless of the presence of an I or I allele. -A lethal allele is one that has the potential to cause the death of an organism. These alleles are typically the result of mutations in essential genes, inherited in a recessive manner. ex. Mouse A allele: yellow coat. Homozygous recessive they die but with only one copy the Y yellow coat is present and dominant. A is caused by a deletion, the deletion actually affects neighboring essential gene (Merc) responsible for development of embryo -dominant lethal alleles only need one copy to result in death. Ex. Human H allele causing Huntington disease -Essential genes are those that are absolutely required for survival. Mutations resulting in the synthesis of a nonfunctional protein can often be tolerated in the heterozygote. -How do alleles become lethal? Many lethal alleles prevent cell division so they will kill an organism at an early age. Ex. Yellow color coat in mice -Some lethal alleles exert their effect later in life Ex. Huntington disease. In the heterozygote, the onset of the disease is delayed well into adulthood. Affected individuals undergo gradual nervous and motor degeneration and dementia, until they die. Typical onset is about 40 years of age, by which time the affected individual has already had a family. Each child has a 50 percent chance of inheriting the disorder. For dominant lethal alleles to exist, the affected individual must reproduce before dying -When two modes of inheritance occur together, Mendel’s principle of independent assortment applies as long as the genes are not linked on the same chromosome. A mating between two humans both of whom are heterozygous for albinism -Phenotypic characters are influenced by many different genes and their products -Gene interaction: Several genes influence a particular characteristic or phenotype Ex. Deafness is a phenotype influenced by many genes -epistasis: expression of gene or gene pairs masks or modifies the expression of another gene or gene pair. Sometimes the effect is antagonistic (masking). –Homozygous presence of a recessive allele prevents (epistatic) or overrides the expression of alleles at another locus (hypostatic). Sometimes the effect is complementary or cooperative. –A single dominant allele at the first locus influences the expression or alleles at a second gene locus. Two gene pairs complement one another wherein one dominant allele is required at each locus for phenotype expression. -The Bombay phenotype is an example of the homozygous recessive condition at one locus masking the expression at a second locus. The mutant form of the FUT1 gene masks the expression of I and I alleles. Only individuals having at least 1 copy of the FUT1 wild type gene can make the A or B antigen. -Recessive epistasis ex. B allele: black pigment, A allele: agouti phenotype, aa genotype: all black, bb genotype: no black pigment, even if A or a alleles present Mouse is albino. bb genotype MASKS expression of A allele recessive epistasis -Dominant epistasis: Dominant allele at one loci masks an allele at second loci. Ex. Summer squash fruit color. Dominant allele A White fruit regardless of second loci allele. Absence of A allele Yellow fruit. Genotypes aa, BB, Bb yellow fruit Genotype bb green -many ratios are possible, but the common ones: -15:1This occurs with redundant (duplicate) function genes. either gene product can get the job done in a pathway – gene A OR B carries out same step, requiring either genes to get a WT phenotype -9:3:4Classic epistasis of “upstream” gene in a pathway, sometimes called recessive epistasis. i.e. both genes needed in sequential different steps -9:7 Complimentary gene interaction. Need both genes to get to a phenotype and can be in sequential pathway or at same step requiring both genes to get WT phenotype. Gene A and B required -Complementation analysis allows the determination of whether two mutations yielding similar phenotypes are on the same gene or on separate genes. Two mutant strains are crossed, and the F1 are analyzed. -Pleiotropy occurs when expression of a single gene (or mutation) has multiple phenotypic effects, and it is quite common. Ex. Marfan syndrome: Autosomal dominant mutation in a single gene encoding the connective tissue protein fibrillin, Effects over many tissues: lens, blood vessels, bones, etc -ex of pleiotropy: Porphyria variegata, Autosomal dominant disorder, Cannot metabolize porphyrin (part of the heme group of the hemoglobin), Deep red urine, Becomes toxic to the brain (also abdominal pain, muscular weakness, fever, insomnia, headaches, vision problems, delirium, etc.) -x linkage present in drosophila -The X,Y system is used for sex determination by many animal and some plant species. –X is a large chromosome and encodes many genes. –Y is a small chromosome with few genes (not homologous to X in the traditional sense but has a pairing region for synapsis during meiosis). –Males have a single copy of genes encoded by the X chromosome: hemizygous. –These genes have unique inheritance/expression properties resulting from their X-linkage. -Traits controlled by the X chromosome are X-linked traits. Ex. Red/green color blindness Ex. Hemophilia –Only females are carriers of recessive x linked alleles. -color blindness: inability to distinguish among certain shades of red and green The genes for both red- and green-absorbing pigments lie very close together on the X-chromosome. -Sex-limited inheritance occurs in cases where the expression of a specific phenotype is absolutely limited to one sex. Genes for breast milk production are only expressed in females. -In sex-influenced inheritance, the sex of an individual influences the expression of a phenotype that is not limited to one sex or the other. Ex. Horn formation in certain breeds of sheep. Ex. Pattern baldness in humans, Caused by an autosomal gene, Allele B behaves as dominant in males, but is recessive in females. Females inheriting BB genotype have a less pronounced genotype, expressed later in life. -Because the Y-chromosome is small and does not contain many genes, few traits are Y-linked, and Y-linked diseases are rare. Since the only humans who have a Y chromosome are males, Y-linked traits are passed only from father to son, with no interchromosomal genetic recombination. Y-linked chromosome deletions are a genetic cause of male infertility. -phenotypic expression of a trait may be influenced by environment as well as by genotype -Penetrance: the percentage of individuals that show at least some degree of expression of the mutant genotype in a population -Expressivity: the range of expression of the mutant phenotype -In some instances, a dominant allele is not expressed in a heterozygote individual. Ex Polydactyly: Autosomal dominant trait, Affected individuals have additional fingers and/or toes. A single copy of the polydactyly allele is usually sufficient to cause this condition. In some cases, however, individuals carry the dominant allele but do not exhibit the trait -the range of phenotypes is thought to be due to influences of the environment and or other genes -position effect – the physical location of a gene influences its expression (position relative to other genetic material) Ex. If chromosomal translocation occur (a region of a chromosome is rearranged), the normal expression of some of the genes may change due to the new “environment”. Such as euchromatin and heterochromatin -temperature effects: conditional mutation ex. Siamese cats and Himalayan rabbits have darker fur on cooler areas of body (tail, feet, ears) Enzymes lose catalytic function at higher temperature -nutritional mutations prevent a pheonotype from reflecting the genotype: Nutritional mutations Prevent synthesis of nutrient molecules, Phenotype expressed or not depending upon the diet. Ex.Phenylketonuria: Loss of enzyme to metabolize phenylalanine. Severe problems unless low phenylalanine diet Ex. Galactosemia and lactose intolerance
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