Biology 240 Exam 1 Chapter 2-5 Study guide!
DNA is organized into chromosomes and virtually all cells within the individual contains the same nuclear genetic material. Gene regulation determines gene expression.
In eubacteria such as E coli, genetic material is present as a long circular DNA molecule compacted in an unenclosed nucleoid. Extends to a large part of the cell and does not undergo extensive coiling, nor is it associated extensively with proteins. They do contain genes specifying rRNA.
The remainder of a eukaryote within the plasma is the cytoplasm, containing colloidal material called the cytosol, encompassing cellular organelles. Contain tubules and filaments, composing of cytoskeleton to provide support structures consisting of microtubules, which are composed of protein tubulin and microfilaments, derived from the protein actin. It maintains the shape and facilitates mobility for the cell, anchoring organelles. The endoplasmic reticulum compartmentalizes the cytoplasm, increasing surface area. The ER is smooth where fatty acids and phospholipids are synthesized; in others rough because of ribosomes.
Ribosomes are sites where genetic information in mRNA is translated into proteins. Mitochondria are found in most eukaryotes and are sites of oxidative phases of cell respiration. It generates ATP. Chloroplasts (mostly found in plants) are associated with photosynthesis. Both contain DNA in a form distinct from the nucleus; it is also capable of duplicating, transcribing and translating its own genetic information. They closely resemble prokarya. These organelles were once primitive free-living organisms that established a relationship with primitive eukaryaꟷendosymbiont hypothesis.
Centrioles, located in centrosomes, are associated with spindle fibers that function during cell division. The organization of spindle fibers by centrioles occurs early in mitosis/meiosis and plays a role in movement of chromosomes as they separate during cell division. They are composed of microtubules consisting of tubulin.
Don't forget about the age old question of What is a corporation?
Each chromosome contains a constricted region of the centromere which establishes the appearance of each chromosome. Don't forget about the age old question of What is an orientalism?
Somatic cells derived from the same species contain the same number of chromosomes, representing a diploid number (2n). With the exception of sex chromosomes, these exist in pairs; members of each pair being homologous chromosomes. There are exceptions like bacteria, viruses, certain plants, yeast, mold (for they live haploid for most of their life). Humans have 2n number of 46
chromosomes. Each of the 46 is a double structure consisting of 2 sister chromatids connected by a centromere. Don't forget about the age old question of What are the characteristics of emerging economies?
Haploid number (n) is equal to half of the diploid. The genetic information contained in the haploid set of chromosomes constitutes the genome, which includes a copy of all genes including noncoding ones.
Homologous chromosomes have identical gene sites, called loci (singular: locus). These are identical in traits and genetic potential. In sexual organisms, one member of each pair is derived from ovum and sperm. Each diploid organism contains two copies of each gene as bi-parental consequence/inheritance, which come from both parents. Members of each pair of genes can influence each pair of genes that are not identical. Alleles are different versions of the same gene.
During meiosis, the diploid number of chromosomes becomes haploid. Gametes or spores contain 1 member of each homolog pair-1 complete haploid set. Fusion re establishes the diploid set and becomes a zygote. Constancy of genetic material is thus maintained. In many species, one pair of homologs, sex-determining chromosomes are often not homologous in size, centromere placement, etc. (Ex: X and Y chromosomes). We also discuss several other topics like What is the purpose of the trial?
Multicellular diploid organisms begin life as single-celled fertilized eggs (zygotes). The mitotic activity of the zygote is the foundation for development and growth of the organism. In adults, mitotic activity is the basis for wound healing and cell replacement. Karyokinesis is the partition of genetic material into daughter cells during nuclear division. Nuclear division is very exact; results in the production of daughter nuclei with exact same chromosome composition. This is followed by cytokinesis where it parts and splits the plasma membrane. As the cytoplasm is reconstituted, organelles replicate themselves and arise from existing membrane structures or synthesized de novo in each cell.
The cell cycle is the continuous alternation between non-division and division. The events in between constitute the cell cycle: If you want to learn more check out What is hussein-mcmahon?
Interphase: the initial stage of the cell cycle, the interval between divisions and includes the replication of DNA in each chromosome.
S Phase: DNA synthesis before mitosis. G1∧G2 are gap phases in which no DNA is
synthesized. It includes the metabolic activity, cell growth and cell differentiation. By the end of G2 , the cell has doubled in size and DNA has been replicated. If you want to learn more check out What is the purpose of 4th ammendment?
Mitosis begins. During G1 cells either enter the G0 stage or proceed through G1 . Those in G0 do not proliferate.
Prophase: characterized by migration of centrioles to the ends of the cell outside the nuclear envelope. Centrosomes organize microtubules into spindle fibers, creating an axis on which the chromosome separation occurs. Fungi, plants, and some algae lack this. The nuclear envelope begins to break down and the nucleolus to disintegrate. Chromatin fibers begin to condense. Two parts of each chromosome are called sister chromatids because DNA in each is genetically identical. These are held together by cohesin. The molecular complex formed between and during the S Phase is when DNA is replicated.
The next stages are leg by migration of the chromosomes; which in turn are led by centromere regions to the equatorial plane/metaphase plateꟷperpendicular to the axis established by spindle fibers. Prometaphase is the period of chromosome movement and metaphase is applied to the chromosome configuration following immigration.
Migration is possible by the binding of the spindle fibers to the kinetochore, an assembly of multilayered proteins associated with centromeres. This forms on opposite sides of each paired centromere in association with sister chromatids. Once attached the cohesin is disintegrated by separase and sisters disjoin except at the centromere. The protein family of shugoshin protects cohesion from being degraded at the centromere.
Mutations of the kinetochore proteins can potentially lead to errors during chromosome migration along the diploid content of daughter cells. Kinetochore microtubules from spindle fibers lengthen and shorten as a result of addition or reduction of polarized tubulin subunits. They each have one end near the centrosome region and another at the kinetochore.
Anaphase is the event critical to chromosome distribution and the shortest stage of mitosis. Sister chromatids of each chromosome disjoin in disjunctionꟷpulled to opposite ends of the cell:
1. Shugoshin must degrade
2. Cohesin must be cleaved by separase
3. Chromatids must go to opposite end of the cell.
Each migrating chromatid is now a daughter chromosome. Movement is dependent on the kinetochore-spindle fiber attachment. Results from motor proteins. Molecular motors use energy generated by hydrolysis of ATP, and its effect shortens spindle fibers, drawing chromosomes to opposite ends.
Steps in anaphase are critical. In human cells, there would be 46 chromosomes at each pole of the cell, one from each sister pair.
Telophase: the final stage in mitosis. At the beginning, 2 complete sets of chromosomes are at each pole. The most significant event is cytokinesis, partitioning of the cytoplasm. In plant cells, the cell plate is synthesized and laid
down across the region of the metaphase plate. Animal cells undergo constriction of the cytoplasm, the cell plate in plants is laid down and becomes the middle lamella. Primary and secondary layers of the cell as are deposited between the membrane and m. lamella in each daughter cell. In animal cells, constriction produces a cell furrow. Chromosomes begin to uncoil, the nuclear envelope reforms, spindle fibers disperse, etc. The cell enters Interphase.
Cell division cycle (cdc) mutations: Normal products of many mutated genes are kinases that can add phosphates to other proteins. Some act as a “master control” molecules functioning with the protein cyclin. Cyclin binds to kinase activating at times during the cycle. Then it phosphorylates to other target proteins that regulate the process of the cell cycle. Cdc mutations help establish 3 checkpoints in the cycle:
G1/S checkpoint: monitors cell size and condition of DNA. If DNA is damaged or
growth is inadequate, progress is arrested. If adequate, the cell moves into Synthesis.
G2/ M checkpoint: DNA is monitored prior to mitosis. If replication is incomplete or if DNA is damaged, the process is arrested.
M checkpoint: Occurs during mitosis, successful spindle fiber formation and spindle fibers to kinetochores necessary.
Crossing over results in genetic exchange between members of each homologous pair of chromosomes. Sexual reproduction reshuffles genetic material; major source of recombination within species.
Early in meiosis, homologous chromosomes form pairs, synapsis. Each synapsed structure, bivalent, gives rise to tetrad, composed of 4 chromatids, demonstrating both homologs duplicated. First division in meiosis I is reduction division. Components of each tetrad separate into dyads, which are composed of 2 sister chromatids joinder at the centromere. Second division in meiosis II is equatorial division. Each dyad splits into 2 monads of once chromosome each. Both divisions produce 4 haploid cells.
Prophase I: same as mitosis except each homologous pair of chromosomes pair through synapsis, crossing over occurs. 5 substages: leptonemia, zygonema, pachynema, diplonema, diakinesis.
Leptonema: interphase; chromatin material condenses, chromosomes visible. Along each chromosome are chromomeres, localized condensations resembling string.
Homology search precedes and is initial pairing of homologs, begins during leptonema.
Zygonema: chromosomes continue to shorten and thicken. Rough pairing complete at end of zygonema. During meiosis, lateral elements along chromosomes increases and synaptonemal complex begins to form between homologs. It is the vehicle responsible for proper alignment during pairing of homologs. In diploids, synapsis acts in zipper-like fashion, beginning at ends. Upon completion, homologs are referred to as bivalents. The number of bivalents equals the haploid number.
Pachynema: chromosomes continue to coil and shorten, and the development of the synaptonemal complex continues between 2 members of each bivalent. This leads to synapsis, and referred to as a tetrad.
Diplonema: apparent tetrad has 2 pairs of sister chromatids. Each pair begins to separate, but some areas remain intertwined (chiasma) at a point of crossing over.
Diakinesis: chromosomes pull apart, but non-sister chromatids remain loosely associated at the chiasmata. The chiasmata move to end of the tetrad in separation.
Terminalization begins at later diplonema and completed in diakinesis, in the nucleolus and nucleus breakdown; two centromeres of each tetrad form spindles. By the end of prophase I, the centromeres are present on the metaphase plate.
Metaphase I: terminal chiasmata hold non-sister chromatids together, tetrad interacts with spindles, alignment of each tetrad is random. Half of the tetrad will be pulled to one end of the pole.
Anaphase I: cohesin is degraded between sister chromatids, except at centromere. . The dyad is then pulled to each pole, disjunction occurs. When separation is not achieved, nondisjunction occurs.
Telophase I: reveals nuclear membraned forming around the dyads. In this case, the nucleus enters a short interphase. If it occurs, the chromosomes do not replicate because they already have two chromatids. In others, they enter meiosis II. Much shorter than mitotic telophase.
Meiosis II: essential if each gamete/spore relieves one chromatid from each original tetrad.
Prophase II: each dyad has 1 pair of sister chromatids attached to the centromere. Metaphase II: centromeres positioned on metaphase plate.
Anaphase II: occurs when shugoshin is degraded and sister chromatids pull apart. Telophase II: results in 4 haploid gametes, each chromosome being a monad.
Spermatogenisis: takes place in testes, enlargement of an undifferentiated diploid germ cell-spermatogonium. Grow into a primary spermatocyte, which undergoes the first meiotic division. Products: secondary spermatocytes, which contain a haploid number of dyads. They undergo meiosis II for spermatids and spermiogenesis to become sperm. To become sperm, there must be equal amounts of cytoplasm when dividing.
Oogenesis: same process as spermatogenesis, but there are not equal amounts of cytoplasm in each cell. Most occurs in the primary oocyte that is derived from the oogonium which is concentration in 1 of the daughter cells. Necessary for nourishment of the embryo. In the first polar body of telophase I, most of the cytoplasm is in the secondary oocyte, which then divides to become an ootid and second polar body. These divisions may not be continuous like sperm. The second meiotic division of the secondary oocyte produces a mature ovum.
Transmission genetics: study of how genes are transmitted from parents to offspring. Derived directly from Mendel’s experimentation. 1856-Mendel performed first set of hybridization experiments with garden pea. Continued until 1868.
Traits: contrasting forms of an organism-characteristics are visible features. Mendel used these to compare garden peas in experimental fertilization.
Monohybrid cross: cross involving only one pair of contrasting traits. Done by mating true-breeding individuals from two parent strands, each exhibiting one out of two contrasting forms of character in study. First generation examined, ton of offspring selfing: self-fertilization.
Parental generation ( P1¿: original parents. First filial generation ( F1 ): offspring of Parental Generation. Second filial generation ( F2 ): offspring of selfing. Ex: breed between tail and dwarf plants.
Genetic data usually expressed/analyzed in ratios.
Reciprocal crosses: when patterns of inheritanceꟷfrom one trait (sperm) and the other (egg) are the same no matter who fertilized. Ex: results of Mendel’s peas are not sex-dependent.
To explain, unit factors for each trait are applied and serve as basic units of heredity unchanged from generation to generation.
1. Unit factors are in pairs: genetic characters are controlled by unit factors existing in pairs of individual organisms. In a monohybrid cross, the specific unit factor for each trait. Each diploid receives one from each parent. Ex: 2 for tall, 2 for dwarf, one of each.
2. Dominance/recessiveness: when two unlike unit factors responsible for a single character are present in an individual, one unit is dominant to the other. Traits that are not usually expressed are recessive. Ex: tall-D, dwarf-R.
3. Segregation: During formation of gametes, paired units separate/segregate randomly so one of each gamete receives with equal likelihood. Ex: probability for tall and dwarf traits.
Phenotype: physical expression of a trait.
Gene: units of inheritance (unit factors). For any given character (such as height), the phenotype is determined by the alternative forms of a single gene; alleles/
Genotype: alleles written in pairs to represent unit factors. Designates genetic makeup of an individual. By reading the genotype, we know the phenotype (dominant/recessive traits). When both alleles are the same, they are homozygous. When the alleles are different, they are heterozygous.
Test cross: method used to distinguish the genotype. Organism with a dominant phenotype but unknown genotype is crossed with a homozygous recessive individual.
Dihybrid cross: two factor cross that analyzes 2 characters simultaneously. Best thought of as 2 separate monohybrid crosses. Ex: Yellow and green seeds independent of round and wrinkled.
Product law of probabilities: when two independent events occur simultaneously, probability of two outcomes occurring in combination is equal to the product of their individual probabilities of occurrence. Ex: (3/4)(3/4)=9/16--- ¾ should be yellow, ¾ should be round.
Independent Assortment: during gamete formation. Segregating pairs of unit factors assort independently of each other. Stipulates segregation of any pair of units occur independent of others. As a result, each gamete receives 1 member of every pair of units. For 1 pair, factor does not influence segregation of others. All combinations
should be in equal frequency.
Mendel’s 9:3:3:1 ratio/dihybrid ratio: ideal ratio based on probability that events involve segregation, independent assortment and random fertilization. Actual results are unlikely to match the ideal.
Trihybrid cross: segregation and independent assortment applies to this 3-factor cross as well. In this, all individuals produced are heterozygous.
Another method developed--fork-line method/branch diagram. Each gene pair is assumed to behave independently during gamete formation can be used for any number of gene pairs, provided all gene pairs sort independently from one another.
For two or more gene pairs:
1. Determine the number of different heterozygous gene pairs (n) involved in the cross. Ex: AaBb x AaBb (n=2), AaBBCcDd x AaBBCcDd (n=3)---because B is not heterozygous.
2. Once n is determined, 2n is the number of different gametes that can be formed, 3n is the number of different genotypes of fertilization; 2n is the phenotype from genotypes. Applied only if independently assorted.
Continuous variation: held by students of evolutionary theory; describes offspring as a blend of their parent’s phenotypes. Ex: Darwin.
Discontinuous variation: held by Mendel due to the dominant-recessive relationship of genes.
Chromosomal theory of inheritance: deduced by Sutton and Boveri, the idea that genetic material in living organisms is contained in chromosomes.
-Following independent segregation of each pair of homologs, each gamete receives one member from each pair. All possible combinations are formed with equal probability. Independent behavior of unit factors is due to separate pairs of homologous chromosomes. Chromosomes are composed of a large number of linearly ordered information containing genes. The location of any particular gene on a chromosome is called a locus. Different alleles of a given gene contain slightly different genetice information that determines the same character. Most genes have more than 2 allelic forms.
Probabilities range from 0.0---certain not to occur---to 1.0----certain to occur.
Product law: probability of 2 or more events occurring simultaneously is equal to the product of their individual probabilities.
Sum law: calculation of probability when possible outcomes of 2 events are independent of one another can be accomplished in more than one way. The probability of obtaining any single outcome, where that outcome can be achieved by 2 or more events, is equal to the sum of the individual possibilities of the event.
Binomial theorem: analyzes cases where there are alternative ways to achieve a combination of events, a+b ¿n=1
¿, a and b being probabilities of alternative
outcomes and n number of trials.
(s!)(t !)asbt where n equals the number of trials, s the number one variable, t Ex:
the same, and a and b is the probability of the outcomes of s and t.
Mendel’s independent assortment and random fertilization is influenced by chance deviation, like observing the ratio of heads/tails coin. As the number of tosses is reduced, the impact of chance deviation increases--independent assortment and random fertilization is subject to random fluctuations from the predicted outcome due to chance deviation. As sample size increases, the chance deviation decreases.
Null hypothesis: in observing deviation it assumes no difference between measured and predicted values. Any difference is chance. If rejected, the deviation observed is not due to chance. If not rejected, deviations are due to chance.
Chi square analysis: test that assesses goodness of fit of null-hypothesis. Takes into account the observed deviation in each component of a ratio (expected) as well as
sample size and reduces to single number value x2 then estimates frequency of deviation: x2=εd2
e where e is the expected value, ε the sum of each ratio
Interpreting x2 is evaluated by degrees of freedom (df)= n-1 where n is the number of different categories data is divided into.
**The greater the number of categories, the more deviation by chance that is expected.
Probability of value (p): form of interpretation of x2 after df is determined. The
percentage indicates the percentage expected for the experiment to exhibit the chance deviaton as great or greater than in initial, or less. Less=more chance More=less chance. The p-value should be less than .05
Pedigree: family tree that observes presence/absence of traits for each member. By analyzing the pedigree, allowed to predict how a trait is inherited.
-Siblings are in order of birth from left to right. Generations are in Roman numerals. Identical twins are monozygotic. Fraternal twins are dizygotic. An individual whose phenotype is brought attention to the family is proband. In albinism the synthesis of pigment of melanin is obstructed. Autosomal dominant diseases like Huntington’s is rare. If so, the parent always exhibits the trait, usually offspring too.
Gene Interaction: situation in which a single phenotype is affected by more than one set of genes.
-Neo-Mendelian genetics investigates observations of genetic data that did not conform precisely to expected Mendelian alleles.
Allele: alternative form of a gene. Allele that occurs most frequently (or is normal) is referred as the wild type allele(represented by a +, ex: abc+). It is often but not always dominant.
**The process of mutation is the source of new alleles. For a new allele to be recognized by observation of an organism, the allele must cause a change in phenotype. A new phenotype is formed from a change in functional activity of cellular products specified by that gene.
Loss of function mutation: causes a gene to create an enzyme that loses/reduces the affinity for substrate. A null allele results if loss is complete.
Gain of function mutation: enhances the function of the wild-type product and increases the quantity of gene product. Results in dominant alleles. Ex:Proto oncogens changed to oncogens, which override regulation of gene product. Function is always turned on. Neutral mutations cause no phenotypic change or evolutionary fitness change.
-Traits are influenced by many gene products. Mutations can have a common effect in metabolic pathways―with a failure to produce an end product.
Incomplete/Partial Dominance: no one gene dominates the phenotype. Creates an intermediate phenotype. There can be alleles that are neither dominant nor recessive.
Haplo-insufficiency: when a single allele is not sufficient to produce the wild type phenotype and mask the other allele. (The dominant allele does not produce enough gene product/dose). Ex: red and white snapdragons―
RR-red (anthocyamin-receives two doses)-1/2 rr-white (receives no doses of pigment)-1/4
Rr-pink (receives one dose of anthocyamin-not enough to make red)-1/4
-Examination of gene product and activity often reveals intermediate gene expression (not in the phenotype). Tay-Sachs disease is related to lipid-storage deficiency. Those that are homozygous are affected, but heterozygotes are only partially affected. Threshold effect: normal phenotypic expression occurs any time a certain level of gene product is attained. In Tay Sachs, it is less than 50% of the time.
Codominance: occurs when joint expression of both alleles occurs. 2 alleles of a single gene are responsible. Ex: MN blood group
Both alleles expressed.
-Codominant inheritance is characterized by distinct expression of gene products of both alleles. Different from incomplete dominance, where products of both alleles. Different from incomplete dominance, where heterozygotes express intermediate blended phenotype.
-When mutation modifies information on a gene, each change produces a different allele.
Multiple Alleles: when present in a population, inheritance may be unique. Multiple alleles can only be studied in populations―one organism may have at most 2 homologous gen loci coupled by different alleles of the same gene. Among members of a species, numerous alternate forms of the same gene can exist. Ex: ABO Blood Groups―characterized by the presence of antigens on the surface of red blood cells―different from MN antigens. Once combination exhibits co-dominant forms of inheritance. IAIB for AB blood type are codominant to each other but
dominant to i, (for O blood type). All individuals possess H substance, to which half of sugars are added (carbohydrates are terminal sugars that determine A or B blood type). Type O, unlike A, cannot add the H substance.
-However, there are mutants of FUT1 gene that cannot express the sugar for the H substance even though they are type A or B functionally―Bombay Phenotype
-Mutations resulting in nonfunctional genes can be tolerated in heterozygotes, where the wild type might be enough to produce the essential product.
Recessive Lethal Allele: mutations resulting in a nonfunctional gene; lethal to homozygotes. May respond as phenotype in heterozygotes. Ex: yellow pigment in mice. Dominant in coat to wild-type, so heterozygotes are yellow. Homozygotes die before birth.
AYAY−homozygousrecessive (death) A A−homozygous dominant (agouti ) AYA−heterozygote( ¿)
2:1 ratio signals lethal gene
Dominant lethal allele: the presence of just one copy of the allele results in death. Ex: Huntington’s disease: Delayed until adulthood in heterozygotes, that results in motor and neurological degradation. (Dominant lethal alleles are rarely observed).
-For dihybrid corsses such as albinism risk and blood type, the 9:3:3:1 ratio converts to 2:6:3:1:2:1, establishing the probability of each phenotype. Phenotype in many cases is affected by more than one gene. Genetic influence on phenotype is more complex than Mendel’s encounters. Gene Interaction expresses the idea that several genes influence a particular characteristic. DOES NOT MEAN THAT TWO OR MORE GENES INTERACT DIRECTLY TO INFLUENCE. DOES MEAN THAT CELLULAR
FUNCTION OF NUMEROUS GENE PRODUCTS CONTRIBUTES TO GENE DEVELOPMENT OF A COMMON PHENOTYPE. Epigenesis: a point in which each step of development increases the complexity of an organ or feature and is under the control/influence of many genes. Ex: formation of inner ear in mammals. Forms as a result of a cascade
of intricate developmental events influenced by many genes. Mutations that interrupt these events leads to hereditary deafness. In a sense, many genes “interact” to produce a common phenotype. Mutant phenotype then becomes the heterogeneous trait.
Epistasis: the expression of one gene pair masks/modifies the effect of another. Sometimes the genes involved influence the same general phenotypic characteristic in an antagonistic manner, which leads to masking. But other times, the genes involved exert influence on one another in a complementary fashion. Ex: recessive homogeneous allele (epistatic) may override the expression of other alleles (hypostatic) at another locus. 2 gene pairs may also complement one another such that at least one dominant allele in each pair is required to express a particular phenotype. (9:7 ratio or 9:3:4).
**When a single character is being studied, a ratio of 16 parts (ex: 3:6:3:4) suggests that two gene pairs are “interacting” in the expression of the phenotype under consideration (epistasis has an effect of combining one or more of 4 phenotypic categories in various ways).
1. In each case, a distinct phenotype classes produced, disassemble from others. Illustrate discontinuous variation―phenotypic traits are discrete and different from each other.
2. Genes considered in each cross are on different chromosomes and assort independently during gamete formation . Ex: A,a,B,b.
3. When we assume complete dominance (AA,Aa,BB,Bb) is equivalent to genetic effects of A- or B- (- means other alleles present)
4. P1 crosses are with homozygous individuals (AABB, aabb,AAbb, aaBB). Each F1 has heterozygote (AaBb)
5. F2 is the main focus. When 2 genes are involved, genotypes fall into 9/16
A-B-, 3/16 A-bb and 1/16 aabb. All genotypes are equivalent on the effect for the phenotype. Ex: mice A- agouti aa-black bb-albino.
bb masks A or a alleles (recessive epistasis)
-Novel Phenotypes may be expressed in the F2 generation in addition to the modified dihybrids Ex: squash shape. F1 = AaBb x AaBb (both disc). A-bb and aaB- yield sphere shape (new phenotype). (gene interaction)
**When 16th in ratios of dihybrid crosses where inheritance pattern is unknown think 2 gene pairs involved.
Heterogeneous trait: go back to hereditary deafness.
Complementation analysis: allows to determine whether 2 independently isolated mutations are in the same gene (alleles) or represent mutations in separate genes. May reveal if only a single gene is involved in a mutation or multiple genes. Ex: Drosophila bugs F1 generation.
Case 1: All offspring have wings―2 recessive mutations (that make bugs wingless) in separate genes and not alleles―being heterozygotes, both genes have a normal copy of each gene and complement.
Case 2: No wings―2 mutations affect the same gene and are alleles of one another. No complementation, homozygous for 2 mutant alleles. No normal product produced.
Complementation group: is for all mutations present in a single gene. Complement mutations are in all other groups. Helps predict the number of genes involved in the determination of a trait.
Pleiotropy: when a single gene has multiple phenotypic effects. Ex: Marfan Syndrome: results from autosomal dominant mutations in the gene encoding fibrillin. Also, Poryphyria variegate: does not allow for the metabolization of hemoglobin when the respiratory pigment is broken down. Leads to toxic buildup.
-A major portion of the Y chromosome is inert genetically. There is only a small portion that is homologous to the X chromosome in order to separate during meiosis. X-linkage―genes present on the X chromosome exhibit patterns of inheritance different from those with autosomal genes. One of the first examples was the white eye of the Drosophila female, which was an X-linked mutation that was expressed in male offspring only.
-Since the Y chromosome lacks homology with almost all genes on the X alleles present on the X will be directly expressed in the phenotype. Males cannot be homozygous or heterozygote for X-linked possession of only one copy of a gene in diploid is heterozygosity. One result of X-linkage is in a crisscross pattern, where recessive X-linked genes passed from homozygous mom to all males. Occurs because females express recessive mutant allele on both chromosomes. Ex: colorblindness.
-If X-linked disorder debilitates/lethal to individuals prior to reproduction, disorder exclusively in males―only source of lethal allele in heterozygote female “carriers”. Pass on to ½ of males. Ex: Duchenne muscular dystrophy.
-In sex-linked inheritance and sex-influenced inheritance: autosomal genes responsible for contrasting phenotypes but expression dependent on hormone constitution of the individual. So, heterozygotes may exhibit a phenotype in males and contrast in females. Ex: domestic fowl plumage distinctly different in males and females, demonstrating sex limited inheritance―controlled by a single pair of autosomal alleles modified by hormones.
Sex influenced patterns of inheritance include pattern baldness in humans, horn formation in sheep and coat patterns in cattle. Autosomal genes are responsible, while trait may show in both sexes, expression dependent on hormone constitution.
-Most gene products function within a cell, cells interact with one another in various ways. Organisms exist under diverse environmental influences. **Gene expression and resultant phenotype modified through interaction between individuals’ genotype and external environment. Degree of expression can be shown by determining penetrance and expressivity.
Penetrance: Percentage of individuals that show at least some degree of expression of a mutant genotype. Ex: If 15% of flies have a mutant genotype with wild-type appearance, penetrance of mutant gene is 85%
Expressivity: range of expression of mutant genotype. Ex: eyeless gene in flies can range from normal to partial to none―experiments show genetics and environmental factors influence expression.
Position effect: effect of genetic background; where physical location of a gene in relation to other genetic material may influence expression. Ex: if a region of chromosome is relocated, normal expression of genes in that region may be modified more. True if gene is relocated to areas condensed and genetically inert (heterochromatin) Ex: white eye of X-linked Drosophila―if a region of X chromosome with w+ allele is relocated, caused red and white phenotype.
-Temperature affects/influences phenotypes through chemical activity―dependent on kinetic energy. Ex: a primrose is red at 23 degrees Celsius and white at 18
degrees. Siamese cat and Himalayan rabbit-fur black in cold areas and white in warmer ones. (temperature sensitive mutations). These are rexamples of conditional mutations. Permissive conditions allow an organism to grow, restrictive conditions require and organism to use essential genes and arrests. Nutritional mutations are crucial in bacteria―mutations that prevent synthesis of nutrient molecules may kill microorganisms. They can also prevent organisms from metabolizing a substance Ex: lactose intolerance.
-Some genes are expressed at different phases of life: prenatal, childhood, pre-adult and adult. Genetic Anticipation: a form of heritable disorders that exhibit progressively earlier ages of onset and increased severity of the disorder in each successive generation. Es: Myotonic dystrophy.
-Genomic/Parental Imprinting: process of selective gene silencing that occurs during early development, impacting subsequent phenotypic expression. Impact depends on parental origin of genes/regions involved. Leads to direct phenotypic expression of allele(s) on homologs that are not silenced.
-Goals for Bio: DNA is organized into chromosomes which are units of heredity; chromosomes are segregated (you should be able to describe linkage and what it does to segregation ratios)
Most chromosomes contain a large number of genes. Those that are part of the same chromosome are linked and demonstrate linkage in genetic crosses. Because chromosomes and not genes are the units of transmission in meiosis, linked genes are not free to undergo independent assortment. Instead, alleles at all loci are transmitted as a unit. In many cases, this does not occur.
Crossing over or reshuffling/recombination shuffles alleles between genes and always occurs during the tetrad stage (prophase-the exact moment is still unknown). This is currently viewed as a physical breaking and rejoining process. The frequency of crossing over is proportional to the distance between genes. Chromosome maps indicate the relative locations of genes on chromosomes.
Chiasma shows consequences of crossing over―exchange of material of 2 homologous chromatid segments.
Genes on the same chromosome may be inherited together.
Complete linkage produces only parental/noncrossover gametes. Two parental gametes are formed in equal proportions when genes are linked on the same chromosome and only two genetically different kinds of gametes are formed. Crossover between linked genes involves non-sister chromatids―this generates 2
new allele combinations―recombinant/crossover gametes. As distance between 2 genes increases, the proportion of recombinant gametes increases and parental gametes decreases.
Recombination can occur between any 2 of 4 chromatids of a homologous pair. Genetically occurs between sister chromatids. The only way to tell if recombination has occurred in homologous chromosomes is if the alleles are different.
If complete linkage exists between 2 genes because of close proximity and organisms that are heterozygous at both loci are mated, a unique F2 phenotypic ratio results―a linkage ratio.
Ex: Drosophila; hv-heavy wing vein (mutant recessive) hv+-thin wing vein (wild type)
bw-brown eye(mutant recessive) bw+-red eye (wild type) 1:2:1- one heavy wing, 2 wild type, 1 brown eyed
When large numbers of mutant genes in a species are investigated, genes on the same chromosome are linked―linkage groups can then be identified (one for each chromosome). Genes on a single chromosome comprise a linkage group (there are 24 linkage groups in the human genome (22 autosomes, 2 sex chromosomes)). The number should correspond to the haploid number of chromosomes.
Alternate possibilities of genetic recombination―independent assortment. unlinked
partially linked (like unlinked) Crossing over indicated by ×
-Morgan( who discovered X linkage) postulated that linked genes are arranged in a linear sequence along the chromosome and that variable frequency of exchange occurs between any 2 genes during gamete formation. 2 genes relatively close to each other alonge a chromosome are less likely to have a chaisma form between them than if the 2 genes are farther apart on the chromosome. The closer the genes are, the less likely that an exhacne will occur between them.
The frequency of crossing over(recombination) between 2 genes is proportional to the distance between the genes.
Ex: Drosophila; rate of occurrence for phenotype 1. Yellow, white .5% → sum of 1 and 2=3
**Recombination frequencies between linked 2. white, minute 34.5% genes are additive 3. yellow, minute 35.4%
The distance between yellow and white is .5 (mu-map units) and the distance between yellow and minute is 35.4 mu. The distance between white and minute is 35.4-.5=34.9 mu
X-linkage and crossing over are not restricted to the X chromosome, but can apply to autosomes. In Drosophila, there is more crossing over in females than males.
-During meiosis, a limited number of crossover events occur in each tetrad. These recombinant events occur randomly along the length of the tetrad. The closer that two loci reside along the axis of the chromosome, the less likely that any single crossover event will occurs between them → occurs between 2 nonsister
chromatids, but not the two loci that are being studied. Where two loci are far apart, crossover does occur. When crossover occurs, the other two chromatids of the tetrad are not involved. These are used to determine the distance between two linked genes.
The percentage of tetrads involved in an exchange between 2 genes is 2x as great as the percentage of recombinant gametes produced. Theoretical limitation of observed recombination due to crossing over is 50%.
Double exchanges of genetic material result from double crossovers (DCOs). 3 genes pairs must be investigated to do this. 2 independent and separate events/exchanges must occur (refer to the product law). Ex: single crossover between A and B is 20% (p=.20); B and C is 30% (.30). The chance of a double crossover is (.20)(.30)=.06 or 6%. Double exchanges of genetic material are used to determine the distance between 3 linked genes. Genes must be heterozygous for both alleles.
A test cross is used to test whether an organism is a heterozygote or homozygote for certain traits.
Ex: Aa × aa (all linkage studies use this to study recombination in a heterozygote) 1:1 ratio
Class Question: If no crossing over occurs between these two, what phenotypes result?
pr-purple eyes pr+-wild type vg- vestigial wing vg+-wild type
Answer: Wild type pr and vg recessive Class Question: If crossing over does occur, what phenotypes?
Answer: wild type, purple vestigial (recessive), vestigial (wild type) and purple eye **If a trait is not specifically stated, assume wild type
Class Question: If the genes are unlinked?
Answer: wild type, purple vestigial (recessive), vestigial and purple eye For a successful mapping cross:
1. The genotype of the organism producing the crossover must be a heterozygote at all loci under consideration.
2. The cross must be constructed so genotypes of all gametes can be accurately determined by observing phenotypes of the resulting offspring.
3. A sufficient number of offspring must be produced.
To diagram a 3 point cross successfully, ** we must assume some theoretical sequence, even though we do not yet know if it is correct.
Ex: Drosophila; white eye, yellow body, echinis eye shape.
P1 -males hemizygous for all 3 wild type; females homozygous-recessive
F1 -females are heterozygous, males hemizygous recessive
F2 - females and males will express genotype from F1
-To create a chromosome map, we must determine which F2 phenotypes correspond to noncrossover and crossover categories. For noncrossover in F2
phenotypes, we must identify individuals derived from parental gametes from the F1 female. Each such gamete contains an X chromosome unaffected by crossing
over. Genotypes of parental gametes and F2 phenotypes complement one
another. Ex: if one is wild type, the other is a mutant for all 3 genes. In other situations, if one chromosome shows a mutant allele, the second shows the other two mutant alleles―these are the reciprocal classes of gametes and phenotypes.
-2 noncrossover phenotypes are most easily recognized because ** they occur in the greatest proportion of offspring.
-The 2nd category is easily represented by double-crossover phenotypes, ** present in the least numbers.
-The remaining categories for the 4 phenotypic classes result from single crossovers. Double crossovers represent 2 simultaneous single crossovers.
To determine gene sequence (Method I):
1. Assume any of the 3 orders (w-y-ec, y-ec-w, y-w-ec), then determine the arrangement of alleles along each homolog of the heterozygous parent giving rise to noncrossover and crossover gametes.
2. Determine whether a double-crossover even occurring within the arrangement will produce the observed double-crossover phenotypes (which occur least frequently).
3. If this order does not produce the correct phenotypes, try each of the other 2 orders.
Method II: assumes that following a double crossover event, the allele in the middle position will fall between the outside/flanking alleles that were present on the opposite parental homolog.
Interference (I): the inhibition of further crossover events by a crossover event in a nearby region of the chromosome causes the reduction. To quantify discrepancies that result from interference, we calculate the coefficient of coincidence (C):
expected DCO . To quantify interference: I=1-C. If interference is complete and
no double crossovers occur, I=1 . If fewer DCOs than expected occur, I is positive and positive interference has taken place. If more DCOs than expected occur, negative interference has occurred. Positive interference is most commonly observed in eukaryotes.
Lod score method: helps to demonstrate linkage of 2 genes when they are separated to the degree that recombinant gametes are formed. Assesses the probability that a particular pedigree involving 2 traits reflects genetic linkage between them. A value of of 3 or above indicates linkage―-2 or below does not. If the result is between -2 and 3 → inconclusive.
DNA markers: short segments of DNA whose sequence and location are known, making them useful for mapping purposes. Ex: Cystic Fibrosis ― a gene located using markers.
Sister chromatid exchanges: occur during mitosis, do not produce new allelic combinations. Frequency of SCEs significant but unknown why.
Cis and Trans configuration: The way alleles of a gene are organized on chromosomes
Cis configuration: Trans configuration:
Crossover depends on whether
mother is cis or trans!
Cis crossover: Trans crossover:
P1 AABB x aabb
F1 AaBb If A and B genes are linked, the offspring has Cis configuration
If the genotype of the offspring is recombinant:
If A and B are trans, the parental crossover of the offspring looks like this: Ab
**In linkage studies, you are really looking at meiosis of the mother
Noncrossover gametes: y+ ec+ y ec
Crossover gametes: y+ ec ** Difference between unlinked genes and linkage
y ec+ with crossing over: crossing over is much rarer
than noncrossing over
To determine the distance of genes: divide the sum of crossovers by the total number of offspring to get the percentage. Ex: there are 10,000 flies for offspring. To find the percentage of noncrossover gametes: [275+273]/10000= .05 or 5% (created numbers).
Percent crossover is equivalent to the crossover map units.