General Biology Essay Sample
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General Biology Essay Sample
Explain what free energy is and how it can be used to predict the energetic outcome of chemical reactions.Free energy is defined as the energy available to do work in any system. The free energy is denoted by the symbol G. G = H – TS* H: the energy contained in a molecule’s chemical bonds, called enthalpy.* TS: the energy term related to the degree of disorder in the system. T is the absolute temperature (K), and S is the entropy.We can use the change in free energy to predict whether a chemical reaction is spontaneous or not:- G positive: the products contain more free energy than the reactants. The bond energy (H) is higher, or the disorder (S) is lower. Therefore, the reaction is NOT spontaneous because it requires the input of energy endergonic. – G negative: the products have less free energy than the reactants. Either the bond (H) is lower, or the disorder (S) is higher; or both. Such reactions tend to proceed spontaneously. These reactions release the excess of free energy as heat exergonic. State the first and second laws of thermodynamics and explain how those laws relate to chemical processes.1) First Law of Thermodynamics: it concerns the amount of energy in the universe. Energy cannot be created or destroyed; it can only change from one form to another.
The amount of energy in the universe remains constant. Potential energy can be transformed into kinetic energy; and some energy dissipates into the environment as heat, which is the measure of the random motion of molecules. Heat can only be harnessed to do work only when there is a heat gradient – a temperature difference between two areas. The energy available to do work decreases, as more of it is progressively lost as heat.2) Second Law of Thermodynamics: it concerns the transformation of potential energy into heat, or random molecular motion. It states that the disorder in the universe, entropy, is continuously increasing. Energy transformations proceed spontaneously to convert matter from a more ordered, less stable form to a less order, more stable form. Explain entropy.Entropy is the disorder in the universe. As time elapses, things get more disorganized. Entropy increases, and it takes energy to restore things to an ordered state. Describe what a catalyst is and what they do.Catalysis: The process of influencing chemical bonds in a way that lowers the activation energy needed to initiate a reaction.
The substances that accomplish this are the catalysts.Catalysts affect an intermediate stage in the reaction, the transition state. The energy needed to reach this transition state is the activation energy, and catalysts lower it to stabilize this transition state. By reducing the activation energy, a catalyst accelerates both the forward and the reverse reactions by the same amount; it does not alter the product. Describe the function of ATP in the cells and how it works.Adenosine Triphosphate (ATP) powers almost every energy-requiring process in cells (making sugars, supplying activation energy, transporting substances across membranes, moving through the environment, growing, etc.)ATP is used as the building block for RNA molecules, and it also has a critical function as a portable source of energy on demand for endergonic cellular processes.* ATP structure:- 5-carbon sugar- Ribose: framework to which other two subunits are attached.- Triphosphate group: The unstable bonds holding the phosphates together have low activation energy, and are easily broken by hydrolysis. When they break they can transfer a considerable amount of energy (It has a negative G). When broken, ATP becomes ADP+P, and energy is released. AMP * The use of ATP can be thought of as a cycle: Cells use exergonic reactions to provide energy to synthesize ATP from ADP+P then use hydrolysis of ATP to provide energy to drive endergonic reactions.
Describe the function of enzymes in biological reactionsEnzymes are the agents that carry out most of the catalysis in living organisms. Most enzymes are proteins, although some are RNA molecules.* Functions of enzymes:- Altering the activation energy of a reaction: its unique 3D shape enables it to stabilize a temporary association between substrates of a reaction. By bring two substrates together in the correct orientation or by stressing particular chemical bonds of a substrate, an enzyme lowers the activation energy required for new bonds to form; quickly.The enzyme is not changed or consumed in the reaction; it is reusable.- Active sites: the enzyme’s pockets. Substrates bind to the active sites, forming enzyme-substrate complex. Substrate fits amino acid groups are very close to the substrate’s bonds side groups interact chemically with substrate, stressing or distorting a particular bond and lowering the activation energy required to break it substrate bonds broken new bonds formed products leave.- Multienzyme complexes: Noncovalently bonded enzyme assemblies that are good because:1- the product of one reaction can be delivered to the next enzyme without releasing it to go.2- the substrate does not leave the complex so unwanted reactions are prevented.3- all reactions are controlled as a unit.- Ribozymes: They have folded structures and catalyze reactions on themselves (intramolecular catalysis).
Others catalyze other molecules (intermolecular catalysis). Plays a key role in ribosome function; the ribosome itself is a ribozyme. List the factors that affect enzyme activity.-Temperature: The rate of an enzyme-catalyzed reaction increases with temperature, because it increases random molecular movement. However, this is only until its optimum temperature. Below this temperature the bonds and interactions are not flexible enough to permit the optimum catalysis. At higher temperatures, these forces are to weak to maintain the enzyme shape, and it denatures. -PH: Ionic interaction between oppositely charged amino acids hold the enzymes together. When hydrogen ion concentration is shifted, the balanced is shifted as well (+/-) Most enzymes optimum pH is between 6-8. – Inhibitors and activators: They regulate when enzymes are active/inactive at a given time. Competitive inhibitors compete with the substrate for the same active site, preventing substances from binding. Noncompetitive inhibitors bind to the enzyme in a location other than the active site, changing the shape. Allosteric inhibitors bind to the allosteric site, and can also be activators increasing the enzyme’s activity. Schematize the relationships between: energy, biological reactions, and enzyme function-Anabolism: YES energy, to create larger molecule.-Catabolism: NO energy, to create smaller molecules.Metabolism is the sum of all chemical reactions in a cell. Biochemical pathways organize chemical reactions, using the product of one as the substrate for the other.
Interpret the role of electrons, electron carriers, and ATP in energy metabolism*Electrons: Energy in metabolism flows in terms of electrons. If electrons are lost, this is called oxidation. If electrons are gained, this is called reduction. Oxidation is coupled to reduction.*Electron carriers: During respiration, glucose is oxidized to CO2. This means that the cell transfers the electrons to intermediate electron carriers, and then eventually O2. If it were given directly to O2, the reaction would be combustion and cells would burst into flames. Many forms of electrons are used:- Soluble carriers that move electrons from one molecule to another- Membrane-bound carriers that form a redox chain.- Carriers that move within the membrane.NAD+ is one of the most important electron/proton carriers. It is made up of AMP (adenosine monophosphate) and NMP (nicotinamide monophosphate). AMP is the core and it provides a shape recognized by many enzymes; NMP is the active part that is readily reduced and easily accepts electrons. When NAD+ acquires two electrons + a proton = NADH.
The ability to supply high-energy electrons is critical both to energy metabolism and to the biosynthesis of many organic molecules.*ATP: Cells use ATP to power most of those activities that require work – one of the most obvious of which is movement. This happens due to the expenditure of energy by ATP hydrolysis, which is used also to drive endergonic reactions that would otherwise not occurs spontaneously. How?The catalyst enzyme has two binding sites, one for the reactant and one for ATP the ATP site splits the ATP molecule, liberating energy This energy pushes the reactant at the second site, reaching the activation energy and driving the endergonic reaction Explain the purpose of oxygen in respirationOxygen acts as electron acceptor and permits the production of a large number of ATP. In the presence of oxygen the following processes can take place: pyruvate oxidation, Krebs’s cycle, ETC and chemiosmosis. Describe the structure of mitochondria- An outer membrane that covers the organelle and contains it like a skin. – An inner membrane that folds over many times and creates layered structures called cristae.- The mitochondrial matrix (the fluid inside) with ribosomes and DNA floating in it.
There are also granules, which control the concentration of ions. Summarize the steps and outcomes of glycolysis1) Two high-energy phosphates from two molecules of ATP are added to glucose.2) Glucose is broken down into 2 molecules of G3P (glyceraldehyde 3-phosphate) 3) Each G3P is oxidized, thus forming 1 NADH each. Each NADH transfers a phosphate to ADP, to form ATP. Therefore each G3P produces 2 ATP.4) A molecule of inorganic phosphate is added to each G3P to form 1,3 bysphosphoglycerate, then converted to a pyruvate.*OUTCOMES: 2 NADH, 2 ATP, 2 pyruvates.*NET YIELD: 2 ATP. Describe the steps of the Krebs cycle and its outcomes1) Acetyl CoA (2C) is combined with Oxaloacetate (4C) to form citrate (6C) through condensation.2) An –H and an –OH group change positions when water is removed from one carbon and added to another. This forms an isocitrate, through isomerization.3) Isocitrate is oxidized, reducing NAD+ to NADH. The central carboxyl group splits off to form CO2, yielding α-ketoglutarate (5C) 4) α-ketoglutarate is decarboxylated.
The succinyl group joins to Coenzyme A succinyl-CoA. Another molecule of NAD+ is oxidized to NADH.5) The succinyl group and CoA are separated, and the energy released is used to phosphorylate GDP (guanosine diphosphate) into GTP (guanosine triphosphate). ATP is formed. Succinate (4C) remains.6) Succinate is oxidized to fumarate (4C). FAD is reduced to FADH2.7) A water molecule is added to fumarate, forming malate (4C)8) Malate is oxidized to form oxaloacetate (4C). NAD+ is reduced to NADH. The cycle begins again.*OUTCOMES (2 cycles): 6 NADH, 2 FADH2, 4 CO2*NET YIELD (2 cycles): 2 ATP Illustrate the purpose of the electron transport chain, where those electrons come from and where do the go at then end.ETC:1) High-energy electrons from catabolized molecules (NADH/FADH2) are transported by electron carriers between three complexes of membrane proteins (NADH dehydrogenase, bc1 complex and cytochrome oxidase complex).2) These complexes use portions of the electrons’ energy to pump protons out of the matrix and into the intermembrane space.3) The electrons are used to reduce oxygen, forming water. Chemiosmosis:4) A concentration gradient is created, + on the intermembrane space and – on the matrix.
This gradient is used by the enzyme ATP synthase, which couples the reentry of protons to the phosphorylation of ADP to form ATP. *OUTCOMES: 30 NADH/ 4 FADH*NET YIELD: 36 ATP (6 per carbon atom) Contrasts the two mechanisms for producing ATP and their relative efficiency.1) In substrate-level phosphorylation, ATP is formed by transferring a phosphate group directly to ADP from a phosphate-bearing intermediate, or substrate.2) In oxidative phosphorylation, ATP is synthesized by the enzyme ATP synthase, using energy from a proton gradient. The high energy electrons in the ETC are donated to oxygen. Distinguish the process and the outcomes between aerobic and anaerobic respiration- Aerobic respiration: Oxygen is necessary. The process includes glycolysis, pyruvate oxidation, Krebs’s cycle, ETC and chemiosmosis. The usual ATP yield ranges from 30 to 38. Its products include NADH, FADH, CO2, H2O…- Anaerobic respiration: No oxygen. This type of respiration uses organic molecules. This process only includes glycolysis and fermentation (lactic/ethanol) The net yield is only 2 ATP.
Contrast binary fission to mitosisIn both bacterial en eukaryotic cells produces two new cells with the same genetic information as the original. The essentials in both processes are the same: duplication and segregation of genetic information into two daughter cells and division of cellular contents. In bacteria most genomes are made up of a single, circular DNA molecule. It is located in the nucleoid, and its compaction and organization involves SMC proteins. The chromosome is replicated and the two products are partitioned to the ends of the cell prior to the actual cell division. In contrast to mitosis, this is a concerted process. Describe the composition of chromatin and the structure of chromosomes- Chromosomes are composed of chromatin, a complex of DNA and proteins; also RNA, because here is where RNA synthesis happens.- Heterochromatin is not expressed. Euchromatin is expressed.*Structure: Every 200 nucleotides the DNA is coiled around a core of eight histone proteins (positively charged). These are attracted to negatively charged phosphate groups of the DNA. DNA + Histone proteins nucleosomeThe DNA wrapped in nucleosomes is coiled in solenoids. Many of these form chromatin loops, then rosettes of chromatin loops, then chromosomes.
This is not clear yet though. Distinguish between homologous chromosomes and sister chromatids- Homologous chromosomes: maternal and paternal chromosomes (homologue each). Different sister chromatids.- Sister chromatids: Two identical DNA molecules held together by a complex of proteins called cohesins. List the phases and the events that occur at each phase of the interphaseInterphase:1) G1 (gap phase 1): primary growth phase of the cell. Longest phase. (G0)2) S (synthesis): the cell synthesizes a replica of the genome.3) G2 (gap phase 2): second growth phase. Preparation for separation of the newly replicated genome. Microtubules begin to reorganize to form a spindle. Describe the events that occur at each phase of mitosisMitosis:1) Prophase: – Chromosomes condense and become visible. – Chromosomes appear as two sister chromatids held at the centromere.- Cytoskeleton is disassembled: spindle begins to form.- Golgi and ER are dispersed.- Nuclear envelope breaks down2) Prometaphase:- Chromosomes attach to microtubules at the kinetochores.- Opposite kinetochores are attached to opposite poles.
Chromosomes move to the equator of the cell.3) Metaphase:- Chromosomes are aligned at the equator of the cell (metaphase plate).- Chromosomes are attached to opposite poles and under tension.3) Anaphase:- Proteins holding sister chromatids together are degraded, freeing individual chromosomes.- Chromosomes are pulled to opposite poles.- Spindle poles move apart.4) Telophase: – Chromosomes are clustered at opposite poles and decondense.- Nuclear envelopes begin to form around chromosomes.- Golgi and ER re-form Contrast the differences in cytokinesis between plant and animal cells- In animals, cleavage furrow forms to divide cells. – In plants, cell plate forms to divide cells. Explain the implications of cell cycle control for multicellular organismsThere are positive regulators of mitosis, and these proteins are produced in synchrony with the cell cycle (cyclins). These are cyclin-dependet kinases (cdks). Cdks = complexes of kinases + cyclin (regulatory molecule). They phosphorylate proteins to drive the cell cycle.APC/C (anaphase-promoting complex) triggers anaphase, removing cohesins.
It also destructs cyclins to exit mitosis.In multicellular organisms growth factors, like PDGF, stimulate cell division. This acts through a MAP kinase cascade that results in the production of cyclins and the activation of Cdks to stimulate cell division in fibroblasts after tissue injury. Only certain cells divide at its appropriate time. G0 phase is also a factor that helps to control. Connect the factors of cell cycle control and cancerCancer is a failure of cell cycle control, because it is the unrestrained growth of cells.- The p53 gene monitors the integrity of DNA, checking that is its undamaged if there is damage it has got to be repaired or the cell has to kill itself. This is a tumor-suppressor gene.- Proto-oncogenes: they become oncogenes – genes that can cause a cell to become cancerous – when mutated. Their mutation can lead to loss of growth in multiple ways. Only one copy needs to undergo mutation for uncontrolled division to take place. – Tumor suppressor genes: Both copies of t-s gene must lose its function for a cancerous phenotype to develop.
Explain why meiosis is fundamental in sexual reproductionMeiosis reduces the number of chromosomes. Eggs and sperm are haploid, which contain one set of all chromosomes, and products of meiotic division. Only germ-line cells are capable of meiosis. Contrast meiosis I to meiosis II, describing how homologous chromosomes pair and then divideMeiosis I:1) Prophase I: – The chromosomes condense, and the spindle begins to form. – Homologous chromosomes pair during synapsis.- Crossing over occurs, forming chiasmata, which holds chromosomes together.2) Metaphase I: – Homologous chromosomes align at the equator of the cell. – Microtubules from each pole attach to sister kinetochores at each homologue, producing tension. 3) Anaphase I: – Kinetochore microtubules shorten, and homologous pairs are pulled apart.4) Telophase I: – The separated homologues form a cluster at each pole- Nuclear envelopes re-form.- Cytokinesis may occur.Meiosis II: does not include S phase in the interphase. It resembles a mitotic division. Describe the process of crossing over and its importance in genetic recombination A structure appears at the same time recombination occurs: recombination nodules.
They are thought to contain machinery to break and rejoin chromatids of homologous chromosomes. – During crossing over, DNA segments are exchanged between non-sister chromatids. On or a few crossovers occur during meiosis. This is important for the random traits of the daughter cells. List the event that occur at each phase of meiosis I and II that result in genetic variation- crossing over during prophase I- random alignment of chromosomes/ sister chromatids at metaphase I and II- separation of randomly aligned chromosomes/ sister chromatids at anaphase I and II. Identify the differences between meiosis and mitosisMeiosis is characterized by four distinct features:1- Homologous pairing and crossing over joins maternal and paternal homologues during meiosis.2- Sister chromatids remain connected at the centromere during Anaphase I.3- Kinetochores of the sister chromatids are attached to the same pole in meiosis I and to opposite poles in meiosis II.4- DNA replication is suppressed between two meiotic divisions. Explain the consequences that mistakes during meiosis can have for the future offspringFailure of chromosomes to move to opposite poles during either meiotic division is called nondisjunction. This produces a gamete that lacks one chromosome and another that has two copies aneuploid gametes. In humans, this is the most common cause for spontaneous abortion.
Explain the principle of segregation and its relation to meiosisPrinciple of segregation/ Mendel’s first law: Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization.Mendel’s five-element model:1- Parents do not transmit physiological traits directly to their offspring. Rather they transmit discrete information for the traits: factors, or factor genes.2- Each individual receives one copy of each gene from each parent. 3- Not all copies of a gene are identical. The alternative forms are called alleles. From two haploid gametes with the same allele the offspring is homozygous. From two haploid gametes with different alleles, the offspring is heterozygous.4- The two alleles remain discrete; they do not blend with each other.5- The presence of a particular allele does not ensure the expression of the trait. . In heterozygous individuals only one allele is expressed, and the other one is just present. Distinguish between: gene, allele and gene locus- Gene: A hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism.- Allele: one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
Gene locus: the specific location of a gene, DNA sequence, or position on a chromosome Predict the outcome of a monohybrid crossA monohybrid cross is a cross that follows only two variations of a single trait (white/purple)F1 generation: All purple flowers. The trait expressed is the dominant trait; the alternative is the recessive trait. Purple= dominant. White= recessive.F2 generation: Most have purple flowers, but some have white flowers. 3:1 ratio.F3 generation: 1:2:1 genotypic ratio. Distinguish between phenotype and genotypeGenotype: The total set of alleles that an individual contains; the blueprint.Phenotype: The physical appearance or observable characteristics of an individual that result from an allele’s expression; the visible outcome. Identify the dominance or recessiveness of alleles using pedigreesA pedigree is a consistent graphical representation of matings and offspring over multiple generations for a particular trait.* Dominant pedigrees: The dominant nature of this trait is obvious from the fact that every generation shows the trait.
This is unlikely for a recessive trait, as it would require large numbers of unrelated individuals to be carrying the disease allele.(Juvenile glaucoma, Huntington’s disease, polydactyly, hypercholesterolemia…)* Recessive pedigrees: Most affected individual have unaffected parents. A single parent usually does not have affected offspring, and affected offspring are more frequent when parents are related.(Albinism, cystic fibrosis, hemophilia, sickle cell anemia…) Explain the principle of independent assortment and its relation to meiosisPrinciple of independent assortment/ Mendel’s second law of heredity:In a dihybrid cross, the alleles of each gene assort independently. The segregation of different allele pairs is independent. – The independent alignment of different homologous chromosomes pairs during metaphase I lead to the independent segregation of the different allele pairs. Predict the outcome of a dihybrid crossWe have dihybrid heterozygotes (doubly heterozygous). YELLOW/green + ROUND/wrinkledF1 generation: Yellow round.
All dominate traits.F2 generation: Two genes each with two alleles can be combined in four ways to produce these gametes: RY, ry, Ry, rY. 9:3:3:1 ratio. 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green. Apply the rules of probability to infer genotypes from testcrossesProbability allows us to predict the likelihood of the outcome of random events.Two probability rules apply to genetics:1) The rules of addition: The probability of two different numbers is the sum of the individual probabilities. The probability of being heterozygous (pP= ¼, Pp= ¼) = ½.2) The rule of multiplication: The probability of two independent events both occurring is the product of their individual probabilities. For heterozygous parents (Pp), the probability of having homozygous recessives (pp) is ¼ = ½ p (mother) x ½ p (father) Define the concepts of polygenic inheritance and pleiotropy*Polygenic inheritance: More than one gene can affect a single trait. The greater the number influencing a character, the more continuous the expected distribution of the versions of that character. (Quantitative traits)*Pleiotropy: A single gene can affect more than one trait. An allele that has more than one effect on phenotype is said to be pleiotropic. A pleiotropic allele may be dominant with respect to one phenotypic consequence and recessive with respect to another.
These are difficult to predict. (Cystic fibrosis, anemia…) List and explain other factors that interfere with Mendel’s predictions.1) Genes may have more than two alleles: There may be more than two alleles of a gene in a population. Given the possible number of DNA consequences, this is not a surprise.2) Dominance is not always complete: – In incomplete dominance, the heterozygote exhibits and intermediate phenotype (pink). The genotypic ratio is the same as the phenotypic ratio.- Codominant alleles each contribute to the phenotype of a heterozygote. Some aspect og both alleles is seen in the heterozygote, not blended.3) Phenotypes may be affected by the environment: this means both external and internal. In Siamese cats, temperature affects the pigment of the color of their bodies.4) In epistasis, interactions of genes alter genetic ratios: A gene can interfere with the expression of another. Genes encoding enzymes that act in a single biochemical pathway is not independent. 9:7 ratio.
Relate the events that occur in meiosis with Mendel’s Principles of Independent Assortment and Segregation1) The Law of Segregation: A parent may have two distinct alleles for a certain gene, each on one copy of a given chromosome. Mendel’s first law states that these two alleles will be separated from each other during meiosis. Specifically, in meiosis II the two copies of each chromosome will be separated from each other, causing the two distinct alleles located on those chromosomes to segregate from one another. 2) The Law of Independent Assortment: Mendel’s second law states that the way an allele pair gets segregated into two daughter cells during meiosis II has no effect on how any other allele pair gets segregated. In other words, the traits inherited through one gene will be inherited independently of the traits inherited through another gene because the genes reside on different chromosomes that are independently assorted into daughter cells during meiosis.
Predict the outcome of a cross of a trait with sex-linkage inheritanceRed eyed female fly + white eyed male fly (mutant)F1 generation: All of the progeny had red eyes, the dominant trait. Leaving white as recessive.F2 generation: Crossing F1 flies, a ratio a little bit greater than 3:1. Only males have white eyes, so we can predict that the white trait is in the X. This trait is sex-linked/x-linked.White eyed male + female red eyed offspring = 1:1:1:1 Identify sex-linkage inheritance in a pedigreeWith the example of hemophilia, we realize some women carry the gene, but none is affected by it because they have to X genes; this is an X-linked recessive allele. Only males are affected by it. Explain dosage compensation and its effects on the phenotypeFemales have two X as sex chromosomes. However, they do not produce twice as much of the proteins encoded by genes on the X chromosome. One of these chromosomes is inactivated in the early embryonic development. This is an example of dosage compensation, which ensures an equal level of expression from the sex chromosomes despite a differing number of sex chromosomes in males and females.
The inactivated X chromosome is highly condensed, making it visible as an intensely staining Barr body (short and oval-shaped).Females that are heterozygous for X-chromosome are genetic mosaics: their individual cells may express different alleles, depending on which chromosome is inactivated. (Ex: calico cat) Describe the inheritance process for mitochondria and chloroplasts*Mitochondrial genes: The zygote inherits all of its mitochondria from the egg cell, which is bigger. This mode of uniparental inheritance from the mother is called maternal inheritance. A mother with a mitochondrial disease will pass it to all progeny, whereas the father won’t.*Chloroplast genes: This is usually maternal as well, although both paternal and biparental inheritance of chloroplasts may be observed in some species. The offspring receives the phenotype from the female parent, regardless of the father’s genotype.- These two do not follow the Mendelian rules.
Explain why distance in the genetic map affects recombination and its relation to crossing over during meiosisWhen proposing the idea of crossing over, Morgan hypothesized that the frequency of recombination was related to the distance between the genes on a chromosome, and that the interchange of genetic information broke the linkage between genes. The closer two genes were to one another on a chromosome, the greater their chance of being inherited together. In contrast, genes located farther away from one another on the same chromosome were more likely to be separated during recombination.- Recombination is the basis for genetic maps. Genes close together on a single chromosome are said to be linked. The farther apart two linked genes are, the greater the frequency of recombination.
A map unit is expressed as the percentage of recombination progeny.- The probability of multiple crossovers increases with distance between two genes and results in and underestimate of recombination frequency. The maximum of recombination frequency is 50%, the same as for independent assortment.- By evaluating intervening genes with less separation, more accurate distances can be obtained. Explain genomic imprinting and how it affects the phenotype Genomic imprinting depends on the parental origin of alleles. The expression of the gene depends on whether it passes through the maternal or paternal germ line. These appear to be inactivated by methylation. Imprinting produces a haploid phenotype.