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You have just read about two pathways in cellular respiration—glycolysis and the citric acid cycle—that generate ATP. Rather, it is derived from a process that begins with moving electrons through a series of electron transporters that undergo redox reactions: the electron transport chain. The current of hydrogen ions powers the catalytic action of ATP synthase, which phosphorylates ADP, producing ATP.
The electron transport chain (Figure 1) is the last component of aerobic respiration and is the only part of glucose metabolism that uses atmospheric oxygen. Oxygen continuously diffuses into plants; in animals, it enters the body through the respiratory system. Electron transport is a series of redox reactions that resemble a relay race or bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain where the electrons reduce molecular oxygen, producing water. There are four complexes composed of proteins, labeled I through IV in Figure 1, and the aggregation of these four complexes, together with associated mobile, accessory electron carriers, is called the electron transport chain. The electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and the plasma membrane of prokaryotes. Note, however, that the electron transport chain of prokaryotes may not require oxygen as some live in anaerobic conditions. The common feature of all electron transport chains is the presence of a proton pump to create a proton gradient across a membrane.
To start, two electrons are carried to the first complex aboard NADH. This complex, labeled I, is composed of flavin mononucleotide (FMN) and an iron-sulfur (Fe-S)-containing protein. FMN, which is derived from vitamin B2, also called riboflavin, is one of several prosthetic groups or co-factors in the electron transport chain. A prosthetic group is a non-protein molecule required for the activity of a protein. Prosthetic groups are organic or inorganic, non-peptide molecules bound to a protein that facilitate its function; prosthetic groups include co-enzymes, which are the prosthetic groups of enzymes. The enzyme in complex I is NADH dehydrogenase and is a very large protein, containing 45 amino acid chains. Complex I can pump four hydrogen ions across the membrane from the matrix into the intermembrane space, and it is in this way that the hydrogen ion gradient is established and maintained between the two compartments separated by the inner mitochondrial membrane.
Q and Complex II
Complex II directly receives FADH2, which does not pass through complex I. The compound connecting the first and second complexes to the third is ubiquinone (Q). The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane. Once it is reduced, (QH2), ubiquinone delivers its electrons to the next complex in the electron transport chain. Q receives the electrons derived from NADH from complex I and the electrons derived from FADH2 from complex II, including succinate dehydrogenase. This enzyme and FADH2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass and thus do not energize the proton pump in the first complex, fewer ATP molecules are made from the FADH2 electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane.
The third complex is composed of cytochrome b, another Fe-S protein, Rieske center (2Fe-2S center), and cytochrome c proteins; this complex is also called cytochrome oxidoreductase. Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. As a result, the iron ion at its core is reduced and oxidized as it passes the electrons, fluctuating between different oxidation states: Fe++ (reduced) and Fe+++ (oxidized). The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, giving slightly different characteristics to each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes (cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time).
The fourth complex is composed of cytochrome proteins c, a, and a3. This complex contains two heme groups (one in each of the two cytochromes, a, and a3) and three copper ions (a pair of CuA and one CuB in cytochrome a3). The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to make water (H2O). The removal of the hydrogen ions from the system contributes to the ion gradient used in the process of chemiosmosis.
In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions (protons) across the membrane. The uneven distribution of H+ ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient), owing to the hydrogen ions’ positive charge and their aggregation on one side of the membrane.
If the membrane were open to diffusion by the hydrogen ions, the ions would tend to diffuse back across into the matrix, driven by their electrochemical gradient. Recall that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase (Figure 2). This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.
Dinitrophenol (DNP) is an uncoupler that makes the inner mitochondrial membrane leaky to protons. It was used until 1938 as a weight-loss drug. What effect would you expect DNP to have on the change in pH across the inner mitochondrial membrane? Why do you think this might be an effective weight-loss drug?
[reveal-answer q=”346232″]Show Answer[/reveal-answer]
[hidden-answer a=”346232″]After DNP poisoning, the electron transport chain can no longer form a proton gradient, and ATP synthase can no longer make ATP. DNP is an effective diet drug because it uncouples ATP synthesis; in other words, after taking it, a person obtains less energy out of the food he or she eats. Interestingly, one of the worst side effects of this drug is hyperthermia, or overheating of the body. Since ATP cannot be formed, the energy from electron transport is lost as heat.[/hidden-answer]
Chemiosmosis (Figure 3) is used to generate 90 percent of the ATP made during aerobic glucose catabolism; it is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation. Recall that the production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation. The overall result of these reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen attract hydrogen ions (protons) from the surrounding medium, and water is formed.
Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you expect the pH of the intermembrane space to increase or decrease? What effect would cyanide have on ATP synthesis?
[reveal-answer q=”26135″]Show Answer[/reveal-answer]
[hidden-answer a=”26135″]After cyanide poisoning, the electron transport chain can no longer pump electrons into the intermembrane space. The pH of the intermembrane space would increase, the pH gradient would decrease, and ATP synthesis would stop.[/hidden-answer]
The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport chain complexes can pump through the membrane varies between species. Another source of variance stems from the shuttle of electrons across the membranes of the mitochondria. (The NADH generated from glycolysis cannot easily enter mitochondria.) Thus, electrons are picked up on the inside of mitochondria by either NAD+ or FAD+. As you have learned earlier, these FAD+ molecules can transport fewer ions; consequently, fewer ATP molecules are generated when FAD+ acts as a carrier. NAD+ is used as the electron transporter in the liver and FAD+ acts in the brain.
Another factor that affects the yield of ATP molecules generated from glucose is the fact that intermediate compounds in these pathways are used for other purposes. Glucose catabolism connects with the pathways that build or break down all other biochemical compounds in cells, and the result is somewhat messier than the ideal situations described thus far. For example, sugars other than glucose are fed into the glycolytic pathway for energy extraction. Moreover, the five-carbon sugars that form nucleic acids are made from intermediates in glycolysis. Certain nonessential amino acids can be made from intermediates of both glycolysis and the citric acid cycle. Lipids, such as cholesterol and triglycerides, are also made from intermediates in these pathways, and both amino acids and triglycerides are broken down for energy through these pathways. Overall, in living systems, these pathways of glucose catabolism extract about 34 percent of the energy contained in glucose.
The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor of the electrons removed from the intermediate compounds in glucose catabolism. The electron transport chain is composed of four large, multiprotein complexes embedded in the inner mitochondrial membrane and two small diffusible electron carriers shuttling electrons between them. The electrons are passed through a series of redox reactions, with a small amount of free energy used at three points to transport hydrogen ions across a membrane. This process contributes to the gradient used in chemiosmosis. The electrons passing through the electron transport chain gradually lose energy, High-energy electrons donated to the chain by either NADH or FADH2 complete the chain, as low-energy electrons reduce oxygen molecules and form water. The level of free energy of the electrons drops from about 60 kcal/mol in NADH or 45 kcal/mol in FADH2 to about 0 kcal/mol in water. The end products of the electron transport chain are water and ATP. A number of intermediate compounds of the citric acid cycle can be diverted into the anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids. These same molecules can serve as energy sources for the glucose pathways.
8.6: Electron Transport Chain - Biology
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Embedded in multiple folds of the inner membrane of mitochondria are numerous copies of the electron transport chain, a series of four protein complexes and associated organic molecules that are critical for extracting energy. Electrons enter the chain using the carrier molecules nicotinamide adenine dinucleotide, or NADH, and flavin adenine dinucleotide, or FADH2, which are produced during the citric acid cycle.
To begin, NADH carries two electrons into complex I, oxidizing NADH to NAD+. These electrons are transferred to the cofactor flavin mononucleotide, or FMN, which is then oxidized as it passes the electrons to an iron-sulfur protein. The cluster then passes the electrons to a carrier molecule, ubiquinone, or Q, which uptakes two protons as it carries the electrons to complex III. As a result of the energy release, four protons are actively pumped through complex I into the intermembrane space, producing a proton gradient across the inner membrane.
FADH2 carries two electrons directly to complex II, oxidizing FADH2 to FAD+. These electrons are transferred to another iron-sulfur protein, and then to the carrier Q which also uptakes two protons from the mitochondrial matrix as it carries the electrons to complex III.
In the third complex, there's a sequence of electron transfers known as the Q cycle. First, an electron is transferred from Q to an iron-sulfur protein, then the two protons carried by Q are pumped into the intermembrane space. After passing through an intermediate cytochrome molecule called cytochrome C1 the electron passes to and reduces a cytochrome c electron carrier. Next, the second electron carried by Q is passed to a cytochrome b complex, and then to a Q molecule, which then binds two protons from the matrix. Now another Q molecule binds complex III, and the first part of the cycle is repeated, pumping two more protons into the intermembrane space for a total of four protons per Q cycle. The second electron from the newly-bound Q molecule is transferred to cytochrome b, and then to the Q molecule that previously received an electron. Now that this Q has two electrons, it's released from complex III and can donate its electrons in a new Q cycle.
Finally, the cytochrome c electron carriers attach to complex IV, and two electrons reduce a cytochrome a3 molecule and a copper atom, allowing an oxygen molecule to bind. Once the oxygen molecule is completely reduced, it picks up four hydrogen ions and splits to form two molecules of water. During this process, four more protons are pumped into the intermembrane space. Thus, the electron transport chain creates a proton gradient by pumping protons into the intermembrane space of the mitochondria. These protons can then flow back down the gradient into the mitochondrial matrix through ATP synthases, generating ATP in a process known as chemiosmosis. The oxidized electron carriers can return to the citric acid cycle to pick up more electrons.
8.9: Electron Transport Chains
The final stage of cellular respiration is oxidative phosphorylation, which consists of (1) an electron transport chain and (2) chemiosmosis.
The electron transport chain is a set of proteins and other organic molecules found in the inner membrane of mitochondria in eukaryotic cells and the plasma membrane of prokaryotic cells. The electron transport chain has two primary functions: it produces a proton gradient&mdashstoring energy that can be used to create ATP during chemiosmosis&mdashand generates electron carriers, such as NAD + and FAD, that are used in glycolysis and the citric acid cycle.
Generally, molecules of the electron transport chain are organized into four complexes (I-IV). The molecules pass electrons to one another through multiple redox reactions, moving electrons from higher to lower energy levels through the transport chain. These reactions release energy that the complexes use to pump H + across the inner membrane (from the matrix into the intermembrane space). This forms a proton gradient across the inner membrane.
NADH and FADH2 are reduced electron carriers produced during earlier cellular respiration phases. NADH can directly input electrons into complex I, which uses the released energy to pump protons into the intermembrane space. FADH2 inputs electrons into complex II, the only complex that does not pump protons into the intermembrane space. Thus, FADH2 contributes less to the proton gradient than NADH. NADH and FADH2 are converted back into electron carriers NAD + and FAD, respectively.
Both NADH and FADH2 transfer electrons to ubiquinone, a mobile electron carrier that passes the electrons to complex III. From there, the electrons are transferred to the mobile electron carrier cytochrome c (cyt c). Cyt c delivers the electrons to complex IV, which passes them to O2. Oxygen breaks apart, forming two oxygen atoms that each accept two protons to form water.
Guo, Runyu, Shuai Zong, Meng Wu, Jinke Gu, and Maojun Yang. &ldquoArchitecture of Human Mitochondrial Respiratory Megacomplex I2III2IV2.&rdquo Cell 170, no. 6 (September 7, 2017): 1247-1257.e12. [Source]
Biology: Electron Transport Chain Help
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6.3.7: Electron Transport Chain
- Contributed by Brian Lindshield
- Associate Prof (Department of Food, Nutrition, Dietetics and Health) at Kansas State University
The electron transport chain is located on the inner membrane of the mitochondria, as shown below.
Figure 6.261 The pathways involved in aerobic respiration 1
The electron transport chain contains a number of electron carriers. These carriers take the electrons from NADH and FADH2, pass them down the chain of complexes and electron carriers, and ultimately produce ATP. More specifically, the electron transport chain takes the energy from the electrons on NADH and FADH2 to pump protons (H + ) into the intermembrane space. This creates a proton gradient between the intermembrane space (high) and the matrix (low) of the mitochondria. ATP synthase uses the energy from this gradient to synthesize ATP. Oxygen is required for this process because it serves as the final electron acceptor, forming water. Collectively this process is known as oxidative phosphorylation. The following figure and animation do a nice job of illustrating how the electron transport chain functions.
Figure 6.262 Location of the electron transport chain in the mitochondria 2
2.5 ATP/NADH and 1.5 ATP/FADH2 are produced in the electron transport chain. Some resources will say 3 ATP/NADH and 2 ATP/FADH2, but these values are generally less accepted now.
For one molecule of glucose, the preceding pathways produce:
Transition Reaction: 2 NADH
Citric Acid Cycle: 6 NADH, 2 FADH2
Multiply that by the amount of ATP per NADH or FADH2 to yield:
10 NADH X 2.5 ATP/NADH = 25 ATP
2 FADH2 X 1.5 ATP/FADH2 = 3 ATP
The first video does a nice job of illustrating and reviewing the electron transport chain. Note that it uses 3 ATP/NADH and 2 ATP/FADH2 so the totals from each cycle are different from those listed above. The second video is a great rap video explaining the steps of glucose oxidation.
ROS Can Modify and Damage Lipids, Proteins, and DNA
Peroxidation of polyunsaturated fatty acids by a ROS attack can lead to chain breakage and shortening, which will increase membrane fluidity and permeability. When isolated mammalian mitochondria are exposed to oxidative stress, the membrane phospholipid diphosphatidylglycerol is damaged, presumably via peroxidation of the polyunsaturated fatty acids 18:2 and 18:3, which are the main fatty acids in this lipid. The damage to diphosphatidylglycerol causes an inhibition of cytochrome c oxidase, which appears to require this phospholipid for activity (Paradies et al. 2000).
Proteins can be modified/damaged by ROS either through direct chemical interaction or indirectly, involving end products of lipid peroxidation. A number of amino acids can be modified for example, cysteine can be oxidized to cystine, and both proline and arginine are converted to glutamyl semialdehyde. Such modifications can affect the function of proteins. In some cases, the damaged amino acids are repaired in situ, whereas in other cases, the entire protein is removed and degraded (Dean et al. 1997 Møller and Kristensen 2004 Møller et al. 2007). A number of oxidized proteins have been identified in isolated plant mitochondria, presumably because they are particularly susceptible to oxidative damage in vivo (Kristensen et al. 2004 Møller and Kristensen 2006). We still do not understand the metabolic significance of this oxidation.
Breakdown products of lipid peroxidation, notably 4-hydroxy-2-nonenal (HNE), affect several mitochondrial processes. Decarboxylating dehydrogenases, such as glycine decarboxylase, are inhibited by HNE because it specifically binds to, and inactivates, lipoic acid, an essential cofactor for these enzymes (Millar and Leaver 2000). HNE inhibits the alternative oxidase, so an increasing proportion of the enzyme may become inactivated during oxidative stress. The induction of alternative oxidase gene expression during stress might therefore be necessary to maintain the activity of the enzyme in the face of increasing inactivation rather than to upregulate its activity (Winger et al. 2005). Interestingly, HNE stimulates the uncoupling protein (Smith et al. 2004), which will prevent overreduction of the electron transport chain and thus lower ROS production. The sensitivity of the alternative oxidase to HNE might explain the presence of both of these energy-wasteful enzymes in plant mitochondria (see Web Topic 12.3).
One of the theories of ageing is that oxidatively modified proteins accumulate over time. This does not appear to be the case in Arabidopsis, where the amount of proteins with free carbonyl groups increases during the vegetative phase, but decreases dramatically during the flowering and senescence phases (Johansson et al. 2004).
Finally, ROS can cause mutations in mtDNA. During aging in mammals, mutations in mtDNA accumulate faster than in nuclear DNA, possibly because mtDNA is closer to the site of ROS synthesis. However, plant mtDNA does not have a particularly high rate of mutation in fact, rearrangements are more common. Little is known about ROS-induced DNA modifications in plant mitochondria.
As mentioned in the introduction, hydrogen peroxide could be a messenger from the mitochondria to the nucleus. If formed in the matrix, the hydrogen peroxide would have to cross the inner membrane and this might happen via aquaporins, channel-forming proteins (Bienert et al. 2007). However, ROS-modified fatty acids or proteins or their fragments could also act as messengers (Møller and Sweetlove 2010).
The energetics of electron transport
In discussing the driving forces of electron transport above, we have referred to both the free energy and the redox potential. Before considering the energetics of the respiratory chain in more detail, we will briefly review how exactly these two physical terms relate to one another.
Redox reactions can be compartmentalized to produce a measurable voltage
This slide illustrates the experimental setup for measuring the redox potential of an electron carrier. Left panel: coenzyme Q withdraws electrons from the standard hydrogen electrode, which by definition gives it a positive redox potential (Δ E ). Right: NADH pushes electrons toward the standard electrode, making its Δ E negative.
In the experimental setup, the molecule of interest and a reference solute are contained in two adjacent buffer-filled chambers. Platinum electrodes are immersed in both solutions and connected through a voltmeter ( V ). As electrons are withdrawn from the solute in one chamber and delivered to the other, the voltmeter indicates the direction and magnitude of the potential difference. Protons and other ions can flow across a salt bridge between the chambers so as to preserve electroneutrality. In order to allow the flow of ions but prevent mixing of the chamber contents by convection, this hole is covered with a porous membrane or plugged with agar.
The reference solute commonly used in chemistry is H2 , equilibrated with hydrogen gas at 1 atm above the solution. The corresponding oxidized form, H + , is adjusted to 1 mol/l or pH 0. The immersed platinum electrode not only conducts electrons but also serves as a catalyst for the interconversion between H2 and H + .
The potential of a redox carrier measured against this electrode is defined as its standard redox potential or Δ E 0. For biochemical purposes, the standard electrode solution is buffered at pH 7 rather than pH 0, and the redox potentials measured against this electrode are referred to as Δ E 0 ′ . A pH of 7 is just as arbitrary a reference point as pH 0, but we will stick with it because the textbooks do so, too.
The redox potential (Δ E ) is proportional to the free energy (Δ G )
|Δ G||≡||(frac< ext|
|Δ E||≡||(frac< ext|
|Δ G||=||(frac< ext|
|ΔG||=||(Delta E imes frac< ext|
|Δ G||=||(- Delta E imes n imes ext ||(6.1)|
From the previous slide, it is clear that electrons will flow spontaneously from one redox cofactor to another if the corresponding Δ E is positive. We also know that reactions proceed spontaneously if their Δ G is negative. The two parameters are directly related to one another according to equation 6.1 . Either one is therefore sufficient to describe the energetics of the reaction the reason why redox potentials are more commonly used in this context is that they can be measured more directly than Δ G .
In the equation, Δ E is the difference in the redox potentials between two cofactors. The parameter n is the number of electrons transferred in the reaction for example, NADH feeds two electrons at a time into the chain, which means that n equals two for this reaction. In contrast, heme typically accepts and donates single electrons, which means that n =1. The F in the equation is Faraday’s constant, which tells us how many units of charge are carried by one mole electrons (96,500 coulombs/mol). 36 One can think of a cofactor’s redox potential as its affinity for electrons—the higher it is, the more strongly the cofactor will attract electrons. 37
Redox potentials and free energies in the respiratory chain
This slide shows the redox potentials, and the corresponding free energy levels, of some selected electron carriers in the respiratory chain. The lowest potential is found with NAD + , in keeping with its position at the start of the transport chain. The next carrier in sequence, FMN, is part of complex I. It has a slightly higher potential than NADH and is therefore able to accept its electrons. The redox potential increases continuously along the respiratory chain to reach its highest value at oxygen, which therefore has the highest affinity for the electrons and gets to keep them. Reduced oxygen, which recombines with protons to yield water, thus is the end product of respiration.
The iron-sulfur cluster N2, which occupies the lowermost position within complex I as shown in slide 6.4 , has a significantly higher potential than the FMN. This step in potential corresponds to a significant amount of free energy that is released at some point within complex I as the electrons travel through it from FMN toward N2. Complex I uses this energy to expel protons from the mitochondrion, against their concentration gradient. Major steps in potential that drive proton expulsion also occur within complex III and complex IV.
Only minor steps of potential occur in the delivery of electrons from complex I to complex III via coenzyme Q, and between complexes III and IV via cytochrome C. Likewise, with complex II, the potentials of both entry and exit points must fall into the narrow interval between FADH2 and coenzyme Q, which means that very little energy is released as electrons traverse this complex. Such minor steps in redox potential suffice to jog the electrons along, but they are too small to contribute to proton pumping.
Cellular Respiration Stage II: The Krebs Cycle
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid), which are then converted to acetyl CoA during the short transition reaction. These molecules enter the matrix of a mitochondrion, where they start the Krebs cycle (also known as the Citric Acid Cycle). The reason this stage is considered a cycle is because a molecule called oxaloacetate is present at both the beginning and end of this reaction and is used to break down the two molecules of acetyl CoA. The reactions that occur next are shown in Figure 4.10.6.
Figure 4.10.6 Reactants and products of the Krebs Cycle.
The Krebs cycle itself actually begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate) (see Figure 4.10.6). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle.
After citric acid forms, it goes through a series of reactions that release energy. The energy is captured in molecules of NADH, ATP, and FADH2, another energy-carrying coenzyme. Carbon dioxide is also released as a waste product of these reactions.
The final step of the Krebs cycle regenerates OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvic acid molecules when it splits glucose.
CHAPTER 8, HEREDITY
1. C Hybrids are organisms that have one of each allele, Ss. When two hybrids cross the offspring are SS, Ss, Ss, and ss. Therefore the percent of the offspring that will possess the same genotype as their parents is 50 percent.
2. B This question is a bit tricky. In the chapter we discussed two independently assorting traits, such as AB or ab. This question is asking about three independently assorting traits. The first thing to do is to write out the gametes that are possible. They are, ABC, ABc, AbC, Abc, aBC, aBc, abC, and abc. Notice that only one out of the eight gametes is recessive. Another way to do this problem is to use this equation, x = 2n, where n represents the number of independently assorting traits. In our example, for three traits, there are 23 = 8 gametes produced. For four independently assorting traits, there are 24 =16 gametes. In all of these examples, the number of gametes that are recessive is always 1. Therefore, for three independently assorting traits, 1 out of 8 is recessive.
3. E Sex-linked traits are traits that almost always exist on the X chromosome. They are therefore often passed from mother to son since sons must receive the X chromosome from their mothers. (A), Many traits can skip a generation. (B), Some diseases that have carriers are not sex-linked. (C), Just because a trait appears in all the offspring, it doesn’t necessarily mean that the trait is sex-linked. (D), Sex-linked traits do not necessarily have to be passed from mothers to daughters. Daughters can inherit a good X from their fathers.
4. C The total number of offspring produced is 400. The number of offspring that exhibit the recessive trait is 81. This means that roughly 25 percent of the offspring show the recessive trait. Do a Punnett square to determine the genotype of the parents, Bb × Bb = BB, Bb, Bb, and bb. This would yield phenotypes in approximately the proportions described above (3,1).
5. A Use the product rule to solve this problem, (1/2)(1/2)(1/2) = 1/8.
6. C When two different alleles are present in regard to a trait, the organism is a heterozygote.
7. A The physical appearance of an organism is called the phenotype. In contrast, the genetic makeup is called the genotype.
8. B When two different alleles are both expressed, this is an example of codominance.
9. D A dihybrid cross is a cross that involves two traits that are independently assorting, such as tall, green pea plants versus short, yellow pea plants.
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