Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 581:

  Aerobic respiration is the major process used by oxygen- requiring organisms to generate energy. During respiration, glucose is metabolized to generate chemical energy in the form of ATP:

  

  The biochemical machinery necessary for cellular respiration is found in the mitochondria, small organelles scattered throughout the cytoplasm of most eukaryotic cells. The number of mitochondria per cell varies by tissue type and cell function.

  Mitochondria are unusual in that they have their own genetic systems that are entirely separate from the cell's genetic material. However, mitochondrial replication is still dependent upon the cell's nuclear DNA to encode essential proteins required for replication. Despite this fact, mitochondria seem to replicate randomly, out of phase with both the cell cycle and other mitochondria.

  The nature of the mitochondrial genome and protein synthesizing machinery has led many researchers to postulate that mitochondria may have arisen as the result of the ingestion of a bacterium by a primitive cell millions of years ago. It is postulated that the two may have entered into a symbiotic relationship and eventually became dependent on each another; the cell sustained the bacterium, while the bacterium provided energy for the cell. Gradually, the two evolved into the present-day eukaryotic cell, with the mitochondrion retaining some of its own DNA. This is known as the endosymbiotic hypothesis. Because mitochondrial DNA is inherited in a non-Mendelian fashion (mitochondria are inherited from the maternal parent, who supplies most of the cytoplasm to the fertilized egg), it has been used to look at evolutionary relationships among different organisms.

  Which of the following pieces of evidence would NOT support the hypothesis that mitochondria were once independent bacteria that eventually formed a symbiotic relationship with eukaryotic cells?

  A. Mitochondrial DNA is circular and not enclosed by a nuclear membrane.

  B. Mitochondrial ribosomes more closely resemble eukaryotic ribosomes than prokaryotic ribosomes.

  C. Many present-day bacteria live within eukaryotic cells, digesting nutrients that their hosts cannot and sharing the energy thus derived.

  D. Mitochondrial DNA codes for its own ribosomal RNA.

  Correct Answer: B

  This question requires a little bit of reasoning on your part. In the passage you're told that many scientists believe that mitochondria were once independent unicellular entities, possibly prokaryotic in origin, which formed a symbiotic relationship with eukaryotic cells that was mutually beneficial to both parties. This theory is known as the endosymbiotic hypothesis. If mitochondria are believed to have been prokaryotic in origin, then any findings that reveal similarities between mitochondria and bacteria would support the hypothesis. So let's take a look at the answer choices and find the one that does NOT support the hypothesis, since that's what the question asks you to do. Choice A says that mitochondrial DNA is circular and not enclosed by a nuclear membrane, which is, in fact, true. Well, that sounds like the way things are set up in bacteria. Bacteria have a single circular chromosome located in a region of the cell known as the nucleoid, which is not bound by a membrane. So choice A is incorrect because it SUPPORTS the endosymbiotic hypothesis. Choice B states that mitochondrial ribosome more closely resemble those found in eukaryotes than those found in prokaryotes. This does NOT support the hypothesis -- any similarity to eukaryotic cells doesn't support it. So choice B appears to be the correct answer, but let's continue on anyway. Choice C says that there are many present-day bacteria that have symbiotic relationships with eukaryotic cells. This DOES support the concept that the mitochondrial ancestor could have lived within a eukaryotic cell in a mutualistic relationship. So, choice C is wrong because it supports the hypothesis. Choice D says that mitochondrial DNA codes for its own ribosomal RNA. This provides evidence that at some point, mitochondria existed as free entities, capable of directing their own protein synthesis and cell division, and all the cellular activities associated with independently living organisms. So, choice D is also incorrect. By the way, the information contained in choice B is NOT true; mitochondrial ribosomes actually DO resemble prokaryotic ribosomes, thereby providing further support for the endosymbiotic hypothesis. However, if it were true, it would not support the hypothesis, which is why choice B is the correct answer.

• Question 582:

  Aerobic respiration is the major process used by oxygen- requiring organisms to generate energy. During respiration, glucose is metabolized to generate chemical energy in the form of ATP:

  

  The biochemical machinery necessary for cellular respiration is found in the mitochondria, small organelles scattered throughout the cytoplasm of most eukaryotic cells. The number of mitochondria per cell varies by tissue type and cell function.

  Mitochondria are unusual in that they have their own genetic systems that are entirely separate from the cell's genetic material. However, mitochondrial replication is still dependent upon the cell's nuclear DNA to encode essential proteins required for replication. Despite this fact, mitochondria seem to replicate randomly, out of phase with both the cell cycle and other mitochondria.

  The nature of the mitochondrial genome and proteinsynthesizing machinery has led many researchers to postulate that mitochondria may have arisen as the result of the ingestion of a bacterium by a primitive cell millions of years ago. It is postulated that the two may have entered into a symbiotic relationship and eventually became dependent on each another; the cell sustained the bacterium, while the bacterium provided energy for the cell. Gradually, the two evolved into the present-day eukaryotic cell, with the mitochondrion retaining some of its own DNA. This is known as the endosymbiotic hypothesis. Because mitochondrial DNA is inherited in a non-Mendelian fashion (mitochondria are inherited from the maternal parent, who supplies most of the cytoplasm to the fertilized egg), it has been used to look at evolutionary relationships among different organisms.

  Four different human cell cultures -- erythrocytes, epidermal cells, skeletal muscle cells, and intestinal cells -- were grown in a medium containing radioactive adenine. After 10 days, the mitochondria were isolated via centrifugation, and their level of radioactivity was measured using a liquid scintillation counter. Which of the following cells would be expected to have the greatest number of counts per minute of radioactive decay?

  A. Erythrocytes

  B. Epidermal cells

  C. Skeletal muscle cells

  D. Intestinal cells

  Correct Answer: C

  According to the question stem, four different human cell cultures were grown in a medium containing radioactive adenine. So, the first thing that you should be thinking about is DNA replication; while these cells are replicating they're going to incorporate this radioactive adenine into all of their DNA. This includes chromosomal DNA, as well as mitochondrial DNA. Mitochondria replicate independently of their cells. Since all autosomal human cells have the same amount of DNA in their nuclei, the only difference in radioactivity will be the amount that was incorporated into the mitochondrial DNA. And this is why the cells' mitochondria were isolated via centrifugation, and then the radiation from each sample was measure using a scintillation counter. The cells with the greatest number of mitochondria will have the highest radioactive count when their mitochondria are separated. So, you need to determine which of the four cell types would have the greatest number of mitochondria. This number is dependent on the energy needs of the tissue. Given the choices -- erythrocytes, epidermal cells, skeletal muscle cells, and intestinal cells -- you should know that the correct answer is choice C, skeletal muscle cells. Muscle cells need a lot of energy in order to contract. ATP is required every time a molecule of myosin binds to actin in the sarcomeres. In general, muscle cells have a higher content of mitochondria than do any other type of autosomal cell. Choice A, erythrocytes, do not even contain any mitochondria. Choice B, epidermal cells, do not have any special energy requirements. The same thing applies to intestinal cells, choice D.

• Question 583:

  Aerobic respiration is the major process used by oxygen- requiring organisms to generate energy. During respiration, glucose is metabolized to generate chemical energy in the form of ATP:

  

  The biochemical machinery necessary for cellular respiration is found in the mitochondria, small organelles scattered throughout the cytoplasm of most eukaryotic cells. The number of mitochondria per cell varies by tissue type and cell function.

  Mitochondria are unusual in that they have their own genetic systems that are entirely separate from the cell's genetic material. However, mitochondrial replication is still dependent upon the cell's nuclear DNA to encode essential proteins required for replication. Despite this fact, mitochondria seem to replicate randomly, out of phase with both the cell cycle and other mitochondria.

  The nature of the mitochondrial genome and protein synthesizing machinery has led many researchers to postulate that mitochondria may have arisen as the result of the ingestion of a bacterium by a primitive cell millions of years ago. It is postulated that the two may have entered into a symbiotic relationship and eventually became dependent on each another; the cell sustained the bacterium, while the bacterium provided energy for the cell. Gradually, the two evolved into the present-day eukaryotic cell, with the mitochondrion retaining some of its own DNA. This is known as the endosymbiotic hypothesis. Because mitochondrial DNA is inherited in a non-Mendelian fashion (mitochondria are inherited from the maternal parent, who supplies most of the cytoplasm to the fertilized egg), it has been used to look at evolutionary relationships among different organisms.

  What is the net number of ATP molecules synthesized by an obligate anaerobe per molecule of glucose?

  A. 2 ATP

  B. 6 ATP

  C. 8 ATP

  D. 36 ATP

  Correct Answer: A

  Again, this is one of those straight knowledge questions that doesn't really have anything to do with the passage per se. Yes, it's a related topic, but you won't find out the answer by reading the passage. To answer this question, you must know how many ATP are formed from the catabolism of one molecule of glucose by an obligate anaerobe. An obligate anaerobe is an organism that must live WITHOUT oxygen in order to survive. Obligate anaerobes produce ATP via fermentation, which includes both glycolysis and the reactions necessary to regenerate the NAD+ necessary for glycolysis to continue. Fermentation leads to a net production of 2 ATP; this ATP is generated during glycolysis. Therefore, an obligate anaerobe will produce 2 ATP per molecule of glucose. Aerobic organisms produce a net of 36 ATP, choice D, per molecule of glucose, as shown in the equation provided in the passage. So, choice D is wrong

• Question 584:

  Aerobic respiration is the major process used by oxygen- requiring organisms to generate energy. During respiration, glucose is metabolized to generate chemical energy in the form of ATP:

  

  The biochemical machinery necessary for cellular respiration is found in the mitochondria, small organelles scattered throughout the cytoplasm of most eukaryotic cells. The number of mitochondria per cell varies by tissue type and cell

  function.

  Mitochondria are unusual in that they have their own genetic systems that are entirely separate from the cell's genetic material. However, mitochondrial replication is still dependent upon the cell's nuclear DNA to encode essential proteins

  required for replication. Despite this fact, mitochondria seem to replicate randomly, out of phase with both the cell cycle and other mitochondria.

  The nature of the mitochondrial genome and protein synthesizing machinery has led many researchers to postulate that mitochondria may have arisen as the result of the ingestion of a bacterium by a primitive cell millions of years ago. It is

  postulated that the two may have entered into a symbiotic relationship and eventually became dependent on each another; the cell sustained the bacterium, while the bacterium provided energy for the cell. Gradually, the two evolved into the

  present-day eukaryotic cell, with the mitochondrion retaining some of its own DNA. This is known as the endosymbiotic hypothesis. Because mitochondrial DNA is inherited in a non-Mendelian fashion (mitochondria are inherited from the

  maternal parent, who supplies most of the cytoplasm to the fertilized egg), it has been used to look at evolutionary relationships among different organisms.

  A mating type of a wild-type strain of the algae C. reinhardii is crossed with the opposite mating type of a mutant strain of the algae, which has lost all mitochondrial functions due to deletions in their mitochondrial genome. All of the offspring

  from this cross also lack mitochondrial functions. Based on information in the passage, this can best be explained by the:

  A. endosymbiotic hypothesis.

  B. non-Mendelian inheritance of mitochondrial DNA.

  C. recombination of mitochondrial DNA during organelle replication.

  D. presence of genetic material in the mitochondria that is distinct from nuclear DNA.

  Correct Answer: B

  In this question you are presented with a cross between two strains of the algae C. reinhardii. You do not need to know anything about this species of algae to answer the question. From the question stem you know that the mating type of a wild-type strain, which has normal mitochondrial DNA, is crossed with the opposite mating type of a strain that lacks functional mitochondria due to deletions in the mitochondrial genome. This whole thing about "mating types" is another way of saying male and female in species that do not technically have opposite genders, such as algae and yeast. In addition, you're told that the offspring of this cross do not have functional mito- chondria either. What does this mean? Somehow the offspring have the same deleted mitochondrial genome as the mutant strain. Now all you have to do is find the choice that best accounts for this occurrence. Choice A is incorrect because the endosymbiotic theory attempts to explain the derivation of mitochondria in eukaryotic cells, not the inheritance of mitochondria. Choice B is correct. Since you're told that the offspring lack mitochondrial functions, this implies that they inherited their mitochondria from the mutant strain mating type. In other words, the mutant strain was the organelle-donating parent -- the female -- in this cross. Therefore, the non-Mendelian inheritance pattern of mitochondria, as explained in the passage, best accounts for these experimental observations. If the mating type of the wild-type strain had been the organelle-donating parent, then all of the offspring would have normal mitochondrial function. Choice C is wrong because the word recombination implies the formation of new gene combinations due to crossing over events that occur during reproduction. If recombinations did occur, you would expect some of the offspring to regain mitochondrial functions since wild-type mitochondrial DNA would replace the deleted segments of DNA in some offspring. Although choice D is a true statement, it does not explain the inheritance patterns observed in this cross, thus choice D is incorrect.

• Question 585:

  Aerobic respiration is the major process used by oxygen- requiring organisms to generate energy. During respiration, glucose is metabolized to generate chemical energy in the form of ATP: The biochemical machinery necessary for cellular respiration is found in the mitochondria, small organelles scattered throughout the cytoplasm of most eukaryotic cells. The number of mitochondria per cell varies by tissue type and cell function.

  

  Mitochondria are unusual in that they have their own genetic systems that are entirely separate from the cell's genetic material. However, mitochondrial replication is still dependent upon the cell's nuclear DNA to encode essential proteins required for replication. Despite this fact, mitochondria seem to replicate randomly, out of phase with both the cell cycle and other mitochondria.

  The nature of the mitochondrial genome and protein synthesizing machinery has led many researchers to postulate that mitochondria may have arisen as the result of the ingestion of a bacterium by a primitive cell millions of years ago. It is postulated that the two may have entered into a symbiotic relationship and eventually became dependent on each another; the cell sustained the bacterium, while the bacterium provided energy for the cell. Gradually, the two evolved into the present-day eukaryotic cell, with the mitochondrion retaining some of its own DNA. This is known as the endosymbiotic hypothesis. Because mitochondrial DNA is inherited in a non-Mendelian fashion (mitochondria are inherited from the maternal parent, who supplies most of the cytoplasm to the fertilized egg), it has been used to look at evolutionary relationships among different organisms.

  Which of the following mitochondrial genome characteristics differs most from the characteristics of the nuclear genome?

  A. Mitochondrial DNA is a double-helix.

  B. Some mitochondrial genes code for tRNA.

  C. Specific mutations to mitochondrial DNA can be lethal to the organism.

  D. Almost every base in mitochondrial DNA codes for a product.

  Correct Answer: D

  To answer this question, you are supposed to pick the characteristic of the mitochondrial genome that differs most from characteristics of the nuclear genome. Now since you probably don't know a lot of information about the mitochondrial genome, and there is little information about it in the passage, let's approach this problem from a slightly different perspective. You do know a lot about the characteristics of the nuclear genome from your introductory biology class. So let's look over the answer choices and find the one that is not a characteristic of the nuclear genome; and this will be our right answer. The nuclear genome is comprised of double-helical DNA that codes for mRNA, tRNA, and rRNA. Therefore, choices A and B are characteristics of the nuclear genome and are therefore incorrect. If a mutation occurred in the nuclear genome that rendered an essential gene non-functional, such as an enzyme involved in glycolysis, the organism would die. Thus, choice C is also a characteristic of the nuclear genome and an incorrect choice. Although the nuclear genome encodes many products, most of the bases of DNA are NON- CODING. That is, they are involved in the regulation of gene expression and do not themselves code for any product. Therefore, choice D is not consistent with the nuclear genome, so choice D is the correct answer. The mitochondrial genome is so small, compared to that of the nucleus, that almost every nitrogen base has to code for a product, the mitochondrial genome doesn't have any DNA to waste!

• Question 586:

  Aerobic respiration is the major process used by oxygen- requiring organisms to generate energy. During respiration, glucose is metabolized to generate chemical energy in the form of ATP:

  

  The biochemical machinery necessary for cellular respiration is found in the mitochondria, small organelles scattered throughout the cytoplasm of most eukaryotic cells. The number of mitochondria per cell varies by tissue type and cell function.

  Mitochondria are unusual in that they have their own genetic systems that are entirely separate from the cell's genetic material. However, mitochondrial replication is still dependent upon the cell's nuclear DNA to encode essential proteins required for replication. Despite this fact, mitochondria seem to replicate randomly, out of phase with both the cell cycle and other mitochondria.

  The nature of the mitochondrial genome and protein synthesizing machinery has led many researchers to postulate that mitochondria may have arisen as the result of the ingestion of a bacterium by a primitive cell millions of years ago. It is postulated that the two may have entered into a symbiotic relationship and eventually became dependent on each another; the cell sustained the bacterium, while the bacterium provided energy for the cell. Gradually, the two evolved into the present-day eukaryotic cell, with the mitochondrion retaining some of its own DNA. This is known as the endosymbiotic hypothesis. Because mitochondrial DNA is inherited in a non-Mendelian fashion (mitochondria are inherited from the maternal parent, who supplies most of the cytoplasm to the fertilized egg), it has been used to look at evolutionary relationships among different organisms.

  Scientists have demonstrated that human mitochondrial DNA mutates at a fairly slow rate. Because mitochondria play such an important role in the cell, these mutations are most likely to be:

  A. point mutations.

  B. frameshift mutations.

  C. lethal mutations.

  D. nondisjunctions.

  Correct Answer: A

  This question tests your knowledge of the things that can go wrong during the replication of mitochondrial DNA, or any DNA for that matter, and the consequences. In addition, you've got to apply this knowledge to the role that mitochondria play in the cell. Mitochondria are responsible for cellular energy production -- they supply the cell with ATP. Mitochondrial DNA directs the synthesis of mitochondrial proteins, which ultimately play a major role in cell survival. Since mitochondria are so essential to eukaryotic cell life, one would therefore expect replication of its DNA to be highly accurate. Mutations that would cause a dramatic change in its DNA and its ability to produce proteins needed for ATP formation would be lethal to the cell. Since mutations do occur, the most likely type to occur would be one that causes the least damage. A point mutation fits the bill. Point mutations are defined as those in which only nitrogenous base is affected; for example, a cytosine is substituted for an adenine during replication. Point mutations are not usually lethal because of the redundancy of the genetic code; that is, each amino acid is typically coded for by more than one codon. Take the amino acid proline, for example. Proline is coded for by four codons: CCU, CCC, CCA, and CCG. Let's say that the codon is CCU; if there's a point mutation at the third base, no matter which of the remaining three bases is substituted for the uracil, the net product will still be proline. So a point mutation is the type of mutation least likely to affect their productivity. Thus, choice A is correct. Choice B, frameshift mutation, is a mutation causing genetic material to be inserted or deleted during DNA replication or transcription. This produces a shift in the reading frame of the mRNA strand being translated, usually leading to the formation of nonsense polypetides. Changes in protein synthesis would most likely be dangerous for the mitochondria and the cell itself; therefore, choice B is incorrect. Lethal mutations, choice C, are those that would cause the mitochondria to become nonfunctional, so choice C is incorrect. Choice D is incorrect because nondisjunction is the failure of homologous chromosomes to separate during meiosis. First of all, mitochondria have one circular chromosome -- there aren't any homologous chromosomes. Second, mitochrondria do not undergo meiosis -- only specialized eukaryotic cells in sexually reproducing organisms undergo meiosis. So, choice D cannot even be a consideration.

• Question 587:

  Hemophilia is a genetically inherited disease that causes the synthesis of an abnormal clotting factor. As a result, hemophiliacs bleed excessively from the slightest injury. The figure below is a partial pedigree for the hemophilia trait in Queen Victoria's descendants. The pedigree indicates no history of hemophilia for either parent prior to the F1 generation.

  

  Based on the pedigree, what is the most reasonable explanation for Rupert's hemophilia?

  A. A mutation occurred on the Y chromosome that he inherited from his father.

  B. His mother was a hemophiliac and transmitted the gene to him.

  C. His father was a carrier of the gene for hemophilia.

  D. His maternal grandfather was a hemophiliac.

  Correct Answer: D

  According to the pedigree, Rupert's father was normal, his mother, Alice was a carrier of the disease, his maternal grandmother was normal, and his maternal grandfather, Leopold, was a hemophiliac. Well, knowing this bit of family history allows us to reconstruct the transmission of the gene for hemophilia through these three generations, and also allows us to rule out choices B and C, since Alice was NOT a hemophiliac and her husband was normal. Okay, Leopold the hemophiliac necessarily passed on the hemophilia gene to his daughter Alice, since she inherited one of her X chromosomes from him. Alice's mother was normal, so Alice's other X chromosome was normal. Alice married a normal man, and gave birth to three children, a normal daughter and son, and a hemophiliac son, Rupert. Now, the reason why Rupert had hemophilia was because he inherited the X chromosome with the hemophilia gene on it from his mother, who in turn, inherited it from her father. So rupert's hemophilia is a direct result of his maternal grandfather's hemophilia, and therefore choice D is the right answer. Choice A is wrong because a mutation of the Y chromosome that Rupert inherited from his father would not affect Rupert's inheritance of hemophilia, since the gene for hemophilia is located on the X chromosome, not the Y.

• Question 588:

  Aerobic respiration is the major process used by oxygen- requiring organisms to generate energy. During respiration, glucose is metabolized to generate chemical energy in the form of ATP:

  

  The biochemical machinery necessary for cellular respiration is found in the mitochondria, small organelles scattered throughout the cytoplasm of most eukaryotic cells. The number of mitochondria per cell varies by tissue type and cell function.

  Mitochondria are unusual in that they have their own genetic systems that are entirely separate from the cell's genetic material. However, mitochondrial replication is still dependent upon the cell's nuclear DNA to encode essential proteins required for replication. Despite this fact, mitochondria seem to replicate randomly, out of phase with both the cell cycle and other mitochondria.

  The nature of the mitochondrial genome and protein synthesizing machinery has led many researchers to postulate that mitochondria may have arisen as the result of the ingestion of a bacterium by a primitive cell millions of years ago. It is postulated that the two may have entered into a symbiotic relationship and eventually became dependent on each another; the cell sustained the bacterium, while the bacterium provided energy for the cell. Gradually, the two evolved into the present-day eukaryotic cell, with the mitochondrion retaining some of its own DNA. This is known as the endosymbiotic hypothesis. Because mitochondrial DNA is inherited in a non-Mendelian fashion (mitochondria are inherited from the maternal parent, who supplies most of the cytoplasm to the fertilized egg), it has been used to look at evolutionary relationships among different organisms.

  In which of the following phases of the cell cycle could mitochondrial DNA replicate?

  I. G1

  II. S

  III. G2

  IV.

  M

  A.

  IV only

  B.

  I and III only

  C.

  II and IV only

  D.

  I, II, III, and IV

  Correct Answer: D

  In the passage you're told that mitochondria replicate in a seemingly random pattern out of phase with both other mitochondria and the cell itself. And remember that mitochondrial DNA must replicate in order for the mitochondrion itself to replicate. So basically, it can be inferred that mitochondrial DNA replicates throughout all of the phases of the cell cycle: G1, S, G2, and M. So, choice D is correct. During the G1 phase, the cell undergoes intense biochemical activity. During the S phase, or synthesis phase, the nuclear DNA replicates. During the G2 phase, the nuclear DNA condenses and the structures used during mitosis begin to assemble. During the M phase, or mitotic phase, mitosis occurs. Finally, the nuclear DNA segregates to opposite poles of the cell, the replicated organelles, mitochondria included, also segregate, along with the nuclear DNA, and the cell divides, forming two identical daughter cells.

• Question 589:

  Hemophilia is a genetically inherited disease that causes the synthesis of an abnormal clotting factor. As a result, hemophiliacs bleed excessively from the slightest injury. The figure below is a partial pedigree for the hemophilia trait in Queen Victoria's descendants. The pedigree indicates no history of hemophilia for either parent prior to the F1 generation.

  

  Theoretically, what percentage of Victoria Eugenia's sons should have been hemophiliacs?

  A. 25%

  B. 33%

  C. 50%

  D. 75%

  Correct Answer: C

  If you look at the pedigree, you'll see that Victoria Eugenia had four sons, three of which were hemophiliacs. So, 75% of her sons turned out to be hemophiliacs. You know that these sons inherited the disease from their mother, as opposed to their father, Alfonso III of Spain, because their mother was a carrier of the disease, as can be seen on the pedigree, and their father was normal. Okay, so 75% was the actual percent, but the question asks you to determine the theoretical probability that Victoria Eugenia's sons would be hemophiliacs. So, what you have to do is just work out the cross between a carrier female and a normal male and look at the results. And, if you do that, you'll find that theoretically, 50% of Victoria Eugenia's daughters should have been carriers, 50% should have been normal; and 50% of her sons should have been hemophiliacs, 50% should have been normal. So the probability that Victoria Eugenia's sons would have been hemophiliacs is 50%.

• Question 590:

  Hemophilia is a genetically inherited disease that causes the synthesis of an abnormal clotting factor. As a result, hemophiliacs bleed excessively from the slightest injury. The figure below is a partial pedigree for the hemophilia trait in Queen Victoria's descendants. The pedigree indicates no history of hemophilia for either parent prior to the F1 generation.

  

  Which of the following best explains why Louis IV was NOT a hemophiliac?

  A. His son Frederick was a hemophiliac.

  B. He did not inherit the gene for hemophilia from his mother.

  C. His father-in-law, Albert, was not a hemophiliac.

  D. Only females can be carriers of the gene for hemophilia.

  Correct Answer: B

  A male can inherit an X-linked trait only from his mother, since he inherits his one X chromosome from her. Thus, Louis IV, who was normal, did not inherit the gene for hemophilia from his mother, who is not shown on the pedigree. Remember, Louis IV was not a blood relative of the Queen, he was her son-in-law. Choice A is incorrect, since Louis IV's son, Frederick, has hemophilia because he inherited the gene from his mother, who was a carrier (although we don't know her name). Besides, whether Louis IV's children were hemophiliacs, carriers, or normal, is irrelevant to the discussion of why Louis himself was normal. Choice C is incorrect because Albert was not a blood relative of Louis IV, so Albert's genotype is independent of Louis' genotype. As for choice D, while it is true that only females can be carriers of the hemophilia gene, or of any X-linked gene for that matter, this does not answer the question.