to Board-Format Practice Questions
Chapter 1
1. B is the correct answer. The other answers are incorrect because:
A. The genome includes DNA outside of the nucleus, such as in the mitochondria.
C. It is the DNA content, not the protein content, of a cell that is its genome.
D. The correlation of genomic size and the amount of information encoded has a poor correlation especially in more complex organisms.
E. You know why.
2. D is the correct answer. The other answers are incorrect because:
A. The current best estimate for humans is 22,000 functioning genes.
B. One of the many surprises has been the marked homology of genes among species.
C. Although the overall sequence has been identified, the actual assignment of sequences to specific genes (and diseases) is nowhere close to complete.
E. The information from the genome project has been made widely available to the public.
3. D is the correct answer. The other answers are incorrect because:
A. If diagnostics and therapies are individualized, costs should go down from less ‘trial and error’ complications.
B. Personalized medicine is not a theoretical science. It has direct clinical application in current day medical practice.
C. This should absolutely improve diagnostics—the launching point of individualized treatments.
E. Personalized medicine is much more applicable in tertiary (and quaternary) prevention.
4. C is the correct answer. The other answers are incorrect because:
A. Medical genetics interacts with all other specialties in a very real sense.
B. Is a recognized specialty in the AMA.
D. In the early days of medical genetics, the primary ‘job’ of the medical geneticist was working with multiple anomaly syndromes. In the current era, it goes into many other avenues of medicine including work with the “common” disorders.
E. Sensational terms are not well accepted by patients and families, so care should be given to using person-friendly language.
Chapter 2
1. A is the correct answer. Different types of RNA play a role in an amazing breadth of functions. RNA participates in DNA replication as described in the text: “DNA polymerase III can use the 3′-OH position of an RNA nucleotide as a point for attaching the first DNA nucleotide.” RNA is not involved in the other four listed processes.
2. C is the correct answer. Pleiotropy refers to multiple effects (consequences) of a genetic change. The other answers are incorrect because:
A. Describes variable expression.
B. Describes incomplete penetrance.
D. Describes site-specific mutation expression.
E. Describes a benign polymorphism.
3. B is the correct answer. The other answers are incorrect because:
A. By definition, pseudogenes do not code for any known functional protein.
C. Gene amplification is a function of the degree of gene expression but does not add diversity.
D. Actually, alternative splicing is what produces increased diversity.
E. Epimerases have nothing to do with this. Epigenetic modifications do. Just a foil with a similar sounding word.
4. D is the correct answer. The other answers are incorrect because:
A. As stated earlier, pleiotropy refers to multiple effects (consequences) of a genetic change.
B. Linkage relates to different loci, not changes in the same gene.
C. Co-dominance is defined as the expression of both alleles independently of each other.
E. This example refers to changes in a single gene, not changes in multiple genes that sit close to each other.
Chapter 3
1. D is the correct answer.
Single transverse palmar creases would meet the definition of a “minor malformation.” It is an abnormality of early (first trimester) development. It is sometimes present in normal individuals. It has no clinical significance.
2. D is the correct answer.
Since porencephaly results from normal cells that are lost due to injury, this would be a disruption.
3. E is the correct answer.
The fourth major category of congenital anomalies is dysplasia (abnormal histogenesis).
4. C is the correct answer. The other answers are incorrect because:
A. The incidence of congenital anomalies is 5%. This is not uncommon. Every practitioner will see patients with birth defects.
B. The overall incidence of congenital anomalies has been very stable at 4 to 6% for decades. For some of the subsets, there have been some changes. Congenital heart disease seems to be increasing. Neural tube defects seem to be decreasing.
D. There are slight regional differences in the reported incidence of congenital anomalies. Higher rates have been reported in the South and parts of the Midwest.
E. When a single congenital anomaly is seen, there is a 50% chance that a second anomaly exists in the same person.
5. B is the correct answer.
Because the kidneys did not form, this represents a malformation (renal agenesis). With low amniotic fluid levels, the fetus is not able to move about the womb freely. The facial changes come from the face being flattened by contact with the uterus. This would be a deformation.
Chapter 4
1. C is the correct answer. The other answers are incorrect because:
A. Like most early concepts in genetics, newer information has shown that the original suppositions need to be changed/updated.
B. The concept is not completely wrong. It does hold true for selected conditions such as inborn errors of metabolism.
D. The principles are the same in plants and animals.
E. Few human diseases are this simple.
2. C is the correct answer. The other answers are incorrect because:
A. The key theme of this chapter is that genetics is not just about protein coding.
B. Gene-gene interactions are common.
D. Gene expression can occur outside of the nucleus, for instance in the mitochondria.
E. Changes in non-coding sequences can affect gene expression.
3. A is the correct answer. The other answers are incorrect because:
B. The centromere is largely composed of alpha satellite material.
C. Changes in satellite DNA can produce many different genetic syndromes.
D. Some types of satellite DNA are classified as mini- and micro-satellites.
E. Satellite DNA is notoriously heterogeneous. This makes it particularly attractive for marker or linkage studies.
4. C is the correct answer. The other answers are incorrect because:
A. As the name implies, the gene for fragile X is on the X chromosome.
B. Fragile X shows X-linked semi-dominant inheritance.
D. Although folate deficient media will express the fragile site, the condition is caused by an expanding tri-nucleotide repeat, not folic acid deficiency.
E. Women are also affected with fragile X.
Chapter 5
1. C is the correct answer. The other answers are incorrect because:
A. Chromosome aneuploidies are not rare at conception. They occur in about 50% of all conceptions.
B. Chromosome abnormalities are frequently lost as miscarriages. The numbers decrease as pregnancy progresses.
C. Trisomy 13, 18 and 21 are compatible with post-natal life.
D. A 45X karyotype is a human monosomy associated with Turner syndrome.
2. E is the correct answer. The other answers are incorrect because:
The incidence of chromosome abnormalities at the time of conception is 50% (1/2).
3. A is the correct answer. The other answers are incorrect because:
A. Turner syndrome is the only whole chromosome monosomy seen in live births in humans.
B. The most common congenital heart disease in Turner syndrome is coarctation of the aorta.
C. Turner syndrome only occurs in females.
D. Girls with Turner syndrome condition are usually short for age.
E. The apostrophe “s” does not go on the end of an eponym.
4. C is the correct answer. The other answers are incorrect because:
A. Chromosome anomalies are common. They have tremendous clinical significance.
B. Chromosomal anomalies show a maternal age effect.
C. In live born infants, aneuploidies for the sex chromosomes occur more commonly than for the autosomes.
D. Whole chromosome aneuploidies can be seen in patients for only the sex chromosomes and autosomes 13, 18, and 21.
E. The most common clinical outcome in a chromosome abnormality is a spontaneous miscarriage.
5. C is the correct answer. The other answers are incorrect because:
A. Clinical syndromes are usually associated with heterozygosity of the deletion.
B. They usually track through a family like an autosomal dominating trait.
C. They usually occur sporadically, but may be familial.
D. They typically show marked variability in the phenotype.
E. Diagnosis by FISH analysis is more sensitive and practical than chromosome testing.
Chapter 6
1. A is the correct answer. The condition is recessive because homozygous individuals are affected. Among the homozygotes, only males are affected. The other answers are incorrect because:
B. Heterozygotes are not affected so it is not dominant.
C. Heterozygotes are not affected so it is not dominant.
D. Heterozygotes are not affected so it is not dominant.
E. There is nothing that suggests maternal/mitochondrial inheritance.
2. D is the correct answer. The increased incidence of chromosome aneuploidy seen with advanced maternal age is due to an increased rate on chromosomal non-disjunction (Chapter 5). The other answers are incorrect because:
A. Older parents have a higher incidence of congenital anomalies.
B. Older fathers have an increased incidence of single gene mutations.
C. Older mother have an increased incidence of chromosome aneuploidy (Chapter 5).
E. Transcription errors increase with advancing paternal age.
3. C is the correct answer. The multiple features (digital, neurologic, genital) are pleiotropic effects of the mutations in this disorder. The other answers are incorrect because:
A. Since patients tend to have a “similar” phenotype, this refers to less variability.
B. Only one gene responsible means locus homogeneity (although there is allelic heterogeneity).
D. Thresholding (incomplete penetrance) is not an issue since all recessive heterozygotes show expression.
E. Lyonization affects X-linked genes, not autosomal genes.
4. D is the correct answer. Males affected with an X-linked dominant condition will only transmit their Y chromosome or their only X with the mutation. Thus all of his daughters will be affected. The other answers are incorrect because:
A. Although the example of Craniofrontonasal Dysplasia in the Clinical Correlation section of this chapter is an example of an X-linked trait that has greater expression in females, this represents the rare exception. For the vast majority of X-linked conditions, males demonstrate more severe clinical expression than females.
B. Females can transmit an X chromosome to their female offspring. In X-linked dominant conditions affected mothers have a 50% chance of having affected daughters.
C. Although the degree of expression would be more in males, the number of affected males and females would be the same.
E. Mitochondrial inheritance (Chapter 13) is such that only females can transmit the condition. Clearly from answer ‘D’ above, descendants of affected males can be affected.
5. B is the correct answer. Dominant conditions typically have more variable expression than recessive disorders. The other answers are incorrect because:
A. In general, dominant conditions are more frequent than recessive conditions. As discussed in this chapter, recessive conditions may be more frequent due to phenomena such as heterozygote advantage or a founder effect. Still they are typically much less common.
C. Incomplete penetrance is typically observed with dominant conditions. This is due to the fact that recessive conditions have less variability. In theory, it is possible that a recessive condition may have incomplete penetrance. We are not aware of any such documented example.
D. Pleiotropism can be seen in dominant or recessive conditions (as per the example of SLO in question 3.
E. Holandric inheritance refers to Y-linked inheritance. There are no confirmed Y-linked disorders in humans.
Chapter 7
1. B is the correct answer.
DNA repair abnormalities increase the risk of cancer, but produce genome instability, premature aging, and immune deficiency. Albinism is not a common feature.
2. C is the correct answer.
There is a polymorphism so it is not a normal sequence. Since it has not been seen in individuals with disease it would be a known benign polymorphism. It is not pathogenic nor is it unknown. Choice E is made up.
3. E is the correct answer.
Mutations are common events and they are unevenly distributed across the genome (hence the term ‘hot spots’). Mutations that are induced versus those that occur spontaneously are no different in their impact. As demonstrated in the clinical correlation section, therapies may be able to ‘correct’ a mutation.
4. A is the correct answer.
Mutagens may be carcinogenic and they may be teratogenic. However, the terms are not interchangeable. Man naturally occurring mutagens exist such as viruses and UV radiation from sunlight. Mutagens may be avoided. Precautions may be taken to avoid exposures to viruses; patients may be shielded from X-rays, etc. Mut0 is a made up term.
5. C is the correct answer.
Mutations occur more often with unprotected DNA. Mutations may be induced by viral (not bacterial) infections. Excuse the poor sense of humor for choice D. The impact of mutations is not possible to quantify. It is simply too far reaching.
Chapter 8
1. E is the correct answer.
The original description of IBEMs was by Garrod in the early 1900s. Metabolic blocks produce a deficiency of product after the block and an excess of precursor before the block.
2. E is the correct answer.
Inborn errors of metabolism are due to monogenic abnormalities. Chromosome testing is almost always normal. The infant described most likely has galactosemia. Thus, family history, feeding history, and maybe even odors will be important clues.
3. B is the correct answer.
While the presentation of IBEMs is quite broad in the range of problems seen, jaundice is the only one of the symptoms listed that is a common indication of a metabolic problem (see Tables 8-4 to 8-7)
4. A is the correct answer.
IBEMs not infrequently cause structural congenital anomalies. Garrod’s description was in the early 1900s. Treatments work, but almost never completely “fix” things. A critical point is that IBEMs have presenting symptoms that are like so many other medical disorders.
5. A is the correct answer.
In the US all states screen for PKU. Hopefully a child never slips through the process and all are picked up in the first few days of life and therapy begun quickly. Dietary therapy is effective, but by no means easy. Also therapy does not result in a completely normal outcome (IQs on average are half a standard deviation below those of sibs). It is only maternal PKU that is teratogenic.
Chapter 9
1. This pedigree shows straightforward X-linked recessive inheritance. There are only affected males. The males share common female ancestors. No male to male transmission is seen. With two affected sons, individual I-1 is a carrier of the condition. (Figure 9-11)
2. The most obvious finding is that only males are affected. The possibility of a Y-linked trait might come to mind. However, there are two females (II-2 and II-5) who have transmitted this trait that they received from their fathers. So it cannot be Y-linked. Could it be X-linked? No, as there is male-to-male transmission. You could hypothesize autosomal dominant with incomplete penetrance and it is just by chance that the females in this family were the ones that were non-penetrant. However, the correct (best) answer would be autosomal dominant with sex-limited expression. (Figure 9-12)
3. Neither parent is affected; there are two affected male children (III-3 and III-5) with the same mother (II-5). One might consider autosomal recessive inheritance. However, the two fathers (II-4 and II-6) would have to be carriers of the same disorder. If they were related, that would then be distinctly possible. However, you are not told that. You should assume that they are not related. Therefore X-linked recessive is the most likely. (Figure 9-13)
4. The correct answer is autosomal dominant with incomplete penetrance. At first glance, X-linked inheritance might be suspected. However, there is male-to-male transmission seen in this pedigree. This rules out X-linked inheritance. (Figure 9-14).
5. The best answer is autosomal recessive inheritance. In coming to this conclusion, there are several considerations. (a) This could be autosomal dominant with incomplete penetrance, but there are quite a few intervening meioses. The condition would have to be very weakly penetrant. (b) It is also possible that this is the chance happening of the same condition in first cousins, given that they are reasonably closely related. Chance would not be the best answer, unless this was a common condition. (c) For this to be a recessive condition the mates (II.2 and II.5) of the cousins (II.1 and II.4) would have to be carriers. The chance of this would depend on the carrier frequency in the population. (d) This is an actual pedigree from one of our clinics. The other important bit of information is not seen in the pedigree as drawn. What the cousins did not tell us at first was that their wives were sisters! This, of course, would make autosomal recessive inheritance extremely likely. (Figure 9-15).
Chapter 10
1. E is the correct answer.
Different genes at different loci producing the same phenotype is genetic heterogeneity (more exactly locus heterogeneity). This is not consistent with the definitions of polygenic or multifactorial inheritance. Although the environment could obviously modify the phenotype, that is not the explanation for the three different loci.
2. C is the correct answer.
Refer to the list of characteristics of multifactorial inheritance in this chapter. For choices A, B, and D the information is the opposite of what is true. Of course E is wrong, because environmental influences are central in multifactorial inheritance.
3. B is the best answer.
Polygenic conditions actually have a small number of major genes that contribute to the phenotype. The offspring tend to be an average of the parents and so B would be a correct assumption. C describes a defined threshold in what is being described as a continuously distributed quantitative trait. The distribution of the phenotype is usually a single normative curve, not a bimodal curve. Sex bias could be at work, but you would not know this unless you were told so.
4. B is the correct answer.
Again refer to the list of characteristics of multifactorial inheritance. The recurrence risk is higher if the least common sex is affected. More relatives affected means increased risk. Multifactorial recurrence risks are not as high as single gene disorders–often on the order of a few percent. The description is actually polygenic.
5. D is the correct answer.
Please do not forget this when you visit with your patients! The other answers are incorrect. Alcohol teratogenesis is common and very important. Only one third of children with alcohol exposures in uterowill have FAS. Of course, genetic factors do play a role.
Chapter 11
1. D is the correct answer.
A. For population screening, the condition should be sufficiently common enough.
B. For population screening, the testing modality should be easy to perform.
C. Population screening should be reserved for conditions that are clinically serious.
E. For population screening, the testing modality should be inexpensive to perform.
2. A is the correct answer.
B. Genetic screening of necessity has to be cost effective.
C. It is absolutely appropriate to identify specific ethnic groups in screening.
D. The whole purpose of screening is to effect changes that prevent disease and disability.
E. Testing can be by many different modalities (biochemical, and so forth).
3. B is the correct answer.
A. After newborn screening identifies a “presumptive positive,” selected biochemical studies are performed to confirm a diagnosis. This, then, is testing.
C. This is diagnostic testing.
D. An examination is not a genetic test.
E. This is diagnostic testing.
4. C is the correct answer.
A. The types of methodology are the same in both.
B. The costs of doing one or the other are the same in both.
D. The laboratories that do one or the other are the same in both.
E. The patient can be of any age for either.
5. C is the closest correct answer.
The current Uniform Screening Panel recommended by the American College of Medical Genetics is 29 (Table 11-8). Most states are in compliance with these recommendations.
Chapter 12
1. B is the correct answer.
A. Uniparental disomy is inheriting two copies of the same chromosome (or locus) from the same parent.
C. This does not describe expanding repeats or transposable elements.
D. Anticipation is the progressive worsening of a condition through the generations.
E. Partial expression would refer to how the trait were expressed, not the selective silencing of one allele.
2. C is the correct answer.
A. Imprinting is a normal phenomenon. It is the way certain genes are supposed to be expressed.
B. Imprinting is an epigenetic mechanism. The gene function, but not the code, is changed.
D. The most common mechanism of imprinting is methylation.
E. Only a small number of loci (maybe 70 or so) are imprinted. The majority of loci have typical di-allelic expression.
3. C is the correct answer.
A. Females, with two X chromosomes have Barr bodies, not males.
B. Since Lyonization is a random process, the average is around 50% for each X chromosome. However, skewing can occur and for any individual, the ratio can vary a lot from 50:50.
D. As with most epigenetic mechanisms, the change is not permanent. It is erased in germ cells to start over in the next generation.
E. Lyonization is an early occurring process in embryogenesis.
4. C is the correct answer.
A. X-linked inheritance is due to the gene being located on the X chromosome.
B. Semi-dominant inheritance refers to the fact that the females can be partially affected.
D. Males are more affected than females, and this is due to fragile X being X linked.
E. Mosaicism means a different genotype in different cells. This would not be related to trinucleotide repeats.
5. B is the correct answer.
A. A chromosome translocation is not epigenetic. It is an actual physical change in the DNA location.
C. Microsatellites are sets of repeated sequences of DNA.
D. A mutation is an actual change in the DNA code.
E. Expanding trinucleotide repeats are changes in the code (number of repeats).
6. A is the correct answer.
B. An insertional mutation is a mechanism that a transposable element might cause.
C. X inactivation is an epigenetic mechanism that is unrelated to trinucleotide repeats.
D. Opposite transcripts are two different gene products from reading the same sequence on both strands of DNA.
E. Modifier genes influence the expression of other genes. The mechanism is not related to trinucleotide repeats.
Chapter 13
1. B is the correct answer.
A. Mitochondrial inheritance shows maternal transmission.
C. Homozygous and heterozygous are terms applied to nuclear genes. Mitochondrial DNA patterns are described as showing homoplasmy or heteroplasmy.
D. Mitochondrial genes have a higher mutation rate than the nuclear genes.
E. The correct term is ‘bottleneck’ phenomenon.
2. D is the correct answer.
A. Clearly she could be affected by any number of mechanisms–either nuclear gene (Mendelian) inheritance or a mtDNA mutation she inherits from her mother.
B. Mitochondrial disorders may have adult onset.
C. Mitochondrial disorders exhibit highly variable expression including intra-familial variation.
E. Of course patient centered counseling would never make such a recommendation.
3. C is the correct answer.
A. Respiratory problems are not a key feature of peroxisomal disorders.
B. Performing a biopsy of a single organ in a multisystem disorder would be unlikely to reveal an answer.
D. You cannot make any assumption at this point. A diagnosis is needed first.
E. Of course testing may give you an answer. That’s what this is all about.
4. A is the correct answer.
B. Mitochondrial disorders usually present with neuromuscular symptoms.
C. Peroxisomal disorders may have structural congenital anomalies, but not typically the types listed here.
D. Lysosomal disorders are storage disorders not associated with structural congenital anomalies.
E. Similar to lysosomal disorders.
5. D is the correct answer.
A. A quick review of basic genetics: X-linked recessive conditions affect males.
B. Lysosomal disorders are progressive storage disorders. The child could look normal now, but still have an emerging phenotype.
C. Remember that screening is not perfect. All screens will have some degree of false negatives. Also, current newborn screening efforts do not include lysosomal storage disorders. However, this may change in the very near future.
E. We are not even sure what “lysosomal transplantation” is, if it even exists.
Chapter 14
1. C is the correct answer.
A. Gene therapy is currently not a standardly available mode of therapy.
B. Congenital malformations are not likely to be helped by gene therapy–this would only work if the defects were known ahead of time and the genetic manipulation performed at or around the time of conception—in the embryo.
D. Gene therapy is not standardly available regardless of the issue of being cost prohibitive.
E. Of course patients should not be counseled in such a directive manner.
2. C is the correct answer.
A. Cloning can be a non-controversial mode of therapy.
B. Even if this technology were available, the ethical issues are yet to be resolved.
D. The cloning of Dolly the sheep was accomplished, but there was the unexpected complication of early aging.
E. Cloning is an extremely controversial issue.
3. E is the correct answer.
A. Normal variant short stature is a licensed use of hGH.
B. The patient declares their disease.
C. hGH therapy is expensive. Just because the FDA has approved its use in normal variant short stature, most third party payers will not cover it.
D. Self-esteem will not necessarily get better with improved height. In fact, it often will not improve at all.
4. C is the correct answer.
A. Clearly other health care providers can be very well suited to help patients with case management.
B. The geneticist can be a “medical home neighbor” in the case management of patients working with the PCP.
D. Surgery is a direct service.
E. Although many third part payers now offer case management services, these are primarily for common disorders (such as diabetes). It is unlikely that an insurance company will have extensive expertise or experience with genetic disorders.
5. A is the correct answer.
B. Chelation of the stored glycosaminoglycans is not a therapeutic option. There is no known agent that can “pull” the GAGs out of the lysozymes in cells.
C. Newborn screening would help detect, but not treat, these disorders.
D. Dietary reduction of glycosaminoglycan does not prevent the accumulation of these compounds.
E. Gene correction is not a currently available treatment modality. It may be in the not so distant future.
Chapter 15
1. D is the correct answer.
According to the Hardy Weinberg distribution:
( p2 + 2pq + q2 = 1)
Thus the genotype frequency of a carrier (heterozygous) of the recessive allele q is 2q(1-q) which is approximately 2q when q is rare as noted in the question.
2. C is the correct answer.
Mrs Smith is a healthy sibling of an affected person. Half of the siblings will be healthy and carriers. One fourth (25%) will be healthy and not carriers and the remaining one fourth will have disease. Therefore, of the healthy siblings the chance of being a carrier is 2/3. This type of question is commonly asked. Be sure you completely understand this. Drawing out a Punnett square may help you visualize the proportions; the answer refers only to the phenotypically normal individuals in the Punnett square.
3. E is the correct answer.
First cousins will have one of their parents who are siblings. Siblings share ½ of the genes. Parents and children also share ½ of their genes. The chance a child from one family will share ½ × ½ of the genes = ¼ with one of their uncle or aunt. Therefore, the first cousins will share ¼ × ½ = 1/8 of the genes.
4. B is the correct answer.
Since the condition is autosomal recessive, males and females are no different. If the condition occurs in 1 in 256 individuals (q2) then the frequency of the allele (q) is the square root of 1/256 or 1/16. The carrier frequency then would be 2pq
or (2)(15/16)(1/16) = 30/256 = 12% which is closest to 1/8.
5. A is the correct answer.
B. If the heterozygous sate produces a disadvantaged situation, this would not increase the frequency of the mutation
C. “Foundation efforts” sounds like “founder effects”, but of course is nonsensical in this context.
D. Random mating would keep specific mutations at a random (low) level.
E. A low mutation rate would of course not increase the occurrence of a specific mutation.
Chapter 16
1. E is the correct answer.
A. Achondroplasia is not caused by a missing copy of the gene.
B. Sequencing the gene would not add any helpful information.
C. Again, the child will not have achondroplasia (and hGH is not effective for achondroplasia anyways).
D. Families will find their own information. There is no way to prevent or dissuade it.
2. B is the correct answer.
A. Haploinsufficiency is a mechanism for dominant conditions. Only one allele has to be affected.
C. Protein interference is likely to cause dominant disorders.
D. Likewise protein suicide will be associated with dominant conditions.
E. Autoregulatory dysfunction is a made-up term.
3. D is the correct answer.
A. It is not outdated, but has limited applications.
B. Is an uncommon pathogenetic mechanism in human disease.
C. Connective tissue disorders are typically dominant disorders that are problems with structural proteins, not enzymes.
E. Homeotic genes regulate other processes. They do not code for enzymes.
4. B is the correct answer.
A. Locus heterogeneity would involve more than one gene.
C. Genetic homogeneity would not apply as this would be the same mutation.
D. Homeotic modulation is a made-up term.
E. Gene-gene mutagenesis is another made-up term.
5. E is the correct answer.
All others can do so. Lyonization involves the X chromosome.