Mastering Genetics (3) Exam II
Although the most frequent forms of Down syndrome are caused by a random error, nondisjunction of chromosome 21, Down syndrome occasionally runs in families. The cause of this form of familial Down syndrome is ________. • a maternal age effect • too many X chromosomes • a translocation between chromosome 21 and a member of the D chromosome group • a chromosomal aberration involving chromosome 1 • an inversion involving chromosome 21
a translocation between chromosome 21 and a member of the D chromosome group
Genes X, Y, and Z are linked. Crossover gametes between genes X and Y are observed with a frequency of 25%, and crossover gametes between genes Y and Z are observed with a frequency of 5%. What is the expected frequency of double crossover gametes among these genes? • 50% • 1.25% • 30% • 5%
1.25% (The probability of a double crossover is the product of the probabilities of the single crossovers: 0.25 x 0.05 = 0.0125, or 1.25%)
A cross is made between a female calico cat and a male cat having the gene for black fur on his X chromosome. What fraction of the offspring would one expect to be calico?
1/4 Female Calico: Xb Xy (display mix colors) Male: Xb Y (black fur) _____|___Xb___|__Xy__ _Xb_|_XbXb _|_XbXy_ __Y__|__XbY__|__XyY XbXy = Calico Xb = Black Xy = Yellow
A human with the karyotype 49, XXXYY forms _______ Barr bodies. • 0 • 1 • 2 • 3
2 (All but one X chromosome is inactivated in every human somatic cell.) N -1 = Barr # 3 - 1 = 2 Barr Bodies
Phenotypically wild-type F1 female Drosophila, whose mothers had light eyes (lt) and fathers had straw (stw) bristles, produced the following offspring when crossed to homozygous light-straw males: Phenotype Number light-straw 22 wild-type 18 light 990 straw 970 Total: 2000 Compute the map distance between the light and straw loci.
2 map units
The sex of birds, some insects, and other organisms is determined by a ZW chromosomal arrangement in which the males have like sex chromosomes (ZZ) and females are ZW (similar to XY in humans). Assume that a recessive lethal allele on the Z chromosome causes death of an embryo in birds. What sex ratio would result in the offspring if a cross were made between a male heterozygous for the lethal allele and a normal female? • 2:1 male to female • 1:2 male to female • 3:1 male to female • 1:1 male to female • 4:1 male to female
2:1 male to female
Assume that the genes from the previous example are located along the chromosome in the order X, Y, and Z. What is the probability of recombination between genes X and Z? • 5% • 50% • 20% • 30%
30% (Recombination frequencies between linked genes along a chromosome are additive, so the recombination frequency between genes X and Z is 25 + 5 = 30.)
If two genes on the same chromosome exhibit complete linkage, what is the expected F2 phenotypic ratio from a selfed heterozygote with the genotype a+b+ ⁄⁄ ab? • 1:1 • 9:3:3:1 • 1:1:1:1 • 3:1
3:1 (Each parent produces two types of gametes, a+b+ and ab, giving the simple Mendelian ratio of 3 a+b+ : 1 ab.)
Klinefelter syndrome in humans, which leads to underdeveloped testes and sterility, is caused by which chromosomal condition? • 47, 21+ • 45, X • triploidy • 47, XYY • 47, XXY
47, XXY
What is the theoretical limit of observed recombination due to crossing over? • 25% • 1% • 100% • 50%
50% (When two genes are very far apart (> 50 mu), then it is theoretically possible for crossing over to occur in 100% of tetrads. Since the other two nonsister chromatids in the tetrad remain unchanged, the maximum percentage of recombinant gametes that could be produced is 50%.)
With respect to the chromosomes involved in the translocation, 14 and 21, what is the total number of different gametes possible for a heterozygous carrier parent to produce? • 6 • 2 • 4 • 3
6 (Correct. During meiosis, the Robertsonian chromosome may sort to one pole of the cell alone, with the normal copy of chromosome 14, or the normal copy of chromosome 21. The other chromosomes migrate to the opposite pole.)
Which of the following arrangements would yield the greatest reduction in gamete viability? • An inversion homozygote for a small inversion • An inversion heterozygote for a small inversion • An inversion heterozygote for a large inversion • An inversion homozygote for a large inversion
An inversion heterozygote for a large inversion (Correct. A large heterozygous inversion will result in greater numbers of genes that are duplicated or deleted when crossing over occurs.)
How many chromosomes would be found in an allopolyploid plant if its parents had diploid numbers of 4 and 6 respectively? • Diploid number of 5 • Haploid number of 12 • Haploid number of 5 • Diploid number of 10
Diploid number of 5
Assume that a species has a diploid chromosome number of 24. The term applied to an individual with 25 chromosomes would be triploid. True or False
False
Dosage compensation is accomplished in humans by inactivation of the Y chromosome. True or False
False
In general, inversion and translocation heterozygotes are as fertile as organisms whose chromosomes are in the standard arrangement. True or False
False
Individuals with familial Down syndrome are trisomic and have 47 chromosomes. True or False
False
The cross GE/ge X ge/ge produces the following progeny: GE/ge 404, ge/ge 396, gE/ge 97, Ge/ge 103. From these data, one can conclude that the G and E loci assort independently. True or False
False
Inversion loops do NOT form during meiosis in paracentric inversion heterozygotes. True or False
False (Correct. Loop formation allows pairing within inverted regions of homologous chromosomes regardless of whether the centromere is included in the inversion.)
Crossing over during prophase I of meiosis occurs between alleles on sister chromatids True or False
False (Crossing over during meiosis occurs between alleles on nonsister chromatids.)
A cross between a tetraploid and a diploid member of the same species will produce offspring that can undergo sexual reproduction. True or False
False (Offspring from this cross would be triploid and produce gametes with an uneven number of homologous chromosomes, making sexual reproduction unlikely.)
In humans, the male is the homogametic sex. True or False
False • Homogametic: Having or producing gametes that possess one type of sex chromosome.
Which of the following statements regarding familial Down syndrome is false? • Heterozygous carriers of the Robertsonian translocation that causes familial Down syndrome can have children who are phenotypically normal and are not carriers of the translocation. • Heterozygous carriers of the Robertsonian translocation that cause familial Down syndrome can have children who are also carriers of the translocation. • It accounts for approximately 5% of all Down syndrome cases. • If one parent is a translocation heterozygote for chromosomes 14/21, a child born to this parent is as likely to have Down syndrome as he is to be normal.
If one parent is a translocation heterozygote for chromosomes 14/21, a child born to this parent is as likely to have Down syndrome as he is to be normal. (Correct. Three viable zygote types are possible when fertilization occurs between one normal and one heterozygous parent. One-third has a normal chromosome complement, one-third is a carrier, and one-third has Down syndrome.)
How do mammals, including humans, solve the "dosage problem" caused by the presence of an X and Y chromosome in one sex and two X chromosomes in the other sex? • Females produce twice the RNA transcripts as males do, but in females half of them are destroyed before being translated. • They don't, but the X chromosome contains genes whose expression is not quantitatively sensitive, so the differences are not important. • Male RNA transcripts from the X chromosome have leaders that cause them to be translated at twice the rate as those in females. • In females, one of the X chromosomes is condensed and largely inactive so that each sex has one active X chromosome. • They don't; the fact that females have twice as much X chromosome protein products as males is critical for sex determination in mammals. • In males, the X chromosome is up-regulated so that its genes are transcribed and translated twice as much as the X chromosomes in females.
In females, one of the X chromosomes is condensed and largely inactive so that each sex has one active X chromosome.
Define the Lyon hypothesis. • Inactivation of a particular X chromosome depends on its genetic content. Inactivated X chromosomes are in some way "marked" such that all clonally related cells have the same X chromosome inactivated. • Inactivation of an X chromosome occurs at random just after fertilization. Inactivated X chromosomes are in some way "marked" such that all clonally related cells have the same X chromosome inactivated. • Inactivation of a particular X chromosome depends on its genetic content and occurs after differentiation of cell type. Different cell types may inactivate different X chromosomes. • Inactivation of an X chromosome occurs at random early in embryonic development. Inactivated X chromosomes are in some way "marked" such that all clonally related cells have the same X chromosome inactivated.
Inactivation of an X chromosome occurs at random early in embryonic development. Inactivated X chromosomes are in some way "marked" such that all clonally related cells have the same X chromosome inactivated.
How does X chromosome dosage compensation in Drosophila differ from that process in humans? • Instead of X chromosome inactivation, X-linked genes in Drosophila are transcribed at twice the rate in males compared to females. • X inactivation in Drosophila females is not random but depends on the quality of genetic products of each X chromosome. • In Drosophila, sex is determined by the balance between X chromosomes and sets of autosomes. X chromosome inactivation occurs both sexes. • Drosophila females have three X chromosomes; two of them must be inactivated instead of one as in human females.
Instead of X chromosome inactivation, X-linked genes in Drosophila are transcribed at twice the rate in males compared to females.
Which of the following are properties of a Barr body? Select all that apply. • The number of Barr bodies per cell is always equal to half the number of X chromosomes present. • It is formed as part of a mechanism for dose compensation. • It is the same X chromosome in every cell. • It is a darkly stained region near the nuclear envelope of interphase cells. • It is an inactivated X chromosome.
It is formed as part of a mechanism for dose compensation. It is a darkly stained region near the nuclear envelope of interphase cells. It is an inactivated X chromosome.
Which of the following statements about nondisjunction is false? • It may fail to separate maternal chromatids from one another or paternal chromatids from one another. • It separates maternal from paternal chromosomes. • It occurs more frequently in human females over age 35. • It may fail to separate maternal from paternal chromosomes.
It separates maternal from paternal chromosomes. (This statement is false. Nondisjunction may fail to separate maternal chromatids from paternal chromosomes.)
It is believed that any male-determining genes contained on the Y chromosome in humans are not located in the limited region that synapses with the X chromosome during meiosis. What might be the outcome if such genes were located in this region? • It would lead to non-disjunction of sex chromosomes during stage 1 of meiosis. • It would be detrimental to sex-determining mechanisms due to crossing over. • It would lead to non-disjunction of sex chromosomes during stage 2 of meiosis. • It would lead to very high frequency of Turner syndrome.
It would be detrimental to sex-determining mechanisms due to crossing over.
How can the order of three linked genes (A, B, and C) on the same chromosome be determined? • Look for parental phenotypes. • Gene order cannot be determined by looking at the results of a cross. • Look for single-crossover phenotypes involving the wild-type and mutant alleles of genes A, B, and C. • Look for double-crossover phenotypes involving the wild-type and mutant alleles of genes A, B, and C.
Look for double-crossover phenotypes involving the wild-type and mutant alleles of genes A, B, and C. (Offspring with a double-crossover phenotype will occur less frequently and will therefore be easy to identify. The three possible gene orders are then compared to the double-crossover genotypes to determine which gene order fits the observed pattern.)
Make sure to go over question 53 and 54!
Make sure to go over question 53 and 54!
Why do mapping experiments become less accurate when the distances between genes become large? • Multiple crossovers are more common. • Recombination occurs less frequently in long chromosomes. • Crossover gametes become more common. • Interference is greater when the distance between genes is large.
Multiple crossovers are more common. (Multiple recombination events that do not separate the genes being studied will not be recovered during mapping, leading to an underestimation of the distance between genes.)
A boy with Klinefelter syndrome (47,XXY) is born to a mother who is phenotypically normal and a father who has the X- linked skin condition called anhidrotic ectodermal dysplasia. The mother's skin is completely normal with no signs of the skin abnormality. In contrast, her son has patches of normal skin and patches of abnormal skin. Using the appropriate genetic terminology, describe the meiotic mistake that occurred. Be sure to indicate in which division the mistake occurred. • Non-disjunction must have occurred during meiosis I. • Non-disjunction must have occurred during meiosis II.
Non-disjunction must have occurred during meiosis I.
How does nondisjunction in human female gametes give rise to Klinefelter and Turner syndrome offspring following fertilization by a normal male gamete? • Nondisjunction results in ova that carry either two X chromosomes or none. The former results in Klinefelter syndrome when fertilized by a Y-containing sperm, and the latter results in Turner syndrome when fertilized by an X-containing sperm. • Nondisjunction of complete sets of chromosomes leads to ova that are either diploid, or completely devoid of chromosomes. A diploid ovum results in Klinefelter syndrome when fertilized by a Y-containing sperm, and results in Turner syndrome when fertilized by an X-containing sperm. • Nondisjunction results in ova that carry either two X chromosomes or none. The former results in Turner syndrome when fertilized by a Y-containing sperm, and the latter results in Klinfelter syndrome when fertilized by an X-containing sperm. • Nondisjunction results in ova that carry either two X chromosomes or none. An ovum with no X chromosomes results in Klinefelter syndrome when fertilized by a Y-containing sperm, and in Turner syndrome when fertilized by an X-containing sperm. • Nondisjunction results in ova that carry either two X chromosomes or none. An ovum with two X chromosomes results in Klinefelter syndrome when fertilized by a Y-containing sperm, and in Turner syndrome when fertilized by an X-containing sperm.
Nondisjunction results in ova that carry either two X chromosomes or none. The former results in Klinefelter syndrome when fertilized by a Y-containing sperm, and the latter results in Turner syndrome when fertilized by an X-containing sperm.
Trisomy 21, or Down syndrome, occurs when there is a normal diploid chromosomal complement but one (extra) chromosome 21. Although fertility is reduced in both sexes, females have higher fertility rates than males. Van Dyke et al. (1995; Down Syndrome Research and Practice 3[2]:65-69) summarize data involving children born of Down syndrome individuals. Assume that children are born to a female with Down syndrome and a normal 46-chromosome male. What proportion of the offspring would be expected to have Down syndrome? • One-half of the offspring would be expected to have Down syndrome. • One-third of the offspring would be expected to have Down syndrome. • None of the offspring would be expected to have Down syndrome. • All the children would be expected to have Down syndrome. • Two-thirds of the offspring would be expected to have Down syndrome.
One-half of the offspring would be expected to have Down syndrome.
Which of the following statements about gamete formation during meiosis is false? • Recombinant gametes contain combinations of alleles not found in the parent cell. • Complete linkage results in the formation of only parental gametes. • Parental gametes contain the same combinations of linked genes as found in the parent cell. • Parental gametes can be formed only if there is no crossing over during meiosis.
Parental gametes can be formed only if there is no crossing over during meiosis. (This statement is false. If crossing over occurs, half of the gametes formed are parental and the other half are recombinant.)
The primrose, Primula kewensis, has 36 chromosomes that are similar in appearance to the chromosomes in two related species, P. floribunda (2n = 18) and P. verticillata (2n = 18). Which one of the following statements best describes how P. kewensis likely arose from P. floribunda and P. verticillata? • Primula kewensis likely formed from the hybridization from a cross between the other two species. • Primula kewensis, with its 36 chromosomes, likely formed from chromosome tripling of P. floribunda with its 12 chromosomes. • Primula kewensis, with its 36 chromosomes, likely formed from chromosome quadrupling of P. floribunda with its 9 chromosomes. • Primula kewensis likely formed from the hybridization and subsequent chromosome doubling of a cross between the other two species.
Primula kewensis likely formed from the hybridization and subsequent chromosome doubling of a cross between the other two species.
Cat breeders are aware that kittens expressing the X-linked calico coat pattern and tortoiseshell pattern are almost invariably females. Which one of the following statements supports this conclusion? • Since females normally have two X chromosomes, random X inactivation during development of a heterozygous female will create a mottled pattern. Normal males are not mottled, because they only have one X chromosome and therefore can only express one of the two potential X-linked alleles. • Since females normally have two X chromosomes, random X inactivation during development of a homozygous female will create a mottled pattern. Normal males are not mottled, because they are heterozygous and therefore express each one of the two potential X-linked alleles. • Since males normally have one X and one Y chromosome, random X inactivation during development of a male will create a mottled pattern. Normal females are mottled, because they have two X chromosomes. • Since females normally have two X chromosomes, expression of both potential X-linked alleles at the same time in all cells will create a mottled pattern. Normal males are not mottled, because they only have one X chromosome and therefore can only express one of the two potential X-linked alleles.
Since females normally have two X chromosomes, random X inactivation during development of a heterozygous female will create a mottled pattern. Normal males are not mottled, because they only have one X chromosome and therefore can only express one of the two potential X-linked alleles.
Which one of the following statements best describes the evidence that suggests that Down syndrome is more often the result of nondisjunction during oogenesis rather than during spermatogenesis? • The incidence of Down syndrome due to increasing paternal age is higher than the incidence due to increasing maternal . • Mothers, but not fathers with Down syndrome can have children with Down syndrome. • Fathers, but not mothers with Down syndrome can have children with Down syndrome. • The incidence of Down syndrome due to increasing maternal age is higher than the incidence due to increasing paternal age.
The incidence of Down syndrome due to increasing maternal age is higher than the incidence due to increasing paternal age.
A boy with Klinefelter syndrome (47,XXY) is born to a mother who is phenotypically normal and a father who has the X- linked skin condition called anhidrotic ectodermal dysplasia. The mother's skin is completely normal with no signs of the skin abnormality. In contrast, her son has patches of normal skin and patches of abnormal skin. Which parent contributed the abnormal gamete? • Both parents must have contributed the abnormal X-linked gene. • The mother must have contributed the abnormal X-linked gene. • The father must have contributed the abnormal X-linked gene.
The father must have contributed the abnormal X-linked gene.
Which of the following statements about XYY males is true? • They are sterile. • They have underdeveloped secondary sex characteristics. • They are generally taller than average. • They are likely to end up in prison.
They are generally taller than average. (This is the only common feature among XYY males.)
Which of the following statements about allopolyploid individuals is true? • They are generated from exposure to colchicine. • They result from a mating between individuals of the same species. • They can be formed when two sperm simultaneously fertilize an ovum within the same species. • They may be sterile and unable to produce offspring.
They may be sterile and unable to produce offspring. (Polyploid individuals are sterile if they produce genetically unbalanced gametes; both autopolyploid and allopolyploid individuals may be sterile.) (online) Allopolyploid: A polyploid individual or strain having a chromosome set composed of two or more chromosome sets derived more or less complete from different species.
A boy with Klinefelter syndrome (47,XXY) is born to a mother who is phenotypically normal and a father who has the X- linked skin condition called anhidrotic ectodermal dysplasia. The mother's skin is completely normal with no signs of the skin abnormality. In contrast, her son has patches of normal skin and patches of abnormal skin. Using the appropriate genetic terminology, explain the son's skin phenotype. • This son's mosaic phenotype is caused by Y-chromosome inactivation, a form of dosage compensation in mammals. • This son's mosaic phenotype is caused by X-chromosome inactivation, a form of dosage compensation in mammals.
This son's mosaic phenotype is caused by X-chromosome inactivation, a form of dosage compensation in mammals.
Chorionic villus sampling (CVS) is sometimes preferred to amniocentesis because results can be provided earlier in the pregnancy. True or False
True
If two gene loci are on nonhomologous chromosomes, genes at these loci are expected to assort independently. True or False
True
Nondisjunction is viewed as a major cause of aneuploidy. True or False
True
The cross GE/ge X ge/ge produces the following progeny: GE/ge 404, ge/ge 396, gE/ge 97, Ge/ge 103. From these data, one can conclude that the recombinant progeny are gE/ge and Ge/ge. True or False
True
The cross GE/ge X ge/ge produces the following progeny: GE/ge 404, ge/ge 396, gE/ge 97, Ge/ge 103. From these data, one can conclude that there are 20 map units between the G and E loci. True or False
True
Two genes that are separated by 10 map units show a recombination percentage of 10%. True or False
True (One map unit is equal to 1% recombination between two genes; 10 map units would be equal to 10% recombination between the genes.)
Assume that an organism has a diploid chromosome number of 14. There would be 28 chromosomes in a tetraploid. True or False
True 2n = 14 4n = 28 n = 7 n = 7
Assume that a species has a diploid chromosome number of 24. The term applied to an individual with 36 chromosomes would be triploid. True or False
True 2n = 24 3n = 36 n = 12 n = 12
An individual with Klinefelter syndrome generally has one Barr body. True or False
True N - 1 = Barr Klinefelter: XXY 2 -1 = 1 Barr Body
An individual with Turner syndrome has no Barr bodies. True or False
True Turner Syndrome: Xø (only 1 X chromosome, affects only females)
Which of the following hypotheses could explain why so many more human male conceptions than female conceptions occur? Select all that apply. • Male fetuses grow faster than female fetuses. • Y-containing sperm are faster than X-containing sperm. • Female fetuses survive pregnancy at higher frequencies than male fetuses. • The surfaces of human ova preferentially bind to Y-containing sperm.
Y-containing sperm are faster than X-containing sperm. The surfaces of human ova preferentially bind to Y-containing sperm.
The primrose, Primula kewensis, has 36 chromosomes that are similar in appearance to the chromosomes in two related species, P. floribunda (2n = 18) and P. verticillata (2n = 18). Which one of the following best describes P. kewensis in genetic terms? • autotetrapoidy • endopolyploidy • aneuploidy • allotetraploidy
allotetraploidy (online) allotetraploidy: Having two diploid sets of chromosomes each derived from a different parental species, resulting in an allopolyploid organism.
What is the general term for a condition in which the chromosome number is not a multiple of a complete set? • aneuploidy • triploidy • trisomy • polyploidy
aneuploidy (The term aneuploid is derived from roots that mean "not true ploidy.") (pp) Mistakes in mitosis or meiosis can lead to changes in chromosome number. (online) Most common chromosome abnormality in humans, and is the leading genetic cause of miscarriage and congenital birth defects. (online) A condition in which a person has one or a few chromosomes above or below the normal chromosome number.
All of the following could result from meiosis in a pericentric inversion heterozygote in which a single crossover occurred within the inversion loop except a _______. • completely normal chromosome • chromosome with some duplicated regions • chromosome with some deleted regions • chromosome with two centromeres
chromosome with two centromeres (Correct. When the centromere is contained within the inversion, it is neither duplicated nor deleted.)
Assume that a cross is made between AaBb and aabb plants and that the offspring fall into approximately equal numbers of the following groups: AaBb, Aabb, aaBb, aabb. These results are consistent with ________. • incomplete dominance • independent assortment • alternation of generations • hemizygosity • complete linkage
independent assortment
Individuals have been identified who have two different karyotypes, such as 45,X/46,XY or 45,X/46,XX. Such individuals are called ________.
mosaics • Mosaic: The presence of two different genotypes in an individual which developed from a single fertilized egg. [remember the calico cat]
A Robertsonian translocation is considered non-reciprocal because _______. • an uneven number of gametes is produced in each meiosis • the smaller of the two reciprocal products of translocated chromosomes is lost • for every viable gamete formed, there are two inviable gametes formed • trisomies of chromosome 21 are viable, whereas monosomies of the same chromosome are not
the smaller of the two reciprocal products of translocated chromosomes is lost (Correct. After several cell divisions, only the larger of the two translocated chromosomes remains. Its reciprocal is lost.)
Which of the following terms can be used to describe an individual with Down syndrome? • triploid and aneuploid • triploid and polyploid • trisomic and polyploid • trisomic and aneuploid
trisomic and aneuploid (online) Trisomic: Having one chromosome in addition to the usual diploid number. (online) Aneuploid: Having particular genes or chromosomal regions present in extra or fewer copies than in the normal type.
How can duplications arise? • loss of a telomere • spindle failure during chromosome separation • uneven crossing over during meiotic prophase • a chromosome break and exchange
uneven crossing over during meiotic prophase (Synapsis of homologs and unequal crossing over of chromatids during meiosis I can produce one chromatid with a duplication and one with a deletion.)
A 3-year-old child exhibited some early indication of Turner syndrome, which results from a 45,X chromosome composition. Karyotypic analysis demonstrated two cell types: 46,XX (normal) and 45,X. Propose a mechanism(s) for the origin of this mosaicism. Check all that apply. • dispermic fertilization • meiotic nondisjunction of X chromosomes • inactivation of X-chromosome • mitotic nondisjunction of X chromosomes • embryonic fusion • reciprocal translocation between two X-chromosomes
• mitotic nondisjunction of X chromosomes • embryonic fusion