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Chapter 6 - Chromosome Mutations: Variation in Number and Arrangement
Terms in this set (66)
The failure of chromosomes to properly separate during meiosis results in variation in the chromosome content of gametes and subsequently in offspring arising from such gametes.
The rearrangement of genetic information within the genome of a diploid organism may be tolerated by that organism but may affect the viability of gametes and the phenotypes of organisms arising from them.
Plants often tolerate an abnormal genetic content, and as a result, manifest unique phenotypes. Such genetic variation has been an important factor in plant evolution.
In animals, genetic information is in a delicate equilibrium whereby the gain or loss of a chromosome, or part of a chromosome, in a n otherwise diploid organism often leads to lethality or to an abnormal phenotype.
Chromosomes in humans contain fragile sites - regions susceptible to breakage, which lead to abnormal phenotypes.
chromosome aberration/mutation (distinguished from gene mutations)
any alteration (duplication, deletion, or rearrangement) of the precise diploid chromosomal content of an organism
condition in which the chromosome number is not an exact multiple of the haploid set (2n-1 monosomy, 2n+1 trisomy)
condition in which the chromosome number is not exact multiple of the haploid set (polyploidy >2n, 3n triploid, 4n tetraploid, etc.)
cell division error in which homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate and migrate to opposite to poles, resulting in defects such as monosomy and trisomy
in diploids, the condition in which an individual possesses only one functional copy of a gene (monosomy) with the other inactivated by mutation, leading to an abnormal phenotype due to insufficient protein produced by a single copy to produce a normal phenotypes (present in many human autosomal dominant disorders)
An alternative explanation for monosomic lethality (especially found in larger chromosomes) is that if even one gene of the single full copy is represented by a lethal allele, monosomy unmasks the recessive lethal that is tolerated in heterozygotes carrying the corresponding wild-type allele, leading to death.
Aneuploidy is better tolerated in the plant kingdom, though both plants and animals experienced impaired viability (slow development, reduced body size, etc.) and fertility.
Trisomy produces more viable individuals than monosomy, provided that the chromosome is relatively small. Addition of a large autosome is generally lethal.
Trisomic individuals have altered phenotypes, different from other trisomics and wild-type plants, with trisomics for longer chromosomes being more distinctive due to greater genetic imbalance.
Down syndrome: Trisomy 21 (47,21+)
only human autosomal trisomy in which a significant number of individuals survive longer than a year past birth (1 in 800 live births, currently >250,000 in US)
as a syndrome, 12-14 characteristics exist though on average an affected individual only exhibits a subset (6-8) of them
similar outward appearance: epicanthic eye folds, flat face, round head, characteristically short, protruding furrowed tongue (partially open mouth), short broad hands of characteristic palm and fingerprint patterns
retarded physical, psychomotor, and mental development
poor muscle tone, prone to respiratory disease and heart malformations, incidence of leukemia (abnormally high number of white blood cells) apx. 20x higher than that of the normal population
life expectancy shortened to about 50 years (frequently due to Alzheimer disease), thought individuals are known to survive into their 60s
75% of nondisjunction occurs during meiosis I, 95% additional chromosome derived from the mother
dramatic increase in Down syndrome births due to increase in maternal age (1 in 100 at age 40, 1 in 30 at age 45), possibly due to increased age of ovum
Down syndrome critical region (DSCR)
hypothetical region of chromosome 21 containing the genes that are dosage sensitive in trisomy and responsible for the phenotypes associated with the syndrome; resulted in the findings that though three copies are necessary but themselves not sufficient from Down syndrome phenotypes, and that Down syndrome individuals have a decreased risk of developing a number of cancers due to the presence of an extra copy of the DSCR1 gene that encodes a protein that inhibits tumor formation
Methods of fetal genotyping/prenatal diagnosis in which fetal cells are isolated and cultured to determine karyotype from cytogenetic analysis. Amniocentesis and CVS are familiar approaches, while NIGPD is a noninvasive newer approach.
Therapeutic abortion is an option.
amniocentesis - procedure in which fluid and fetal cells are withdrawn from the amniotic layer surrounding the fetus
chorionic villus sampling (CVS) - chorionic fetal cells are retrieved intravaginally or transabdominally and sued to detect cytogenetic and biochemical defects in the embryo
noninvasive prenatal genetic diagnosis (NIGPD) - uses maternal blood sample to analyze thousands of fetal loci using fetal DNA fragments present in the maternal blood
Patau and Edwards syndromes are the only two human trisomies to survive to term.
Surviving individuals manifest severe malformations and early lethality.
Apx. 20% of all conceptions terminate in spontaneous abortion and about 20% of all spontaneously aborted fetuses demonstrate some form of chromosomal imbalance (6% of conceptions contain an abnormal chromosome complement).
Trisomies for every human chromosome have been recovered. The monosomy with the highest incidence among abotuses (aborted fetuses) is the 45,X condition (Turner syndrome).
Autosomal monosomies are seldom found, even though nondisjunction should produce n - 1 gametes with equal frequency to n + 1 gametes.
This finding suggests that gametes lacking a single chromosome are so functionally impaired that the embryo dies before a fetus is even recovered.
Polyploidy is relatively infrequent in many animal species but is well known in lizards, amphibians, and fish, and is much more common in plant species. Odd numbers of chromosomes are not reliably inherited since an uneven number of homologs often does not produce genetically balanced gametes. For this reason, triploids, polypoids, and so on, are not usually found in plant species that depend solely on sexual reproduction.
A = a1 + a2+ a3 + a4 + ... + an (subscript)
a1, a2, and so on, are individual chromosomes and n is the haploid number.
diploid AA, triploid AAA, tetraploid AAAA
AA x BB produces AB (hybridization)
polyploid condition resulting from the addition of one or more extra sets of chromosomes identical to the normal haploid component of the same species
polyploid condition resulting from the union of two or more chromosome sets from different species due to hybridization
diploid gamete from failure of segregation during meiotic divisions
fertilization of ovum by two sperm (triploid zygote)
cross of diploid with tetraploid (2n + 4n)/2 = 3n
chromosome replication without parent cell division due to:
cold or heat shock application to meiotic cells
colchicine (spindle formation interference alkaloid) application to mitotic cells
Autopolyploids are larger than their diploid relatives due to larger cell size, making such varieties of greater horticultural or commercial value.
Economically important plants include triploid potato species, Winesap apples, commercial (seedless triploid compared to hard seed diploid) bananas, seedless watermelons, and cultivated tiger lilies; and tetraploid alfalfa, coffee, peanuts, and McIntosh apples.
an allopolyploid containing two genomes derived from different species (amphidiploid is preferred in describing an allotetraploid of known original species)
Amphidiploid plants are often found in nature, reproducing with balanced gametes and normal meiosis, though homology of chromosomes is likely in amphidiploids formed from closely related species. Allopolyploids are rare in most animals because mating behavior is generally species-specific, and thus the initial step of hybridization is unlikely to occur.
The hybridization of Old World (13 large pairs of chromosomes) and wild American cotton (13 small pairs of chromosomes) to produce the cultivated species of American cotton (26 pairs of chromosomes, half large and half small) is a classic example of amphiploidy in plants.
Amphidiploids often exhibit traits of both parental species, as in the hybridization of wheat, a high-yielding grain, and rye, a versatile grass for growth in unfavorable environments, to produce the Triticale species of both traits.
Tetraploid wheat is crossed with diploid rye and the F1 plants are treated with colchicine to obtain a hexaploid variety (6n = 42), the hybrid Triticale which represents a new genus.
(2 + 4)/2 x 2 = 6
Structural chromosomal aberrations are due to breaks along the axis of a chromosome, which occur spontaneously but may increase in frequency in cells exposed to hazardous chemicals or radiation.
The ends produced at points of breakage are "sticky" and may rejoin other broken ends, but if the original relationship is not established and if the alteration occurs in germ plasm, the gametes will contain the heritable structural rearrangement
If the aberration is found in one homolog but not the other, the individual is said to be heterozygous for the aberration, causing unique pairing configurations during meiotic synapsis.
If no loss or gain of genetic material occurs, heterozygous individuals are unlikely to phenotypically affected, though may produce duplicated or deficient chromosomal regions in gametes that lead to offspring of the carriers demonstrating increased probability of phenotypic change.
a chromosomal mutation involving the loss of chromosomal material, terminal and intercalary (end or interior of chromosome, respectively)
the unpaired region of a normal homolog that must buckle out for synapsis to occur between a chromosome with a large intercalary deletion and the normal homolog
cri du chat synrome
segmental deletion of a small terminal portion of chromosome 5 (partial monosomy)
anatomic malformations, mental retardation, and abnormal development of the glottis and larynx that causes the clinical symptom of an eerie cry similar to the meowing of a cat
longer deletion results in greater impact on physical, psychomotor, and mental skills levels of surviving children
a chromosomal aberration in which a segment of the chromosome is repeated that may arise as a result of unequal crossing over between synapsed chromosomes during meiosis or through a replication error prior to meiosis
heterozygote pairing produces a compensation loop as in deletion
production of phenotypic variation as similar to deletion
important source of genetic variation during evolution
the process by which gene sequences are selected and differentially replicated either extrachromosomally or intrachromosomally
nucleolar organizer region (NOR)
region of the chromosome for multiple copies of genes of rDNA (DNA that codes for specific RNA sequences) that is selectively replicated to form micronucleoli for gene amplification to provide adequate amounts of rRNA for construction of ribosomes
Bar-eye phenotype mutation in Drosophila
Instead of the normal oval-eye shape, Bar-eyes flies have narrow, slitlike eyes inherited as a dominant X-linked mutation.
Duplications may cause phenotypic variation that might at first appear to be caused by a simple gene mutation.
normal wild-type: 800 facets (B+/B+), heterozygous: 350 facets (B/B+), homozygous: 70 facets (B/B), double Bar: even fewer (BD/B+)
A copy of the region designated as 16A is present on both X chromosomes of wild-type flies is duplicated in Bar flies and triplicated in double Bar flies, providing evidence that the Bar phenotype is not a result of a simple chemical change in the gene but rather a duplication.
Susumu Ohno's Thesis in Evolution by Gene Duplication
Gene products of many genes, present as only a single copy in the genome, are indispensable to the survival of members of any species during evolution.
Unique genes are not free to accumulate mutations sufficient to alter their primary function and give rise to new genes.
However, an essential gene may be duplicated in the germ line, and major mutational changes in this extra copy will be tolerated in future generations because the original gene provides the genetic information for its essential function. The duplicated copy will be free to acquire many mutational changes over extended periods of time, sufficiently changing over long evolutionary periods to assume a different, adaptively advantageous role to organisms.
Major evolutionary jumps, such as the transition from invertebrates to vertebrates, may have involved the duplication of entire genomes, occurring several times in the course of evolution.
Many genes have a substantial amount of common organization and DNA sequence while maintaining distinct gene products, including trypsin and chymotrypsin, and myoglobin and the various forms of hemoglobin.
There is no compelling evidence to convince evolutionary biologists that genome duplication has been responsible.
a chromosomal aberration in which a chromosomal segment has been reversed (180°), possibly by forming a chromosomal loop prior to breakage in which the newly formed "sticky ends" rejoin, paracentric and pericentric (centromere not included and included in inverted segment, respectively)
An inversion loop must be formed for inversion heterozygotes to pair in nonlinear synapsis.
If crossing over does not occur within the inverted segment of the inversion loop, the homologs will segregate, resulting in two normal and two inverted chromatids that are distributed into the gametes.
If crossing over does occur within the inversion loop, abnormal chromatids are produced.
In a meiotic tetrad, a single crossover between nonsister chromatids produces two parental chromatids and two recombinant chromatids.
However, if the crossover occurs within a paracentric inversion, one recombinant dicentric chromatid (two centromeres) and one recombinant acentric chromatid (lacking a centromere) are produced, both contains duplicaiton and deletions of chromosome segments.
During anaphase, an acentric chromatid moves randomly to one pole or the other or may be lost, while a dicentric chromatid is pulled in two directions to produce a dicentric bridge that breaks so that part of the chromatid do to each gamete during reduction division. Gametes containing either recombinant are deficient in genetic material.
In animals, when such a gamete participates in fertilization, the zygote most often develops abnormally, inviable embryos are produced, and lethality is the final result. In plants, gametes fail to develop normally, leading to aborted pollen or ovules that are incapable of achieving fertilization.
In inversion heterozygotes, inversion suppresses recovery of crossover products when chromosome exchange occurs within the inverted region. Up to 1/2 of the viable gametes have the inverted chromosome, repeating the inversion continuously during meiosis in future generations.
Groups of specific alleles at adjacent loci within inversions are preserved. If the alleles of the involved genes are advantageous to fitness, the inversion is beneficial to the evolutionary survival of the species.
In laboratory studies, organisms heterozygous for balancer chromosomes, which contain inversions, are used to preserve desired sequences of alleles during experimental work.
a chromosomal mutation associated with the reciprocal or nonreciprocal transfer of a chromosomal segment from one chromosome to another, and denoting the movement of mRNA through the ribosome during translation
copy number variation (CNV)
DNA segments longer than 1 kb (1000 base pairs) that are repeated a variable number of times in the genome, with duplications and deletions of CNVs resulting in positive or negative effects on various related diseases
Reciprocal translocation involves the exchange of segments between two nonhomologous chromosomes. The least complex way for this event to occur is for two nonhomologous chromosome arms to come close to each other so that an exchange is facilitated.
Terminally, a simple reciprocal translocation requires two breaks, and if the exchange includes internal chromosome segments, four breaks are required, two on each chromosome.
Though genetically unbalanced gametes are produced as a result of unusual synapsis during meiosis of homologs heterozygous for reciprocal translocation, they are a result of random independent assortment rather than crossing over.
condition in which as few as 50% of the progeny of parents heterozygous for reciprocal translocation survive, affected by partial aneuploidy leading to a variety of birth defects
Robertsonian translocation/centric fusion
a chromosomal aberration created by breaks in the short arms of two acrocentric chromosomes followed by fusion of the long arms of these chromosomes at the centromere, producing a new, large submetacentric or metacencric chromosome
a heritable gap, or nonstaining region, of a chromosome than can be induced to generate chromosome breaks, of which several clear association have been established with altered phenotypes including mental retardatio nand cancer
the most common form of inherited mental retardation (1 in 4000 males, 1 in 8000 females)
FMR-1 gene is composed of trinucleotide repeats (recognized in other human disorders as well), a high number of which correlates directly with expression of fragile-X syndrome
normal: 6-54 repeats, carrier: 55-230 repeats, expression: >230 repeats
example of genetic anticipation
the phenomenon in which the severity of symptoms in genetic disorders increases and the age of onset decreases from generation to generation, caused by the expansion of trinucleotide repeats within or near a gene
How is it known that the extra chromosome causing Down syndrome is usually maternal in origin?
Chromosome analysis, as well as the striking correlation between incidence of Down syndrome births and maternal age.
How is it known that human aneuploidy for each of the 22 chromosomes occurs at conception, even though most often human aneuploids do not survive embryonic or fetal development and thus are never observed at birth?
Aborted fetuses of trisomies for every human chromosome have been recovered.
How is it known that specific mutant phenotypes are due to changes in chromosome number or structure?
Representative karotypes of these phenotypes have clear chromosomal aberrations of number and structure compared to those ofnormal individuals.
How is it known that the mutant bar-eye phenotype in Drosophila is due to a duplicated gene region rather than to a change in the nucleotide sequence of a gene?
The studies of Calvin Bridges and Herman J. Muller comparing polytene X chromosome banding patterns revealed that a copy of the region designated as 16A present on both X chromosomes of wild-type flies is duplicated in Bar flies and triplicated in double Bar flies.
For a species with a diploid number of 18, indicate how many chromosomes will be present in the somatic nuclei of individuals that are haploid, triploid, tetraploid, trisomic, and monosomic.
2n = 18 chromosomes
haploid n = 9 chromosomes
triploid 3n = 27 chromosomes
tetraploid 4n = 36 chromosomes
trisomic 2n + 1 = 19 chromosomes
monosomic 2n - 1= 17 chromosomes
Contrast the relative survival times of individuals with Down, Patau, and Edwards syndromes. Speculate as to why such differences exist.
Individuals with Down syndrome live much longer than those with Patau and Edwards syndromes, likely because chromosome 21 is relatively small (actually is the smallest) while the chromosomes affected by Patau and Edwards syndromes are relatively large. Of course, genetic content of the affected chromosomes may have varying impacts on development.
What explanation has been proposed to explain why Down syndrome is more often the result of nondisjunction during oogenesis rather than during spermatogenesis?
Women are born with all the eggs they'll ever have. Aging of ova over time may produce genetic defects, the release of an ovum from an older woman resulting in Down syndrome..
Contrast the fertility of an allotetraploid with an autotriploid and an autotetraploid.
Allotetraploids are the most fertile, have the possibility of producing bivalents at meiosis I. Autotetraploids have an even number of chromosomes and thus are more fertile than autotriploids.
Why do human monosomics most often fail to survive prenatal development?
Lethality may result to insufficient protein produced by a single copy or from an unmasked recessive lethal allele, normally tolerated in heterozygotes carrying the corresponding wild-type allele.
Predict how the synaptic configurations of homologous pairs of chromosomes might appear when one member is normal and the other member has sustained a deletion or duplication.
Sections without a homolog will buckle to form compensation loops.
Inversions are said to "suppress crossing over." Is this terminology technically correct? If not, restate the description accurately.
No. Crossovers still occur, but result in lethality, causing inversion to theoretically suppress crossing over through death of affected zygotes
Predict the genetic composition of gametes derived from tetrads of inversion heterozygotes where crossing over occurs within a pericentric inversion.
Crossing over in the inversion loop of a pericentric heterozygote produces all chromatids with centromeres, but the two chromatids involved in the crossover are genetically unbalanced. The balanced chromatids are either normal or inverted.
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). How could P. kewensis arise from these species? How would you describe P. kewensis in genetic terms?
Primula kewensis likely formed from the hybridization and subsequent chromosome doubling of a cross between the other two species.
Certain varieties of chrysanthemums contain 18, 36, 54, 72, and 90 chromosomes; all are multiples of a basic set of nine chromosomes. How would you describe these varieties genetically? What feature do the karyotypes of each variety share? A variety with 27 chromosomes has been discovered, but it is sterile. Why?
Autopolyploids are the most fertile, have the possibility of producing bivalents at meiosis I. Autotetraploids have an even number of chromosomes and thus are more fertile than autotriploids.
Drosophila may be monosomic for chromosome 4, yet remain fertile. Contrast the F1 and F2 results of the following crosses involving the recessive chromosome 4 trait, bent bristles:
monosomic IV, bent bristles x normal bristles
monosomic IV, normal bristles x bent bristles
Mendelian ratios are modified in crosses involving autotetraploids. Assume that one plant expresses the dominant trait green seeds and is homozygous (WWWW). This plant is crossed to one with white seeds that is also homozygous (wwww). If only one dominant allele is sufficient to produce green seeds, predict the F1 and F2 results of such a cross. Assume that synapsis between chromosome pairs is random during meiosis.
Predict the F2 ratio of a "dihybrid" cross involving two independently assorting characteristics. (e.g. P1 = WWWWAAAA x wwwwaaaa).
In a cross between two varieties of corn, gl1gl1Ws3Ws3 (egg parent) x Gl1Gl1ws3ws3 (pollen parent), a triploid offspring was produced with the genetic constitution Gl1Gl1gl1Ws3ws3ws3. From which parent, egg or pollen, did the 2n gamete originate? Is another explanation possible? Explain.
A couple planning their family are aware that through the past three generations on the husband's side a substantial number of stillbirths have occurred and several malformed babies were born who died early in childhood. The wife has studied genetics and urges her husband to visit a genetic counseling clinic, where a complete karyotype-banding analysis is performed. Although the tests show that he has a normal complement of 46 chromosomes, banding analysis reveals that one member of the chromosome 1 pair (in group A) contains an inversion covering 70% of its length. The homolog of chromosome 1 and all other chromosomes show the normal banding sequence.
(a) Explain the high incidence of past stillbirths.
(b) Predict the probability of abnormality/normality of their future children.
(c) Will the woman have to bring each pregnancy to term to determine whether the fetus is normal? If not else, what can you suggest?
In all probability, crossing over in the inversion loop of an inversion (in the heterozygous state) had produced defective, unbalanced chromatids, thus leading to still births and or malformed children.
A woman who sought genetic counseling is found to be heterozygous for a chromosomal rearrangement between the second and third chromosomes.
(a) What kind of chromosomal aberration is shown?
(b) Using a drawing, demonstrate how these chromosomes would pair during meiosis. Be sure to label the different segments of the chromosome.
(c) This woman is phenotypically normal. Does this surprise you? Why or why not? Under what circumstances might you expect a phenotypic effect of such a rearrangement?
She has had two miscarriages.
(a) Is there a genetic explanation of her frequent miscarriages?
(b) Should she abandon her attempts to have a child of her own?
(c) If not, what is the chance that she could have a normal child?
This is not surprising because the woman is euploid for nearly her entire chromosome complement. The only defective genes are likely associated with the translocation breakpoints and because there are functional copies of each on the intact homologs there should be no phenotype unless the rearranged genes exhibit a dominant effect.
Yes, The miscarriages are the result of the production of unbalanced (aneuploid) gametes produced by the normal meiotic process.
No. It is possible for her to produce an ovum with a normal copy of each chromosome or a translocated copy of each, both of which are viable pairings.
No. If she has a child, he/she will be phenotypically normal. But there is a 50% chance that the child, like her mother, will carry two translocated chromosomes, and will thus also have difficulty in having children
In a recent cryogenetic study on 1021 cases of Down syndrome, 46 were the result of translocations, the most frequent of which was symbolized as t(14;21). What does this designation represent, and how many chromosomes would you expect to be present in t(14:21) Down syndrome individuals?
Twenty-four of them are the most fertile, have the possibility of producing bivalents at meiosis I. Autotetraploids have an even number of chromosomes and thus are more fertile than autotriploids.
A boy with Klinefelter syndrome (47.XXY) is born to a mother 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.
(a) Which parent contributed the abnormal gamete?
(b) Using the appropriate genetic terminology, describe the meiotic mistake that occurred. Be sure to indicate in which division the mistake occurred.
(c) Using the appropriate genetic terminology, explain the son's phenotype.
The father must have contributed the abnormal X-linked gene.
Non-disjunction must have occurred during meiosis I.
This son's mosaic phenotype is caused by X-chromosome inactivation, a form of dosage compensation in mammals.
In a human genetic study, a family with five phenotypically normal children was investigated. Two children were "homozygous" for a Robertsonian translocation between chromosomes 19 and 20 (they have contained two identical copies of the fused chromosome). They have only 44 chromosomes but a complete genetic complement. Three of the children were "heterozygous" for the translocation and contained 45 chromosomes, with one translocated chromosome plus a normal copy of both chromosomes 19 and 20. Two other pregnancies resulted in stillbirths. It was later discovered that the parents were first cousins. Based on this information, determine the chromosome compositions of the parents. What led to the stillbirths? Why was the discovery that the parents were first cousins a key piece of information in understanding the genetics of this family?
A Robertsonian translocation that causes Down syndrome occurs when a large part of chromosome 21 is translocated to chromosome 14. The translocation can be passed on to the gametes, and thus there is always a 1-in-4 chance that a gamete will form with two copies of chromosome 21.
A three-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 for the origin of this mosaicism.
Mitotic nondisjunction of X chromosomes
A normal female is discovered with 45 chromosomes, one of which exhibits a Robertsonian translocation containing most of chromosomes 18 and 21. Discuss the possible outcomes in her offspring when her husband contains a normal karyotype.
The woman is monosomic. (46 - 1 = 45).
Down Syndrome: Trisomy 21
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