Genetics Exam 2 (Chapter 8)1. List the different types of chromosome mutations and define each one.

What is haploinsufficiency?

Haploinsufficiency is the term for a condition where having only one copy of a wild-type gene does not produce a wild-type phenotype in an otherwise diploid organism. For haploinsufficient genes, the gene dosage and relative amounts of gene products are important.

How do translocations in which no genetic information is lost or gained produce phenotypic effects?

Like inversions, translocations can produce phenotypic effects if the translocation breakpoint disrupts a gene or if a gene near the breakpoint is altered in its expression because of relocation to a different chromosomal environment (a position effect).

What is genetic mosaicism and how does it arise?

Mosaicism is a condition in which an individual has patches of cells that are genetically different from other cells. Mosaicism may arise from mitotic nondisjunction during early embryonic divisions, X-inactivation in a heterozygous female, fusion of two zygotes into a single embryo, and other mechanisms.

What is the difference between primary Down syndrome and familial Down syndrome? How does each type arise?

Primary Down syndrome is caused by spontaneous, random nondisjunction of chromosome 21, leading to trisomy 21. Familial Down syndrome most frequently arises as a result of a Robertsonian translocation of chromosome 21 with another chromosome, usually chromosome 14. Translocation carriers do not have Down syndrome, but their children have an increased incidence of Down syndrome. If the translocated chromosome segregates with the normal chromosome 21, the gamete will have two copies of chromosome 21 and result in a child with familial Down syndrome.

Species I has 2n = 16 chromosomes. How many chromosomes will be found per cell in each of the following mutants in this species? a. Monosomic Solution: b. Autotriploid c. Autotetraploid Solution: d. Trisomic e. Double monosomic Solution: f. Nullisomic g. Autopentaploid Solution: h. Tetrasomic

a) 15 b) 24 c) 32 d) 17 e) 14 f) 14 g) 40 h) 18

Which types of chromosome mutations: a. increase the amount of genetic material in a particular chromosome? b. increase the amount of genetic material in all chromosomes? c. decrease the amount of genetic material in a particular chromosome? d. change the position of DNA sequences in a single chromosome without changing the amount of genetic material? e. move DNA from one chromosome to a nonhomologous chromosome?

a)Duplications b)Polyploidy c)Deletions d)Inversions e)Translocations

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Bill and Betty have had two children with Down syndrome. Bill's brother has Down syndrome and his sister has two children with Down syndrome. On the basis of these observations, indicate which of the following statements are most likely correct and which are most likely incorrect. Explain your reasoning. a. Bill has 47 chromosomes. b. Betty has 47 chromosomes. c. Bill and Betty's children each have 47 chromosomes. d. Bill's sister has 45 chromosomes. e. Bill has 46 chromosomes. f. Betty has 45 chromosomes. g. Bill's brother has 45 chromosomes.

solution:The high incidence of Down syndrome in Bill's family and among Bill's relatives is consistent with familial Down syndrome, caused by a Robertsonian translocation involving chromosome 21. Bill and his sister, who are unaffected, are phenotypically normal carriers of the translocation and have 45 chromosomes. Their children and Bill's brother, who have Down syndrome, have 46 chromosomes, but one of these chromosomes is the translocation that has an extra copy of the long arm of chromosome 21. From the information given, there is no reason to suspect that Bill's wife Betty has any chromosomal abnormalities. a. Incorrect. Bill is phenotypically healthy but is a translocation carrier (with 45 chromosomes). b. Incorrect. Betty is phenotypically healthy, and thus, unlikely to have an extra chromosome. c. Incorrect. Familial Down syndrome, as evident in this family pedigree, results from an extra copy of the long arm of chromosome 21 that is translocated to another chromosome. Thus, the overall number of chromosomes in those with familial Down syndrome is still 46 (although they have three functional copies of chromosome 21). d. Correct. It is likely that Bill and his sister, who are unaffected, are phenotypically normal carriers of the translocation and have 45 chromosomes. e. Incorrect. Bill is phenotypically healthy but is a translocation carrier, with 45 chromosomes because one copy of chromosome 21 is fused with another chromosome. f. Incorrect. Betty is phenotypically healthy. The familial Down syndrome is inherited through Bill's translocation (shared with his sister), thus, Betty is unlikely to have a translocation resulting in 45 chromosomes. g. Incorrect. Because Bill's brother has familial Down syndrome, he carries a translocation with an extra copy of chromosome 21, and has a total of 46 chromosomes.

The green-nose fly normally has six chromosomes: two metacentric and four acrocentric. A geneticist examines the chromosomes of an odd-looking green-nose fly and discovers that it has only five chromosomes; three of them are metacentric and two are acrocentric. Explain how this change in chromosome number might have taken place.

A Robertsonian translocation between two of the acrocentric chromosomes would result in a new metacentric chromosome and a very small chromosome that may have been lost.

What is the difference between a paracentric and a pericentric inversion?

A paracentric inversion does not include the centromere; a pericentric inversion includes the centromere.

How can inversions in which no genetic information is lost or gained cause phenotypic effects?

Although inversions do not result in loss or duplication of chromosomal material, inversions can have phenotypic consequences if the inversion disrupts a gene at one of its breakpoints or if a gene near a breakpoint is altered in its expression because of a change in its chromosomal environment, such as relocation to a heterochromatic region. Such effects on gene expression are called position effects.

Red-green color blindness is a human X-linked recessive disorder. A young man with a 47,XXY karyotype (Klinefelter syndrome) is color blind. His 46,XY brother also is color blind. Both parents have normal color vision. Where did the nondisjunction that gave rise to the young man with Klinefelter syndrome take place? Assume that no crossing over took place in prophase I of meiosis.

Because the father has normal color vision, the mother must be the carrier for color blindness. The color-blind young man with Klinefelter syndrome must have inherited two copies of the color-blind X chromosome from his mother. The nondisjunction event took place during meiosis of the egg.

List the different types of chromosome mutations and define each one.

-- Chromosome rearrangements = 1)Deletion: loss of a portion of a chromosome 2)Duplication: addition of an extra copy of a portion of a chromosome 3)Inversion: a portion of the chromosome is reversed in orientation 4)Translocation: a portion of one chromosome becomes incorporated into a different (nonhomologous) chromosome --Ploidy changes= 1)Aneupoloidy: loss or gain of one or more chromosomes so that the chromosome number deviates from 2n or the normal euploid complement 2)Polyploidy: gain of entire sets of chromosomes so the chromosome number changes from 2n to 3n (triploid), 4n (tetraploid), and so on

List four major types of aneuploidy.

1)Nullisomy: having no copies of a chromosome 2)Monosomy: having only one copy of a chromosome 3)Trisomy: having three copies of a chromosome 4)Tetrasomy: having four copies of a chromosome

Some people with Turner syndrome are 45,X/46,XY mosaics. Explain how this mosaicism could arise.

Such mosaicism could arise from mitotic nondisjunction early in embryogenesis, in which the Y chromosome fails to segregate and is lost. All the mitotic descendents of the resulting 45,X embryonic cell will also be 45,X. The fetus will then consist of a mosaic of patches of 45,X cells and patches of normal 46,XY cells. If the gonads are derived from 45,X cells, the individual will develop as a female with Turner syndrome.

Why do extra copies of genes sometimes cause drastic phenotypic effects?

The expression of some genes is balanced with the expression of other genes; the ratios of their gene products, usually proteins, must be maintained within a relatively narrow range for proper cell function. Extra copies of one of these genes cause that gene to be expressed at proportionately higher levels, thereby upsetting the balance of gene products.

What is a Robertsonian translocation?

The long arms of two acrocentric chromosomes are joined to a common centromere through translocation, resulting in a large metacentric chromosome and a very small chromosome with two very short arms. The very small chromosome may be lost.

Species I has 2n = 8 chromosomes and species II has 2n = 14 chromosomes. What would the expected chromosome numbers be in individual organisms with the following chromosome mutations? Give all possible answers. a. Allotriploidy including species I and II b. Autotetraploidy in species II c. Trisomy in species I d. Monosomy in species II e. Tetrasomy in species I f. Allotetraploidy including species I and II

a)Such allotriploids could have 1n from species I and 2n from species II for 3n = 18; alternatively, they could have 2n from species I and 1n from species II for 3n = 15 b)4n = 28 c)2n + 1 = 9 d)2n - 1 = 13 e)2n + 2 = 10 f)Allotetraploids must have chromosomes from both species, and total 4n. There are 3 possible combinations for such allotetraploids: 2n from each: 2(4) + 2(7) = 22 1n from species I + 3n from species II: 1(4) + 3(7) = 25 3n from species I + 1n from species II: 3(4) + 1(7) = 19

Species I is diploid (2n = 8) with chromosomes AABBCCDD; related species II is diploid (2n = 8) with chromosomes MMNNOOPP. What types of chromosome mutations do individual organisms with the following sets of chromosomes have? a. AAABBCCDD b. MMNNOOOOPP c. AABBCDD d. AAABBBCCCDDD e. AAABBCCDDD f. AABBDD g. AABBCCDDMMNNOOPP h. AABBCCDDMNOP

a)Trisomy A b)Tetrasomy O c)Monosomy C d)Triploidy e)Ditrisomy A and D f)Nullisomy C g)Allotetraploidy h)Allotriploidy

A chromosome initially has the following segments: A B * C D E F G Draw the chromosome, identifying its segments that would result from each of the following mutations: a. Tandem duplication of DEF b. Displaced duplication of DEF c. Deletion of FG Solution: d. Paracentric inversion that includes DEFG e. Pericentric inversion of BCDE Solution:

a.) A B * C D E F D E F G b.)A B * C D E F G D E F or other arrangements where the duplicated DEF is not adjacent to the original DEF c.)A B * C D E d.)A B * C G F E D e.)A E D C * B F G

A chromosome has the following segments, where * represents the centromere: A B *C D E F G mutations are required to change this chromosome into each of the following chromosomes? (In some cases, more than one chromosome mutation may be required.) a.) A B A B * C D E F G b.) A B * C D E A B F G c.) A B * C F E D G d.) A * C D E F G e.) A B * C D E f.) A B * E D C F G g.) C * B A D E F G h.) A B * C F E D F E D G i.) A B * C D E F C D F E G

a.) Tandem duplication of AB b.)Displaced duplication of AB c.)Paracentric inversion of DEF d.)Deletion of B e.)Deletion of FG f.)Paracentric inversion of CDE g.)Pericentric inversion of ABC h.)Duplication and inversion of DEF i.)Duplication of CDEF, inversion of EF