Homework 8 - Chromosome Rearrangements

How many chromosomes would you expect to be present in t(14;21) Down syndrome individuals?

46

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 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 arrangements would yield the greatest reduction in gamete viability?

An inversion heterozygote for a large inversion. A large heterozygous inversion will result in greater numbers of genes that are duplicated or deleted when crossing over occurs.

It is likely that when certain combinations of genes are of selective advantage in a specific and stable environment, it would be _________ to the organism to protect that gene combination from disruption through crossing over. By having the genes in an ________, _______ are not recovered and therefore are not passed on to future generations.

Beneficial ; Inversion; Chromatid

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 of either normal or inverted sequence.

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 percent of its length. The homolog of chromosome 1 and all other chromosomes show the normal banding sequence. How would you explain the high incidence of past stillbirths? What can you predict about the probability of abnormality/normality of their future children? How is it possible to determine some of the chromosomal abnormalities during pregnancy?

Crossing over in the inversion loop of an inversion (in the heterozygous state) had produced defective, unbalanced chromatids, thus leading to stillbirths and/or malformed children. It is probable that a significant proportion (perhaps 50 percent) of the children will be similarly influenced by the inversion. by performing an amniocentesis or CVS

In general, inversion and translocation heterozygotes are as fertile as organisms whose chromosomes are in the standard arrangement. (T/F)

False

Individuals with familial Down syndrome are trisomic and have 47 chromosomes. (T/F)

False

Inversion loops do NOT form during meiosis in paracentric inversion heterozygotes. (T/F)

False Loop formation allows pairing within inverted regions of homologous chromosomes regardless of whether the centromere is included in the inversion.

Which of the following syndromes is not paired with its causative chromosomal aberration? Fragile X syndrome - deletion Down syndrome - Robertsonian translocation Cri du chat syndrome - deletion Huntington disease - duplication

Fragile X syndrome - deletion Fragile X syndrome is an example of a trinucleotide repeat disorder, which shows increasing severity with increased copy number of a specific three‑base sequence.

Which of the following statements regarding familial Down syndrome is false? It accounts for approximately 5% of all Down syndrome cases. Heterozygous carriers of the Robertsonian translocation that cause familial Down syndrome can have children who are also carriers of the translocation. 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. 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.

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. 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.

What phenotype would be expected in balanced translocation heterozygotes in the absence of position effects?

Normal, because they have a normal amount of genetic material Balanced translocation heterozygotes have a normal amount of genetic material, but it is in a translocated configuration. As long as there are no position effects, these individuals can be phenotypically normal.

The outcome of a single crossover between nonsister chromatids in the inversion loop of an inversion heterozygote varies depending on whether the inversion is of a pericentric or paracentric type. What differences would be expected in the four chromatids following crossing over and meiosis with a pericentric vs. a paracentric inversion?

Pericentric inversions produce four chromatids with one centromere each, while paracentric inversions produce an acentric and a dicentric chromoatid (both inviable).

Human adult hemoglobin is a tetramer containing two alpha (α) and two beta (β) polypeptide chains. The α gene cluster on chromosome 16 and the β gene cluster on chromosome 11 share amino acid similarities such that 61 of the amino acids of the α-globin polypeptide (141 amino acids long) are shared in identical sequence with the β-globin polypeptide (146 amino acids long). How might one explain the existence of two polypeptides with partially shared function and structure on two different chromosomes?

The clusters arose from the same ancestral gene.

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 _______. chromosome with two centromeres chromosome with some duplicated regions completely normal chromosome chromosome with some deleted regions

This won't happen --> chromosome with two centromeres -This won't happen because when the centromere is contained within the inversion, it is neither duplicated nor deleted.

__________ offers an opportunity for new gene combinations by associations of genes from ___________ chromosomes. Under certain conditions such new combinations may be of selective advantage. Meiotic conditions have evolved so that segregation of ___________ chromosomes yields a relatively uniform set of gametes.

Translocation ; Nonhomologous ; Translocated

A paracentric inversion is one whose breakpoints do not flank the centromere. (T/F)

True

A pericentric inversion includes the centromere. (T/F)

True

Gene duplications provide an explanation for the origin of gene families. (T/F)

True

The chromosomal aberration that causes cri du chat syndrome can be referred to as a segmental deletion. (T/F)

True

rDNA in eukaryotes is typically redundant. (T/F)

True

Duplications and deletions can be detected during meiosis by the presence of extrachromosomal loops that do not pair properly with their homolog. (T/F)

True During meiosis, the presence of a chromosomal loop signals that a duplication or deletion has occurred.

A mechanism for bobbed reversion is likely based on meiotic recombination and occurs through _______ crossing over. When ________ chromosomal regions _______, homologs can misalign. If crossing over occurs in the misaligned segments, one _________ can gain chromosomal material at the expense of the other _________. As a chromosome gains rRNA genes, it harbors a selective advantage and produces flies that outcompete those with _________ bobbed mutations. Eventually, a stock that originally contained bobbed Drosophila contains wild-type flies over time.

Unequal ; Redundant ; Synapse ; Chromatid ; Chromatid ; Nonreverted

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 translocation between chromosome 21 and a member of the D chromosome group

How can duplications arise?

by 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.

In a recent cytogenetic 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 symbol represent?

t(14;21) indicates that a translocation (t) has occurred between chromosomes 14 and 21.

A Robertsonian translocation is considered non-reciprocal because _______.

the smaller of the two reciprocal products of translocated chromosomes is lost After several cell divisions, only the larger of the two translocated chromosomes remains. Its reciprocal is lost.