Lecture 9 Flashcards
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. AAABBCCDD: Solution: Trisomy A b. MMNNOOOOPP Solution: Tetrasomy O c. AABBCDD Solution: Monosomy C d. AAABBBCCCDDD Solution: Triploidy e. AAABBCCDDD Solution: Ditrisomy A and D f. AABBDD Solution: Nullisomy C g. AABBCCDDMMNNOOPP Solution: Allotetraploidy h. AABBCCDDMNOP Solution: Allotriploidy
A large plant arose in a natural population. Qualitatively, it looked just the same as the others, except much larger. Is it more likely to be an allopolyploid or an autopolyploid? How would you test that it was a polyploid and not just growing in rich soil?
It would be more likely an autopolyploid. To make sure it was polyploid, you would need to microscopically examine stained chromosomes from mitotically dividing cells and count the chromosome number.
Is a trisomic an aneuploid or a polyploid?
Aneuploid. Trisomic refers to three copies of one chromosome. Triploid refers to three copies of all chromosomes.
A disomic product of meiosis is obtained. What is its likely origin? What other genotypes would you expect among the products of that meiosis under your hypothesis?
The likely origin of a disomic (n + 1) gamete is nondisjunction during meiosis. Depending whether the nondisjunction took place during the first or second division, you would expect one nullosomic (n – 1) gamete, or two nullosomic gametes and another disomic gamete, respectively.
Can a trisomic A/A/a ever produce a gamete of genotype a?
Yes. You would expect that one-sixth of the gametes would be a. Also, two-sixths would be A, two-sixths would be Aa, and one-sixth would be AA.
Which, if any, of the following sex-chromosome aneuploids in humans are fertile: XXX, XXY, XYY, XO?
Both XYY (male) and XXX (female) would be fertile. XO (Turner syndrome) and XXY (Klinefelter syndrome) are known to be sterile.
In an inversion, is a 5′ DNA end ever joined to another 5′ end? Explain.
No. The DNA backbone has strict 5′ to 3′ polarity, and 5′ ends can only be joined to 3′ ends.
If you observed a dicentric bridge at meiosis, what rearrangement would you predict had taken place?
A crossover within a paracentric inversion heterozygote results in a dicentric bridge (and an acentric fragment).
Why do acentric fragments get lost?
By definition, an acentric fragment has no centromere, so it cannot be aligned or moved during meiosis (or mitosis). Consequently, at the end of a cell division, it gets left in the cytoplasm where it is not replicated.
Compare and contrast the origins of Turner syndrome, cri du chat syndrome, and Down syndrome. (Why are they called syndromes?)
urner syndrome is a monosomy in X chromosomes (45 XO) due to a meiotic nondisjunction. Down syndrome (47 or trisomy 21) results from meiotic nondisjunction or from a Robertsonian translocation (with 46 chromosomes but a translocation between 21 and 14). Cri du chat syndrome is the result of a deletion of a significant portion of the short arm of chromosome 5 (specifically bands 5p15.2 and 5p15.3). The term syndrome is used to describe a set of phenotypic changes (often complex and varied) that generally occur together with a specific human chromosomal aberration. All four of these syndromes often include mental retardation and unique body and facial features. In addition, fatality rates are low and most children with these syndromes reach adulthood.
The normal sequence of nine genes on a certain Drosophila chromosome is
123 · 456789, where the dot represents the centromere. Some fruit flies were found to have aberrant chromosomes with the following structures:
a. 123 · 476589
b. 123 · 46789
c. 1654 · 32789
d. 123 · 4566789
Name each type of chromosomal rearrangement, and draw diagrams to show how each would synapse with the normal chromosome.
a. 123 · 476589
Paracentric inversion
b. 123 · 46789
Deletion
c. 1654 · 32789
Pericentric inversion
d. 123 · 4566789
Duplication
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.
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.
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.
What is uniparental disomy and how does it arise?
Uniparental disomy refers to the inheritance of both copies of a chromosome from the same parent. This may arise originally from a trisomic conception in which the early embryo loses one of the three chromosomes, and the two remaining copies are from the same parent.
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.
Explain why autopolyploids are usually sterile, whereas allopolyploids are often fertile.
Autopolyploids arise from duplication of their own chromosomes. During meiosis, the presence of more than two homologous chromosomes results in faulty alignment of homologues in prophase I, and subsequent faulty segregation of the homologues in anaphase I. The resulting gametes have an uneven distribution of chromosomes and are genetically unbalanced. These gametes usually produce lethal chromosome imbalances in the zygote. Allopolyploids, however, have chromosomes from different species. As long as they have a diploid set of chromosomes from each species, as in an allotetraploid or even an allohexaploid, the homologous chromosome pairs from each species can align and segregate properly during meiosis. Their gametes will be balanced and will produce viable zygotes when fused with other gametes from the same type of allopolyploid individual.
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. increase the amount of genetic material in a particular chromosome?
Solution: Duplications
b. increase the amount of genetic material in all chromosomes?
Solution: Polyploidy
c. decrease the amount of genetic material in a particular chromosome?
Solution: Deletions
d. change the position of DNA sequences in a single chromosome without changing the amount of genetic material?
Solution: Inversions
e. move DNA from one chromosome to a nonhomologous chromosome?
Solution: Translocations
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.
A wild-type chromosome has the following segments: A B C · D E F G H I Researchers have found individuals that are heterozygous for the following chromosome mutations. For each mutation, sketch how the wild-type and mutated chromosomes would pair in prophase I of meiosis, showing all chromosome strands.
Refer to Answer key
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 (sperm or egg) 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.
Humans and many complex organisms are diploid, possessing two sets of genes, one inherited from the mother and one from the father. However, a number of eukaryotic organisms spend most of their life cycles in a haploid state. Many of these eukaryotes, such as Neurospora and yeast, still undergo meiosis and sexual reproduction, but most of the cells that make up the organism are haploid. Considering that haploid organisms are fully capable of sexual reproduction and generating genetic variation, why are most complex organisms diploid? In other words, what might be
the evolutionary advantage of existing in a diploid state instead of a haploid state? And why might a few organisms, such as Neurospora and yeast, exist as haploids?
The most obvious advantage to being a diploid is genetic redundancy. Most mutations reduce or eliminate gene function, and are recessive. Having two copies means that the organism, and its component cells, are able to survive the vast majority of mutations that would be lethal or deleterious in a haploid state. This redundancy is especially important for organisms that have large, complex genomes, complex development, and relatively lengthy life cycles. Another advantage to diploidy is that gene expression levels can be higher than in haploid cells, often leading to larger cell sizes and larger, more robust organisms. Indeed, many cultivated polyploid crop plants have higher growth rates and yields than their diploid relatives. A third advantage is that the ability to carry recessive mutations in a masked state allows diploid populations to accumulate and harbor much more genetic diversity. Variant forms of genes that may be harmful to the organism and selected against in the current environment may prove advantageous when the environment changes. These advantages are less important for organisms that have small genomes and simpler, shorter life cycles. Yeast and Neurospora genomes are highly adapted to their ecological niches, and haploid growth exposes and weeds out less favorable genetic variants. The ability to replicate their haploid genomes more quickly may give them a selective advantage over diploids. Indeed, these species become diploid and undergo meiosis only when conditions become less favorable for growth.
