|
Homologs fail to separate during _______ |
Meiosis I |
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Homologs failing to separate during meiosis I is a possible _____ event |
Nondisjunction |
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All of the following events occur during normal meiosis except___ 1. Two haploid gametes fuse to form a diploid cell |
1. Fusion of haploid gametes occurs AFTER meiosis |
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T/F: In order to create the possibility of generating a trisomy, nondisjunction must occur during meiosis II. |
False. Nondisjunction during either meiosis I or meiosis II creates gametes that will generate trisomies if fertilized |
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During ___ of the first round of meiotic division, homologous chromosomes separate and migrate to opposite poles |
Anaphase |
|
During ____ of the second round of meiosis (and mitosis), sister chromatids separate and move to opposite poles |
Anaphase |
|
Meiosis is responsible for producing __ haploid cells from __ original parent cell |
4, 1 |
|
Failure of chromosomes to separate from one another during either round of meiosis results in ___ |
Nondisjunction |
|
When does nondisjunction occur? |
Meiosis or mitosis in both sexes |
|
Normal chromosome separation begins with a ___ cell at the start of meiosis |
Diploid |
|
Each chromosome consists of 2 _______ joined at their centromere |
Sister chromatids |
|
During the first meiotic division, homologous chromosomes separate, resulting in 2 cells, each containing ____ |
One pair of sister chromatids |
|
During the second meiotic division, what happens? |
The centromeres split and the 2 sister chromatids segregate to different cells |
|
The end result of meiosis is? |
4 haploid cells, each with one sister chromatid, now called a chromosome |
|
In animals, the 4 haploid cells are called |
Gametes |
|
When gametes fuse with a gamete from the opposite sex, what is produced? |
Normal diploid cells |
|
What happens if nondisjunction occurs during meiosis I? |
One cell is produced with both homologues and the other cell has no copies of this chromosome. When the cell divides during meiosis II, the centromeres of both sister chromatids split and they segregate to each of the 2 progeny cells. These 2 cells are unusual in that they contain 2 copies of this chromosome instead of one. In the other 2 cells, there are no copies of this chromosome instead of the usual one |
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Upon fusion with a normal haploid gamete, all of the cells produced are ____ for this chromosome (if nondisjunction happens during meiosis I) |
Abnormal. Half of the cells will have 3 copies of the chromosome, instead of the normal 2. The other half will only have one copy of the chromosome |
|
What are trisomic cells? |
Cells that have 3 copies of a chromsome instead of two |
|
What are monosomic cells? |
Cells that have 1 copy of a chromosome instead of 2 |
|
What happens if nondisjunction occurs during meiosis II? |
2 abnormal cells are produced: one having 2 copies of the chromosome and the other having none. Normal segregation occurs in the other cell, giving rise to 2 normal haploid cells |
|
What is Turner’s syndrome? |
A female with only one X |
|
What is Klinefelter’s syndrome? |
A male with 2 X’s and one Y |
|
Nondisjunction of the autosomes is responsible for ___ |
Down Syndrome (trisomy 21) |
|
The incidence of Down syndrome, also known as trisomy 21, increases with increasing maternal age. What error most likely produces this condition? |
Nondisjunction during either meiosis I or II in the female gamete |
|
T/F: Females with only one X chromosome do not develop; this condition is lethal |
FALSE. They have Turner syndrome. but MEN that lack an X chromosome do not develop and are lethal |
|
Which of the following statements about the products produced when nondisjunction occurs during meiosis I is true? 1. Both products of the first meiotic division have too many chromosomes |
4. |
|
Before meiosis begins, the chromosomes are duplicated, resulting in ___ chromatids per chromosome |
two |
|
What is disjunction? |
Homologous chromosomes synapse, giving rise to tetrads consisting of 4 chromatids each. The components of the tetrad then separate into 2 dyads |
|
Each dyad consists of.. |
Two sister chromatids joined at a common centromere |
|
Meiosis I is a ____ division |
reductional |
|
The second meiotic division is similar to the division in mitosis in that each dyad splits into ____ |
two monads (also called disjunction) |
|
Meiosis II is an ____ division |
Equational |
|
What is aneuploidy? |
A condition in which a cell lacks an exact multiple of a haploid set |
|
(2n-1) cells are.. |
Monosomic |
|
(2n+1) cells are.. |
Trisomic |
|
Both monosomy and trisomy are ____ conditions |
Aneuploid |
|
Characteristics of Klinefelter for men |
Large breasts, small testes, may be mentally retarded |
|
A female with an extra X chromosome is considered.. |
"47,XXX" |
|
What are the characteristics of "47,XXX"?/ |
Most females go through their lives without being aware of it and are essentially normal |
|
Characteristics of Turner syndrome |
Sterile due to underdeveloped ovaries |
|
What is the general term for a condition in which the chromosome number is not a multiple of a complete set? |
Aneuploidy |
|
Which of the following statements about nondisjunction is false? 1. It separates maternal from paternal chromatids |
1. |
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T/F: A cross between a tetraploid and a diploid member of the same species will product offspring that can undergo sexual reproduction |
False. Offspring from this cross would be triploid and produce gametes with an uneven number of homologous chromosomes, making sexual reproduction unlikely |
|
Which of the following statements about allopolyploid individuals is true? 1. They can be formed when two sperm simultaneously fertilize an ovum within the same species |
4. Both autopolyploid and allopolyploid individuals may be sterile |
|
How many chromosomes would be found in an allopolyploid plant if its parents had diploid number of 4 and 6 respectively? |
Haploid number of . The parents contribute 2 and 3 chromosomes respectively to each of their gametes. |
|
When does polyploidy occur? |
When complete haploid sets of chromosomes are added to a diploid genome |
|
Polyploid is common in ____ and has likely contributed to their extensive evolutionary diversity |
Plants |
|
What is polyploidy? |
Any condition in which entire haploid sets are found in multiples greater than two |
|
Triploids have ___ chromosomes |
3n |
|
Tetraploids have ___ chromosomes |
4n |
|
Why aren’t odd numbers of chromosome sets usually not maintained across multiple generations? |
The odd number of homologues results in chromosomally unbalanced gametes |
|
Two basic types of polyploidy |
Autopolyploidy and Allopolyploidy |
|
What is autopolyploidy? |
Each additional set of chromosomes is identical to the normal haploid set in its species |
|
What is allopolyploidy? |
The multiple sets of chromosomes result from matings between two closely related species |
|
Three ways autotriploidy can occur |
Two sperm may simultaneously fertilize an ovum..OR.. All the chromosomes of an organism may fail to segregate during the first or second meiotic division, which can produce a diploid gamete and if that diploid survives and is fertilized by a normal haploid gamete, a triploid results..OR.. from crosses between diploid and tetraploid member of a species |
|
How can autopolyploids be produced experimentally? |
Through exposure to colchicine (with interferes with spindle formation, with the result that a cell can duplicate its chromosomes but the chromosomes cannot separate from each other to opposite poles) |
|
What happens when colchicine is removed? |
The cells can reenter interphase, this time with double the number of chromosomes- 4n instead of 2n |
|
Why does allopolyploidy occur? |
From the hybridization of two closely related species (species recently derived from a common ancestor may interbreed and produce fertile, diploid offspring.) If the two species have diverged somewhat more, they may cross successfully, but the hybrid offspring are usually sterile. |
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Why are more distantly related species not likely to hybridize at all? |
Because of gamete incompatibility or other prezygotic isolating mechanisms |
|
What happens when a sterile hybrid undergoes meiosis? |
The chromosomes cannot find homologs for pairing and thus produce genetically unbalanced gametes, resulting in the death of the zygote or embryo |
|
What happens after fertilization between members of two species and chromosome doubling? |
A fertile amphidiploid containing two complete diploid genomes is formed |
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Why is this type of allopolyplody important? |
Important in the evolution and artificial development of many plant groups, including important grain crops |
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Amphidiploids produce gametes with a number of chromosomes ____ to the sum of the number in the two original haploid gametes |
Equal |
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What two terms can be used to describe Down syndrome? |
Trisomic and aneuploid |
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Which arrangement yields the greatest reduction in gamete variability? |
An inversion heterozygote for a large inversion, this will result in greater numbers of genes that are duplicated or deleted when crossing over occurs |
|
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… 1. chromosome with some duplicated regions |
3. When the centromere is contained within the inversion, it is neither duplicated nor deleted |
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T/F: Inversion loops do NOT form during meiosis in paracentric inversion heterozygotes |
False. Loop formation allows pairing within inverted regions of homologous chromosomes regardless or whether the centromere is included in the inversion. |
|
What is an inversion? |
A chromosomal mutation wherein a segment within the chromosome breaks at 2 ends, changes orientation by 180 degrees, then reintegrates into the same chromosome |
|
Two types of inversion |
Pericentric and paracentric |
|
Which type of inversion includes the centromere in the inverted segment? |
PERIcentric |
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Is genetic material gained, lost, both, or neither in an inversion? |
Neither |
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An individual who is heterozygous for the pericentric inversion has…. |
One chromosome with the normal order of genes and the other chromsome with an inverted segment |
|
Inversion loop |
The homologous chrosmomes pair to the best of their ability in meiosis, and this means that the pairing of the inverted segment with the normal segment will produce a LOOP |
|
When do problems arise with inverted segments? |
When a crossover occurs within the inverted segment between a normal chromosome and an inverted chromosome during meiosis. Crossovers can give rise to viable and inviable gametes. |
|
If a single crossover occurs within the inversion loop… |
4 different chromosomes are produced |
|
The first chromsome where no crossover event occurred has the ____ order of genes. This is a ____ gamete. |
Normal, viable |
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The second chromsome was involved in the crossover. It lacks ____ and is duplicated for others. Due to the deletions and duplications, the gamete produced is ____. |
Genes, inviable |
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The third chromsoome was not involved in the crossover. This chromosome is identical to the _____ inverted chromosome. Because it contains all the genes, even though they are in a different order, the gamete with this chromosome is ____ |
Original parental, viable |
|
The fourth chromosome was also involved in the corssover. It lacks some ____ and is duplicated for others. Due to the deletions and duplications, it also produces an ____ gamete. |
Inviable |
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A single crossover event within a heterozygous pericentric inversion gives rise to ___ viable and ___ inviable gametes |
2, 2 |
|
What is a deletion? |
The loss of part of a chromosomal segment |
|
What is a duplication? |
The repetition of a segment. The repeated segment may be located next to the original or at a different location, and its orientation may be the same as the original or reverse. |
|
What is an inversion? |
The removal of a segment followed by its reinsertion into the same chromosome in the reverse orientation |
|
What is a translocation? |
The transfer of a segment to a nonhomologous chromosome. They may be reciprocal (two nonhomologous chromosomes exchange segments) or nonreciprocal (one chromsome transfers a segment without receiving one) |
|
Crossing over is rare between genes that are.. |
close together (occurs more frequently between genes that are far apart) |
|
Once genetic information is taken into a recipient cell, it can recombine with the recipient’s chromosome and create an _______ |
Altered bacterial genotype |
|
The main mechanism for transfer of genetic information in bacteria is ___ |
Conjugation |
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Only ____ bacteria are able to grow on minimal medium. What are these called? |
Wild-type, prototrophs |
|
Conjugation is _____ and requires… |
Unidirectional, physical contact between strains |
|
What does F stand for (different strains are F+ and F-) |
Fertility factor (or F factor) |
|
F+ strains are ____ of chromosomal maternal and F- strains are ___ |
Donors, receivers |
|
What is the F factor? |
A plasmid that directs the synthesis of pili, tubelike extensions of the cell that allow contact with other cells |
|
During conjugation, the two DNA strands of the F factor ____ and one strand moves into the recipient cell |
separate |
|
Both strands serve as templates for DNA replication, resulting in |
two F+ cells |
|
The F factor itself is passed to all recipient cells, but on rare occasions what happens? |
The transfer of genes from the bacterial chromosome (genetic recombination) |
|
What are Hfr strains? |
Strains of bacteria that undergo recombination at a high rate in mutagenesis experiments |
|
Hfr strains result from.. |
Integration of the F factor into the bacterial chromosome of an F+ strain |
|
The arrangment of the genes on the bacterial chromosome can be determined by.. |
Using interrupted mating with different Hfr strains (mechanically at specific time intervals before the entire chromosome is transferred), which allows gene order and the distance between genes to be determined |
|
During conjugation between Hfr and F- cells, the position of the F factor determines.. |
The initial point of transfer |
|
The point where transfer originates is determined by.. |
the point of integration of the F factor into the chromosome |
|
Transfer can proceed in what direction? Depending on what? |
Either, the orientation of the F factor in the bacterial chromosome |
|
The F factor can lose its integrated state, causing an Hfr strain to revert to ___ |
F+ |
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An F’ bacterium can initiate conjugation with F- cells and may transfer ___ to the F- cell on the F factor… Is this case, what is formed? |
chromosomal genes, a partially diploid (merozygote) |
|
Transformation provides a mechanism for… |
The recombination of genetic information in bacteria |
|
Cells must be competent. What does this mean? |
Able to take up DNA |
|
In transformation, double-stranded DNA enters the bacterial cell. What happens? |
One of the strands is degraded by nucleaes, whereas the other strand integrates into the bacterial DNA |
|
T/F: A bacterial strain that is: pro+thi+leu-met- will grow on minimal media plus leucine and thiamine |
FALSE. The strain is auxotrophic for leucine and methionine, meaning that it cannot synthesize those nutrients. This, it requires those two nutrients to be added to minimal media for growth |
|
Which of the following statements about conjugation is true? 1. Only competent cells can undergo conjugation |
3. |
|
Bacteriophages |
Viruses that infect bacteria, using them as hosts in which to reproduce |
|
Bacteriophage T4 has a ___ structure |
Complex. |
|
The head of the virus consists of DNA inside an ____ |
Icosahedral protein coat |
|
The head is connected to… |
A tail containing a collar and a contractile sheath with a central core |
|
Tail fibers have.. |
Binding sites in their tips that recognize unique areas on the cell wall of the host, E. coli |
|
Viral assembly takes 3 sequential paths |
1. DNA packaging as the viral heads are assembled, tail assembly, and tail-fiber assembly |
|
Once DNA is packaged into the head, it combines with the ___ components, to which tail fibers are added |
Tail |
|
Total construction is a combination of.. |
Self-assemly and enzyme-directed processes |
|
What is the first step in the life cycle of T4? |
Adsorption of the bacterial host |
|
An ATP-driven contraction of the tail sheath causes… |
The central core to penetrate the cell wall |
|
The DNA is then ___ into the host |
Injected |
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Within minutes, all bacterial ____, ____, and ____ is inhibited and synthesis of _______ begins |
DNA, RNA, protein synthesis, viral molecules |
|
At the same time, _____ of the host DNA is initiated |
degradation |
|
Phage DNA replication results in… |
A pool of viral DNA molecules |
|
The proteins that compose the head, tail, and tail fibers are synthesizes, and the ___ viruses are assembled |
Mature |
|
The phage produces a ____ that ruptures the bacterial cell wall and releases the virsus |
Lysozyme |
|
Lysis occurs when approximately ___ virus particles are produced |
200 |
|
These phages infect other bacterial cells and what happens? |
The cycle continues |
|
What is the plaque assay? |
A technique developed for the study of bacteriophages |
|
T/F: Bacteriophage T4 can adsorb to its host because of binding sits in its tail fibers that recognize areas of the E. coli cell wall |
TRUE |
|
Which of the following statements about the T4 lytic life cycle is false? 1. The phage DNA is injected into the host and integrates into the bacterial chromosome |
1. The phage DNA is inject into the host but is then replicated during the lytic cycle, producing a pool of viral DNA particles. Integration into the bacterial chromosome occurs only during the lysogenic cycle |
|
What is transduction? |
the exchange of genetic material using a bacteriophage |
|
What is conjugation? |
dependent on the F factor, which, by a variety of mechanisms, can direct genetic exchange between two bacterial cells |
|
What is transformation? |
the uptake of exogenous DNA by cells |
|
In an interrupted mating experiment, the purpose of planting cells on a selective medium is.. |
To ensure that only recombinant genotypes are recovered (allow elimination of parental genotypes and recovery of only those whose genotypes result from transfer of donor genes) |
|
Mapping bacterial genes by conjugation is based on which assumption? |
Genes are transferred from donor to recipient in a linear fashion (if this were not true, the distance between genes could not be measured as a function of time) |
|
Which of the following statements about mapping bacterial genes by conjugation is NOT true? 1. Two genes that are very close together may appear to be transferred at the same time |
2. It depends on which side of the origin the gene resides |
|
Gene mapping experiments in bacteria are useful in determining.. |
the organization of genes on the chromosome |
|
Gene mapping experiments rely on.. |
the ability of genes to be transferred between different bacterial strains |
|
What is conjugation? |
A process whereby DNA is transferred through direct contact between a donor bacterium and a recipient bacterium |
|
When are recombinants seen in the recipient? |
Soon after a gene enters |
|
How can the order of genes and their relative distances apart be determined? |
Timing the appearance of recombinants |
|
What is the donor bacterium? |
An Hfr strain, which has the F factor integrated into the chromosome |
|
The F factor controls what 2 things? |
The mating between 2 cells and the transfer of genes from donor to recipient |
|
What is the donor bacterium? |
F- cell (a chromosome but no F factor) |
|
The map units between genes are expressed in ____ |
minutes |
|
E. coli genetic map is ____ minutes |
100 |
|
Low heritabliy = |
most likely to be environmental |
|
How to calculate realized heritability |
(parental mean)/(population mean + meaning training value) In homework: (6)/(8.5+7.5) |
|
What is broad-sense heritability? |
all the genetic contributions to a population’s phenotypic variance (H^2) |
|
How to calculate broad-sense heritability of a trait |
vG/vP |
|
What is narrow-sense heritability? |
The additive genetic portion of a population’s phenotypic variance (h^2) |
|
How to calculate narrow-sense heritability of a trait |
vA/vP |
|
How to know which of two traits will respond best to selection by a breeder? |
HIGHER narrow-sense variability |
|
Heritability for yield is always ___ for inbred, genotypically identical population |
0 |
|
How to calculate mean weight of progeny given narrow-sense heritability, mean population weight and 2nd generation weight |
(2ndgenweight-parentweight) = K K x (h^2) = L L + (parentweight) = MEAN PROGENY WEIGHT |
|
Realized heritability = |
Narrow-sense heritability |
|
Map distance = |
centiMorgans |
|
Chromosome mutations are… |
large-scale changes in the number or structure of chromosomes |
|
What is cri du chat syndrome? |
Deletion of the tip of chromosome 5 |
|
Is Down Syndrome a chromosome mutation? |
YES, 3 copies of chromosome 21 |
|
4 types of chromosome mutations: |
1. translocation 2. deletion 3. duplication 4. inversion |
|
Gain of genetic material |
Duplication |
|
Loss of genetic material |
Deletion |
|
Relocation of genetic material (2) |
Translocation and Inversion |
|
An individual that possesses a multiple of the basic chromosome set is called.. |
Euploid |
|
An organism with more than two chromosome sets is.. |
Polyploid (triploid 3n, tetraploid 4n, pentaploid 5n, hexaploid 6n, etc) |
|
An individual that has only one chromosome set is called |
Monoploid |
|
The ___ number of chromosomes is the number of chromosomes in gametes |
Haploid |
|
Polyploidy is much more common in ____ than in ____ |
Plants, animals |
|
What can cause an increase in plant size? |
Polyploids are often larger than diploids (each cell is bigger and the overall size as well) |
|
What are autopolyploids? |
Multiple chromosome sets that arise within the same species |
|
Autopolyploids may arise when ___ fails to occur at the end of mitosis |
Cytokinesis |
|
Allpolyploids arise from ______ between species |
Hybridization |
|
Allopolyploidy is important in the evolution of ____ |
Plant species |
|
If an allotetraploid plant has 26 chromosomes in its somatic cells and one of its progenitor species has n = 8, what is the chromosome complement of the other species? |
n = 5 Since it’s a tetraploid (2n + 2n) 2n = 10 2n = 16 10 + 16 = 26 |
|
____ involves chromosome replication and separation of sister chromatids without cell division |
Endomitosis |
|
Endomitosis produces ____ tissues |
Polyploid |
|
____ chromosomes result from sister chromosomes that do not separate |
Polytene |
|
An ___ individual has a chromosome number that is NOT an exact multiple of the haploid number |
Aneuploid |
|
2n-2 chromosomes = |
Nullisomy |
|
2n-1 chromosomes = |
Monosomy |
|
2n+1 chromosomes = |
Trisomy |
|
Most aneuploidy is caused by _____ during meiosis |
Nondisjunction |
|
What is nondisjunction? |
The failure of homologous chromosomes or chromatids to move to opposite poles during anaphase |
|
Monosomy can develop from union of a _____ gamete with a gamete lacking _______ |
normal, a particular chromosome |
|
In most diploid organisms, monosomy is _____ |
Deleterious |
|
In humans, monosomy for any autosome causes what type of death? |
Death in utero |
|
Turner syndrome is a _____ human monosomy. There is only one ___ chromosome and no ___ chromosome |
Non-lethal, X, Y |
|
What is Turner syndrome represented by? |
45, XO |
|
Males with Turner syndrome are phenotypically ___, but _____ |
Female, sterile |
|
What problems arise with Turner? |
Learning deficiencies, short stature, webbed neck, cardiovascular abnormalities |
|
Trisomy can develop from… |
Union of a normal gamete with a gamete having an extra copy of one chromosome |
|
In diploid organisms, trisomy is usually ____, but there are many examplse of viable trisomics |
Deleterious |
|
What are the 3 human sex chromosome trisomies? |
47,XXX 47,XXY 47,XYY |
|
Phenotypically normal females but sometimes with reduced fertility and mental impairment |
47,XXX |
|
47,XXY: Phenotypically male, but with some female secondary sexual characteristics, usually sterile, called _____ syndrome |
Klinefelter |
|
Phenotypically male, but tend to be taller than average; usually fertile |
47,XYY |
|
______ is the only common human autsomal trisomy |
Down syndrome |
|
DS usually results from… |
An extra copy of chromosome 21 |
|
Phenotype of DS |
Mental impairment, a broad flat face, short stature, and a distinctive palmar crease |
|
DS females are often ____, but males rarely are |
Fertile |
|
Risk of DS increases with…. |
Maternal age 3/1000 at 35 10/1000 at 40 1/30 at 45 1/15 at 50 |
|
Only two other human trisomics survive to birth…. Most die within a few weeks or months of birth |
Trisomy 13 or trisomy 18 |
|
Plants tend to be more tolerant of ____ |
Aneuploidy |
|
Why does aneuploidy cause severely abnormal phenotype? |
Gene dosage effect: Amount of gene product is proportional to number of copies of the gene |
|
Normal development depends on.. |
Having the proper balance of all gene products |
|
In aneuploids, the normal ratios of gene copies are _____: genes on the affected chromosome are either too abundant or too rare, relative to genes on other chromosomes |
Disrupted |
|
A man is found to have one Barr body in his somatic cells. What is his chromosomal constitution? |
47,XXY |
|
What is a Barr body? |
An inactive X chromosome in the female somatic cell |
|
Small deletions within a gene may produce a.. |
phenotypic effect by inactivating the gene |
|
Large deletions may completely…. causing…. |
Remove several genes, severe phenotypic effects |
|
Phenotype abnormalities of cri-du-chat syndrome |
Abnormally small head, mental retardation, and a distinctive catlike mewing sound made by affected infants |
|
In a _____ ______, one member of a homologous pair is normal and the other has a deletion |
Deletion heterozygote |
|
During meiosis, deletion heterozygotes contain a ____, because part of the normal homolog has nothing to pair with |
Loop |
|
deletion heterozygotes can show _____, which happens when the normal chromosome contains a recessive allele for a gene that is deleted in the other chromosome |
Pseudodominace |
|
Tandem duplications |
Are adjacent to each other |
|
Insertional duplications |
Located elsewhere in the genome |
|
Segmental duplications |
Large duplicated units that include whole genes and the regions in between |
|
Inversion |
The reversion of the orientation of a chromosomal segment; a chromosome segment is detached, flipped around, and re-inserted |
|
Inversion heterozygote |
Has one normal chromosome and one with an inversion |
|
_______ inversions can lead to deletion products |
Pericentric |
|
_____ inversions can lead to duplication-and-deletion products |
Paracentric |
|
Reciprocal translocation |
Two non-homologous chromosomes trade pieces |
|
What does independent assortment create? |
New combinations of alleles not seen in the parental strains |
|
Meiotic recombination |
Any meiotic process that makes haploid gametes with allele combinations different from those in the parental gametes |
|
___ ___ is one source of meiotic recombination |
Independent assortment |
|
T/F Genes on separate chromosomes assort independently |
TRUE. Genes close together on the same chromosome do NOT assort independently |
|
Linkage |
The tendency for genes on the same chromosome to be inherited together |
|
Linkage analysis |
Testing whether two genes are linked or independently assorting |
|
3 reasons for linkage analysis |
1. if genes are linked, we know that they are on the same chromosome 2. the strength of linkage tells us the distance between two genes 2. distance information helps us to draw a map of the genes on each chromosome |
|
Worse way to test whether two genes are linked, and the problem |
Perform a dihybrid cross and test null hypotehsis that offspring have 9:3:3:1 phenotypic ratio Problem: Can’t tell the genotype of offspring with dominant phenotype |
|
Better way to test whether two genes are linked |
Perform a dihybrid testcross: cross a double heterozygote with a tester (an individual homozygous recessive for both genes) |
|
What proportions of each phenotype would you expect in a dihybrid testcross if independent? |
1/4 for each |
|
Two genes are linked when.. |
Dominance and recessiveness for both traits offspring are greater in number than heterozygous offspring |
|
Genes on the same chromosome can recombine during.. |
Crossing over |
|
When does crossing over occurs? |
During synapsis in prophase I, when homologous chromosome dyads pair up |
|
What leads to the exchange of chromosome parts between homologs in crossing over? |
Breakage and re-union |
|
Two ways to get meiotic recombination |
Independent assortment and crossing over |
|
Independent assortment creates new combinations of alleles on ___ chromosome(s) |
Different |
|
Crossing over creates new combinations of alleles on ___ chromosome(s) |
Same |
|
When genes are linked, testcross yields ____ recombinants than parentals |
Fewer |
|
The number of recombinants depends on… |
The distance between the genes |
|
Crossover frequency measures… |
The distance between two genes on a chromosome |
|
The greater the physical distance between two genes, the higher the.. |
Frequency of crossovers between them |
|
Distance (crossover frequency) can be estimated by.. |
Measuring recombination frequency |
|
How to calculate recombination frequency |
(Number of recombinant progeny)/(Total number of progeny) |
|
Distances on gene map are measured in |
centiMorgans (cM) |
|
One cM = |
a crossover frequency of .01 |
|
Gene mad distances _____ |
Additive |
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4 steps in drawing a linkage map |
1. Cross an individual that is heterozygous for 2 genes with an individual that is homozygous recessive (testcross) 2. Classify the offspring according to phenotype 3. Determine which offspring are recombinants and which are non-recombinants 4. Calculate the recombination frequency to estimate the genetic distance between the genes |
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Each crossing-over event creates how many recombinant chromosomes? |
Two |
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If the most offspring are heterozygous, the allele arrangement in the dihybrid parent must be.. |
Both heterozygous!! |
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How can we tell which order of genes on a chromosome is the right one? |
Three-point testcross (cross a triple heterozygote, trihybrid, and a tester homozygous recessive for all 3 genes) |
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What does the 3-point testcross test for? |
Linkage between all 3 genes; can also tell their order and distance between them if they are linked |
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How is gene order revealed? |
Comparing the double recombinant genotypes with the parental genotypes |
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The allele pair for the middle gene will appear _____ in the double recombinants |
"flipped" |
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5 steps in analyzing a 3-point testcross |
1. Determine which 2 offspring classes are non-recombinant (parental) 2. Determine which 2 offspring classes are double recombinants 3. Compare parental and double recombinants to determine gene order 4. Use the gene order to classify single recombinants 5. Calculate the distances between the center gene and each of the outer genes |
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How to calculate the distances between center gene and each of the outer genes |
(4 smallest numbers)/(all) |
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What are the 2 advantages of bacteria as genetic model systems? |
Short reproduction cycle and large clonal cultures |
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Why is E. coli the model bacterial organism? |
Symbiotic bacteria in human and other animal guts, a singular circular chromosome of 4.6Mb (haploid), unicellular and grows by simple cell division |
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Bacteria have small circular DNA molecules called |
Plasmids |
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Wild-type bacteria are also called |
Prototrophic (grow and divide on minimal media such as inorganic salts, carbon source, water) |
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Nutritional mutants are also called |
Auxotrophic (no growth without specific cellular building blocks or energy source, no growth when only one particular energy source is given) |
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3 processes of bacterial genetics |
Transformation, conjugation, transduction |
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Transformation |
Uptake of free DNA |
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Conjugation |
Direct transfer of DNA from one bacterium to another |
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Transduction |
Transfer of bacterial DNA by a bacteriophage |
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When is cell contact required (3 processes)? |
conjugation only |
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Which 3 processes are sensitive to DNase? |
Transformation only |
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Does donor lose DNA in conjugation? |
NO |
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Integration of the F plasmid into chromosomes creates an ____ stran |
Hfr |
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Conjugation is usually ____ before the chromosome transfer is completed |
Interrupted |
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How can the genetic map of bacteria be inferred? |
By identifying genes (alleles) that transferred together |
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How does one generate a chromosome map? |
Tracking time of marker entry |
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Two types of DNA transfer can take place during conjugation |
Chromosome transfer, plasmid transfer |
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Mendelian genetics was developed by studying ___ traits, but most traits are ____ |
Discrete, continuous |
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Quantitative genetics |
The study of the genetics of continuously varying traits (most traits are quantitative) |
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With discrete traits, ratios of phenotypic classes reveal… |
Underlying genotype |
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Quantitative traits do not allow.. |
Classification of offspring into discrete categories |
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What is the phenotype of an individual influenced by? |
Both its genotype and environment |
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Differences between parental strains are due to ____ differences |
Genetic |
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Differences among individuals within each strain are due to ____ |
Environment |
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Variation in ___ generation is also due to environmental effects |
F1 |
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Heritability |
the proportion of the variation in phenotype that is due to variation in genotype |
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Finding out whether a trait is heritable (IS/IS NOT) the same as finding out whether it is influenced by genes at all |
IS NOT |
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If a trait shows continuous phenotypic variation, we ask two big questions: |
1. Is any of the phenotypic variation due to genetic causes? 2. If so, how much of the variation is genetic and how much is environmental? |
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How to calculate variance |
Sum squared deviations from the mean, divided by the number of measurements |
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Phenotypic variation |
Vt = Ve+Vg |
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Broad-sense heritability |
Measures the proportion of total phenotypic variation that is due to genetic variation |
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How to measure H^2 |
Vg/Vt |
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Two subgroups of genetic variance |
Additive variance and dominance variance |
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Additive variance |
Variance that is due to effects of individual alleles |
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Dominance variance |
Variance that is due to dominance relations among alleles |
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Example of trait where all genetic variance is ADDITIVE |
Replacing an a allele with an A allele always has the same effect |
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Example of trait with both additive and dominance variance |
Replacing an a allele with an A allele has a big effect for aa, but not effect for Aa |
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Example of trait where all genetic variance is dominance |
Replacing an a allele with an A allele has an average effect of zero |
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4 components of phenotypic variation |
environmental additive genetic dominance genetic epistatic genetic Vt = Ve + Va + Vd + Vi |
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Narrow sense measures.. |
the proportion of total phenotypic variation that is due to additive genetic variation |
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Epistatic variance |
Due to interactions among alleles at different genes |
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(Va+Vd+Vi)/Vt = |
H^2 |
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Va/Vt = |
h^2 |
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QTLs (quanitative trait loci) |
Genes that influence a quantitative trait |
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Why are QTLs difficult to find? |
There are many for each trait and each one may have only a small effect |
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The chromosomal region containing a QTL can be found by.. |
using linkage maps with many molecular markers |
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Correlation coefficiant measures.. |
The relationship between paired measurements |
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Population genetics |
Causes and consequences of genetic variation within a species |
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Population |
a group of individuals that actually or potentially mate with one another to produce the next generation |
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When does genetic polymorphism occur? |
When more than one allele is observed at a single gene |
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Most populations show very (high/low) levels of polymorphism |
High |
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Polymorphism is created by |
Mutation |
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Polymorphism is lost by (2) |
selection and random events |
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What does selection quickly remove? |
Deleterious alleles |
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Most polymorphisms are ____; no cost or benefit having one allele versus another |
Neutral |
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How are neutral alleles lost? |
Slowly, by random events |
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Gene pool |
Total sum of all alleles of all breeding members of the population at a given time |
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If there are only two alleles, p and q must add up to __ |
1 |
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p(freqA) = |
freq(AA) + 1/2freq(Aa) same as qfreq(a) |
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If a population meets these 3 requirements, then genotypic frequencies remain constant generation after generation |
Members choose mates randomly All genotypes have equal survival and reproduction The population size is sufficiently large |
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For two alleles, the equilibrium frequencies are |
AA: p^2 Aa: 2pq aa: q^2 |
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3 important consequences of hardy-weinberg |
1. Dominant traits do not necessarily increase from one generation to the next 2. Genetic variability can be maintained in populations 3. With the knowledge about the frequency of one genotype, the frequency of other genotypes can be calculated |
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Random mating means that individuals do not choose their mates on the basis of… |
A particular heritable character |
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Positive assortative mating |
Bias toward phenotypically similar mates |
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Negative assortative mating |
Bias toward phenotypically different mates |
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Inbreeding |
Bias toward mating with relatives |
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What 5 factors can change allele frequencies? |
Mutation, migration, selection, random events, nonrandom mating (this one changes genotypic frequencies but not allele frequencies) |
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Inbreeding and positive assortative mating increase ____ relative to the predictions of HW |
homozygosity |
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What is an extreme form of inbreeding? |
Self-fertilization |
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Each generation of selfing reduces heterozygosity by ___ |
50% |
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Inbreeding increases the chances that an indiviudal has two alleles that are ______ ___ ______ from a common ancestor |
Identical by descent |
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The ________ is the probabiltiy that two alleles in an individual are identical by decent from a common ancestor |
Inbreeding coefficient |
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Simple formula for calculate the inbreeding coefficient F for individual I with common ancestor C |
1. Trace inbreeding loop 2. Count the number of individuals (n) in the loop, not including I 3. F = (1/2)^n |
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When there is more than one common ancestor, calculate F separately for each ______ and add the results |
Inbreeding loop F1 + F2 = F |
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HW assumes all genotypes have equal ___ |
Fitness |
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What does fitness mean? |
The average number of offspring produced by an individual |
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Stability of population size depends on.. |
The average fitness across all indiviudals |
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Stable population has average fitness of…Declining has…Growing has… |
1,<1, >1 |
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If genotypes have unequal fitness, then allele frequencies will change over time. This is called.. |
Natural selection |
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Lower fitness of the inferior genotype is measured by the.. |
Selection coefficient (s) |
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Eventually, selection in favor of a recessive allele eliminates what? |
The dominant allele from the population |
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Selection in favor of a dominant allele gradually reduces the frequency of the recessive allele but does NOT lead to.. |
Immediate elimination |
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Relationships among species can be determined using data on ____ ____ at _____ genes |
Sequence divergence, homologous |
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Sequence divergence between species means that a mutation went to ____ in one lineage |
Fixation |
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What does the fossil record tell us? |
How far back in time two lineages diverged |
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What does sequence analysis measure? |
The nucleotide divergence between each species pair |
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For many sequences, the rate of divergence is roughly constant… |
A molecular clock |
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The nucleotide divergence between two species increases at a constant rate… |
u (upsilon) |
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u depends on.. |
How much the mutation affects the fuctionality of the protein |
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A beneficial mutation will probably.. |
Become fixed (this is rare) |
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A deleterious mutation will.. |
Be rapidly eliminated, won’t go to fixation |
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Most neutral mutations will be eliminated by.. |
Drift |
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A few neutral mutations will.. |
Take over the population and become fixed |
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The rate at which mutations become fixed is higher for synonymous than nonsynonymous substitutions. Why? (2) |
Synonymous mutations are almost always neutral Nonsynonymous mutations often change the function of a protein |
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