BIO340 Exam 2

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Homologs fail to separate during _______

Meiosis I

Homologs failing to separate during meiosis I is a possible _____ event


All of the following events occur during normal meiosis except___

1. Two haploid gametes fuse to form a diploid cell
2. One dipoloid cell produced four haploid cells
3. Sister chromatids separate from one another during meiosis II
4. Homologous chromosomes separate from one another during meiosis I

1. Fusion of haploid gametes occurs AFTER meiosis

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

During ___ of the first round of meiotic division, homologous chromosomes separate and migrate to opposite poles


During ____ of the second round of meiosis (and mitosis), sister chromatids separate and move to opposite poles


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 ___


When does nondisjunction occur?

Meiosis or mitosis in both sexes

Normal chromosome separation begins with a ___ cell at the start of meiosis


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


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

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
2. Both products of the first meiotic division contain dyads representing one chromosome of each homologous pair
3. Three products of the second meiotic division have too many chromosomes, while one is missing a chromosome for a particular set
4. Two products of the second meiotic division have both the maternal and paternal chromosomes of a set and the other two products have none for that set


Before meiosis begins, the chromosomes are duplicated, resulting in ___ chromatids per chromosome


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


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


What is aneuploidy?

A condition in which a cell lacks an exact multiple of a haploid set

(2n-1) cells are..


(2n+1) cells are..


Both monosomy and trisomy are ____ conditions


Characteristics of Klinefelter for men

Large breasts, small testes, may be mentally retarded

A female with an extra X chromosome is considered..


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?


Which of the following statements about nondisjunction is false?

1. It separates maternal from paternal chromatids
2. It may fail to separate maternal chromatids from one another or paternal chromatids from one another
3. It may fail to separate maternal from paternal chromatids
4. It occurs more frequently in human females over age 35


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
2. They result from a mating between individuals of the same species
3. They are generated from exposure to colchicine
4. They may be sterile and unable to produce offspring

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


What is polyploidy?

Any condition in which entire haploid sets are found in multiples greater than two

Triploids have ___ chromosomes


Tetraploids have ___ chromosomes


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.

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

Why is this type of allopolyplody important?

Important in the evolution and artificial development of many plant groups, including important grain crops

Amphidiploids produce gametes with a number of chromosomes ____ to the sum of the number in the two original haploid gametes


What two terms can be used to describe Down syndrome?

Trisomic and aneuploid

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
2. completely normal chromosome
3. chromosome with two centromeres
4. chromosome with some deleted regions

3. When the centromere is contained within the inversion, it is neither duplicated nor deleted

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?


Is genetic material gained, lost, both, or neither in an inversion?


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

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

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.


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 ___


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


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 ___


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:


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
2. One strand of DNA from an F+ cell integrates into the chromosome of an F- cell and the other strand is degraded
3. DNA is transferred from an F+ cell to an F- cell
4. The F factor is an element that is found in the chromosome of an F+ cell



Viruses that infect bacteria, using them as hosts in which to reproduce

Bacteriophage T4 has a ___ structure


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


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


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


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


The phage produces a ____ that ruptures the bacterial cell wall and releases the virsus


Lysis occurs when approximately ___ virus particles are produced


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


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
2. The phage takes over the protein synthesis machinery of the bacteria
3. The bacterial DNA is degraded
4. The phage produces lysozyme, which ruptures the host cell wall and releases newly formed viral particles

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. The closer a gene to the Hfr origin, the more likely it will be transferred to the recipient during conjugation
3. For any two genes transferred from donor to recipient, all genes residing between them have also been transferred

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 ____


E. coli genetic map is ____ minutes


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


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


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


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 =


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


Loss of genetic material


Relocation of genetic material (2)

Translocation and Inversion

An individual that possesses a multiple of the basic chromosome set is called..


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


The ___ number of chromosomes is the number of chromosomes in gametes


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


Allpolyploids arise from ______ between species


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 produces ____ tissues


____ chromosomes result from sister chromosomes that do not separate


An ___ individual has a chromosome number that is NOT an exact multiple of the haploid number


2n-2 chromosomes =


2n-1 chromosomes =


2n+1 chromosomes =


Most aneuploidy is caused by _____ during meiosis


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 _____


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


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,XXY: Phenotypically male, but with some female secondary sexual characteristics, usually sterile, called _____ syndrome


Phenotypically male, but tend to be taller than average; usually fertile


______ 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


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 ____


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


A man is found to have one Barr body in his somatic cells. What is his chromosomal constitution?


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


deletion heterozygotes can show _____, which happens when the normal chromosome contains a recessive allele for a gene that is deleted in the other chromosome


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


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


_____ inversions can lead to duplication-and-deletion products


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


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)


Crossing over creates new combinations of alleles on ___ chromosome(s)


When genes are linked, testcross yields ____ recombinants than parentals


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 _____


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

Each crossing-over event creates how many recombinant chromosomes?


If the most offspring are heterozygous, the allele arrangement in the dihybrid parent must be..

Both heterozygous!!

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)

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

How is gene order revealed?

Comparing the double recombinant genotypes with the parental genotypes

The allele pair for the middle gene will appear _____ in the double recombinants


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

How to calculate the distances between center gene and each of the outer genes

(4 smallest numbers)/(all)

What are the 2 advantages of bacteria as genetic model systems?

Short reproduction cycle and large clonal cultures

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

Bacteria have small circular DNA molecules called


Wild-type bacteria are also called

Prototrophic (grow and divide on minimal media such as inorganic salts, carbon source, water)

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)

3 processes of bacterial genetics

Transformation, conjugation, transduction


Uptake of free DNA


Direct transfer of DNA from one bacterium to another


Transfer of bacterial DNA by a bacteriophage

When is cell contact required (3 processes)?

conjugation only

Which 3 processes are sensitive to DNase?

Transformation only

Does donor lose DNA in conjugation?


Integration of the F plasmid into chromosomes creates an ____ stran


Conjugation is usually ____ before the chromosome transfer is completed


How can the genetic map of bacteria be inferred?

By identifying genes (alleles) that transferred together

How does one generate a chromosome map?

Tracking time of marker entry

Two types of DNA transfer can take place during conjugation

Chromosome transfer, plasmid transfer

Mendelian genetics was developed by studying ___ traits, but most traits are ____

Discrete, continuous

Quantitative genetics

The study of the genetics of continuously varying traits (most traits are quantitative)

With discrete traits, ratios of phenotypic classes reveal…

Underlying genotype

Quantitative traits do not allow..

Classification of offspring into discrete categories

What is the phenotype of an individual influenced by?

Both its genotype and environment

Differences between parental strains are due to ____ differences


Differences among individuals within each strain are due to ____


Variation in ___ generation is also due to environmental effects



the proportion of the variation in phenotype that is due to variation in genotype

Finding out whether a trait is heritable (IS/IS NOT) the same as finding out whether it is influenced by genes at all


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?

How to calculate variance

Sum squared deviations from the mean, divided by the number of measurements

Phenotypic variation

Vt = Ve+Vg

Broad-sense heritability

Measures the proportion of total phenotypic variation that is due to genetic variation

How to measure H^2


Two subgroups of genetic variance

Additive variance and dominance variance

Additive variance

Variance that is due to effects of individual alleles

Dominance variance

Variance that is due to dominance relations among alleles

Example of trait where all genetic variance is ADDITIVE

Replacing an a allele with an A allele always has the same effect

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

Example of trait where all genetic variance is dominance

Replacing an a allele with an A allele has an average effect of zero

4 components of phenotypic variation

environmental additive genetic dominance genetic epistatic genetic Vt = Ve + Va + Vd + Vi

Narrow sense measures..

the proportion of total phenotypic variation that is due to additive genetic variation

Epistatic variance

Due to interactions among alleles at different genes

(Va+Vd+Vi)/Vt =


Va/Vt =


QTLs (quanitative trait loci)

Genes that influence a quantitative trait

Why are QTLs difficult to find?

There are many for each trait and each one may have only a small effect

The chromosomal region containing a QTL can be found by..

using linkage maps with many molecular markers

Correlation coefficiant measures..

The relationship between paired measurements

Population genetics

Causes and consequences of genetic variation within a species


a group of individuals that actually or potentially mate with one another to produce the next generation

When does genetic polymorphism occur?

When more than one allele is observed at a single gene

Most populations show very (high/low) levels of polymorphism


Polymorphism is created by


Polymorphism is lost by (2)

selection and random events

What does selection quickly remove?

Deleterious alleles

Most polymorphisms are ____; no cost or benefit having one allele versus another


How are neutral alleles lost?

Slowly, by random events

Gene pool

Total sum of all alleles of all breeding members of the population at a given time

If there are only two alleles, p and q must add up to __


p(freqA) =

freq(AA) + 1/2freq(Aa) same as qfreq(a)

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

For two alleles, the equilibrium frequencies are

AA: p^2 Aa: 2pq aa: q^2

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

Random mating means that individuals do not choose their mates on the basis of…

A particular heritable character

Positive assortative mating

Bias toward phenotypically similar mates

Negative assortative mating

Bias toward phenotypically different mates


Bias toward mating with relatives

What 5 factors can change allele frequencies?

Mutation, migration, selection, random events, nonrandom mating (this one changes genotypic frequencies but not allele frequencies)

Inbreeding and positive assortative mating increase ____ relative to the predictions of HW


What is an extreme form of inbreeding?


Each generation of selfing reduces heterozygosity by ___


Inbreeding increases the chances that an indiviudal has two alleles that are ______ ___ ______ from a common ancestor

Identical by descent

The ________ is the probabiltiy that two alleles in an individual are identical by decent from a common ancestor

Inbreeding coefficient

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

When there is more than one common ancestor, calculate F separately for each ______ and add the results

Inbreeding loop F1 + F2 = F

HW assumes all genotypes have equal ___


What does fitness mean?

The average number of offspring produced by an individual

Stability of population size depends on..

The average fitness across all indiviudals

Stable population has average fitness of…Declining has…Growing has…

1,<1, >1

If genotypes have unequal fitness, then allele frequencies will change over time. This is called..

Natural selection

Lower fitness of the inferior genotype is measured by the..

Selection coefficient (s)

Eventually, selection in favor of a recessive allele eliminates what?

The dominant allele from the population

Selection in favor of a dominant allele gradually reduces the frequency of the recessive allele but does NOT lead to..

Immediate elimination

Relationships among species can be determined using data on ____ ____ at _____ genes

Sequence divergence, homologous

Sequence divergence between species means that a mutation went to ____ in one lineage


What does the fossil record tell us?

How far back in time two lineages diverged

What does sequence analysis measure?

The nucleotide divergence between each species pair

For many sequences, the rate of divergence is roughly constant…

A molecular clock

The nucleotide divergence between two species increases at a constant rate…

u (upsilon)

u depends on..

How much the mutation affects the fuctionality of the protein

A beneficial mutation will probably..

Become fixed (this is rare)

A deleterious mutation will..

Be rapidly eliminated, won’t go to fixation

Most neutral mutations will be eliminated by..


A few neutral mutations will..

Take over the population and become fixed

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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