Test #2 Genetic Linkage and Mapping, Genetic Transfer and Mapping in Bacteria Flashcards
When does crossing over happens?
Crossing over happens with non-sister chromatids. This happens during pachytene stage of prophase 1 of meiosis. The non-sister chromatids of the bivalents of homologous chromosomes exchange DNA segments.
Define genetic linkage.
The phenomenon that genes located close to each other on a chromosome tend to be transmitted as a unit.
Explain what are linkage groups.
Linkage groups refer to chromosomes because they contain a group of genes that are linked and inherited together.
When cross over occur, the haploid cells will contain a combination of alleles that are_____ found in the orignal chromosomes.
NOT
Describe Morgan and Linkage of X-Linked Genes.
Discovery of Sex-linked Traits: Morgan observed that certain traits, such as eye color in fruit flies, were inherited differently depending on the sex of the parent. For example, in fruit flies, the gene for eye color is located on the X chromosome. He noticed that certain eye color traits were passed down from one generation to the next in a pattern consistent with X-linked inheritance.
His Experiment: He used fruit flies in his experiment. He noticed that some of the males had white eyes instead of the normal red eyes. He did a experiment with a red eyes female and white eyed male. The F1 generation produced all red eyes. The F2 generation produced three red eyes and one white eyes. He also found that the white eyes were all male. This doesn’t mean only males are to be associated with white eyes. This proved that the white eye gene is found to be linked to the X chromosome.
25% would be red eyed female(w+ w+), 25% would be red eyed male(w+ w+), 25% would be red eyes female(w+,w), and 25% would be white eyed male(w).
Explain genetic mapping.
Genetic mapping refers to the determination of the linear order of linked genes along the same chromosome.
Genetic mapping experiments are performed by testcross.
Map distance= number of recombinant offspring/ total number of offspring x 100
Units of distance = map unites or centiMorgans, each map unit is equivalent to 1% recombination frequency.
Explain Alfred Sturtevant Experiment.
In his experiment, Sturtevant focused on the concept of genetic linkage, which refers to the tendency of certain genes to be inherited together because they are located close to each other on the same chromosome. He observed that genes located on the same chromosome tend to be inherited together more frequently than genes located on different chromosomes, a phenomenon known as linkage.
To map the relative positions of genes on a chromosome, Sturtevant conducted a series of genetic crosses between fruit flies with different traits controlled by genes located on the same chromosome. By analyzing the frequency of recombinant offspring (those with new combinations of traits not present in the parental generation), he was able to estimate the distances between genes and create a linear map of gene order along the chromosome. This map, known as a genetic or linkage map, provided valuable insights into the organization and structure of chromosomes.
Hypothesis: It is possible to map the order of genes along a chromosome by estimating the distance between genes on the chromosome from the percentage of recombinant offspring.
Extra Info:
First genetic map was constructed in 1911 by Alfred, a student of Thomas Hunt Morgan.
S+ = normal bristles, s = short bristles, e+ = gray body color, e = ebony body color.
One parent must be homozygous recessive for both genes (ss ee) and the other must be heterozygous for the two genes (s+s e+e).
- The s and e alleles are linked on the same chromosome.
- The s+ and e+ are linked on the homologous chromosome.
The percentage of recombinant offspring reaches a value of 50%. This is because the likelihood of multiple crossovers increases so the observed recombination tend to underestimate the actual measure of map distance. Also, a test cross is expected to yield a maximum of only 50% recombiant offspring.
Explain the Creighton and McClintock Experiment.
The Creighton and McClintock experiment involved crossing maize (corn) plants with different genetic markers located on distinct chromosomes. They observed that when genes were physically linked on the same chromosome, the frequency of recombinant phenotypes was low, indicating suppressed crossing over. This provided direct evidence for the physical exchange of genetic material during meiosis, supporting the concept of genetic linkage. Their experiment illuminated the mechanism of genetic recombination and laid the groundwork for further research in genetics.
Additional Facts:
- They wanted to prove the correlation between recombinant offspring and the occurence of cross over.
- They discovered two abnormalities in corn chromosome 9.
- They were two relevant genes: a gene for kernel color and a gene for endosperm texture. (C = colored, c = colorless, wx = waxy endosperm Wx= starchy endosperm.)
- They expected to see a cross over involving a normal and abnormal chromosome 9 that would yield either a knob or translocation but never both.
-Hypothesis: Offspring with non parental phenotypes are the product of a cross over via an exchange between homologous chromosomes.
- The colored/waxy phenotype (Cc wxwx) can occur only if the genetic recombination did not occur.
- If there was no physical change, then Parent A would pass the knob translocation to the offspring.
- Conclusion: Observations were consistent with the idea that crossover happened between the C and wx genes involving a physical exchange of segments between homologous chromosomes.
Crossing over is more likely to result in recombination for genes that are _______ than genes that are closer together.
Farther apart.
The likelihood of recombination for two genes is called…
A recombintion frequency.
What’s a nucleosome?
The nucleosome is made of histone octamer of dimers of H2A, H2B, H3, AND H4. 146 bp of the DNA is wrapped around the octamer, forming a core particle. The 146 bp is enough DNA to wrap 1.7 times around the core particle. The remaining 54 bp DNA(from the original 200 bp) is called linker DNA.
What was the most novel idea Watson and Crick proposed in their model?
DNA replication is semi-conservative.
What effect would an H1 histone mutation have on eukaryotic cells? Would the same effect occur in bacteria?
This would be a lethal mutation in eukaryotes as an H1 responsible DNA packing. The mutation will prevent chromosome formation. Since bacteria does not have chromosomes or H1, the mutation will not occur.
In the dihybrid cross AaBbx aabb, what proportion of individuals are expected to be homozygotic for both genes in the F1 generation?
25%
Two congenital analgesia heterozygous individuals plan to start a family and have 6 kids. What is the probability that the first three kids will have congenital analgesia?
1/4 x 1/4 x 1/4 = 1/64
The transmission of genes located in the same chromosome will____ follow the Mendel’s law of independent assortment.
NOT.
When does cross over occur?
Cross over occur during the Pachytene stage of Prophase I of meiosis, where non-sister chromatids of the bivalents of homologous chromosomes exchange DNA segments.
-In diploid eukaryotes species, linkage can be altered during meiosis by crossing over. Based on the distance between two genes, the cross over rate will vary.
Bateson and Punnett Crosses
-In 1905, Bateson and Punnett conducted a cross in sweet pea involving two different traits (flower color and pollen shape).
-F2 generation of this dihybrid was expected to produce 9:3:3:1 phenotypic ratio but didn’t.
-They suggested that the transmission of the two traits from the parents was somehow coupled. They didn’t realize that the coupling is due to the present of the two genes on the same chromosome(inked).
Morgan made three important hypotheses:
-The genes for body color, eye color, and wing length are all located on the X-chromosome, and they tend to be inherited together.
-Due to crossing over, the homologous X chromosome can exchange pieces, creating a new combination of alleles.
-The likelihood of crossing over depends on the distance between the two genes, where crossing over is more likely to occur between two genes that are far apart from each other.
-Parental phenotypes are the most common offspring while recombinant offsprings are not common because the genes are close together.
Why are genetic maps important?
-It can be used to diagnose or predict inherited human diseases, and perhaps treat them.
-It provides helpful information for improving crops through selective breeding programs.
-It improcves our understanding of the evolutionary relationships among different species.
-It helps molecular biologists clone genes.
-It allows us to understand the overall complexity and genetic organization of a particular species.
Trihybrid Crosses
Step 1: Perform a testcross by mating female heterozygotes to male homozygous recessive for all three alleles.
Step 2: Collect data for the F2 generation.
Step 3: Calculate the map distance between pairs of genes.
Step 4: Construct the map.
Genetic Mapping in Haploid Eukaryotes
-Fungi are unicellular or multicellular organisms, typically haploid that reproduce asexually.
-However, two haploids cells can fuse to form a diploid zygote which then reproduces sexually.
-The sac fungi (ascomycetes) have been useful to geneticists because of their unique style of sexual reproduction.
What’s the key difference between fungal sexual reproduction and other organisms?
The key difference between fungal sexual reproduction and other organisms where meiosis producted are contained within a sac.
Meiosis in fungi produces four haploid cells aka
Spores.
A group of four spores is known as a _____ which are enclosed in a sac termed ____
Tetrad; ascus.
What are the types of tetrads or octads?
Unordered tetrads and octad: ascus provides space for spores to randomly mix together.
Ordered tetrad: tight ascus prevents mixing of spores.
Ordered Tetrad
The key feature in ordered tetrad is that the position and order of spores within the ascus is determined by the divisions of meiosis and mitosis.
The genetic content of spores in ordered tetrads can be used to map the distance between a single gene and the centromere.
The logic of this mapping technique is based on:
- Centromeres of homologous chromosomes separate during meiosis I.
- Centromeres of sister chromatids separate during meiosis II.
The gene is separated from its original centromere only after a crossover in the region between the gene and the centromere.
Map distance= 1/2( number of recombinant asci)/ total number of asci x 100
Unordered Tetrad
Can be used to determine: if 2 genes are linked or independently assorted. to compute the distance between two linked genes.
-If the genes are assorted independently, 50% recombinant and 50% parental spores are produced.
If the two genes are linked, the type of crossover between them determined what type of ascus is produced:
- No crossovers yield the parental ditype.
-Single crossover produce the tetratype.
- Double crossovers can yield any of the three types.
Map distance= single crossover tetrads + (2) (Double crossover tetrads)/ total number of asci x 0.5 x 100
Explain Mitotic Recombination.
Although mitosis does not involve the homologous
pairing of chromosomes to form bivalents, cross-over
does occur in mitosis on rare occasions during the early
developmental stages of the embryo
* When it occurs, the daughter cells containing the
recombinant chromosomes continue to divide,
ultimately resulting in a patch of tissue with
characteristics that are different from those of the rest of
the organism
Curt Stern proposed that unusual patches on the bodies on fruit flies were due to mitotic recombination.
In his experiment, he microscopically observed that there were two adjacent regions that were different from the rest of the body called a twin spot. He believed this was due to a single mitotic recombination within one cell during embryonic development.
Define genetic transfer.
A segment of bacterial DNA is transferred from one bacterium to another.
Transfer of genetic material from one bacterium to another can occur in three ways:
Conjugation: involves direct physical contact.
Transduction: involves viruses.
Transformation: involves DNA uptake from the environment.
Explain the history of conjugation.
Genetic transfer in bacteria was discovered in 1946 by Joshua Lederberg and Edward Tatum.
They were studying strains of E.coli that had different nutritional growth requirements.
The auxotroph strain was designated bio-met-phe+thr+.
- Requires one vitamin(biotin) and one amino acid(methionine).
- It could produce the amino acids phenylalanine and threonine.
The other was designated bio+met+phe-thr-.
- It has the opposite requirements for growth.
The genotype of the bacterial cells that grew has to be bio+ met+ phe+ thr+.
Define auxotrophs.
Auxotrophs cannot synthesize a needed nutrient.
Define prototrophs.
Prototrophs make all their nutrients from basic components.
How did Bernard Davis discover how transfer happened?
Bernard Davis showed that bacterial strains needed to make physical contact for transfer.
He used an apparatus called the U-tube. This contained a filter at the bottom that has pores that were large enough to allow passage of genetic material but small enough to prevent the passage of bacterial cells.
Plasmids transmitted via conjugation are termed as…
Conjugative plasmids.
