Question 1

What is a lod score?

Accepted Answer

A lod score is a statistical calculation used to determine the probability that two genetic traits are linked by analyzing pedigrees. It stands for logarithm of the odds and involves comparing the likelihood of observing certain pedigree patterns if the two traits are linked versus if they are not linked. A lod score greater than 3 indicates strong evidence for linkage, while a score less than -2 indicates strong evidence against linkage.

Question 2

In a three-point mapping experiment,which class do you expect to find the least?

Accepted Answer

Double crossover events are the least frequent class of recombination events, as they require two crossovers to occur in between two markers. Therefore, we would expect to find the least number of double crossovers in a three-point mapping experiment.

Question 3

In the fruit fly,Drosophila melanogaster,a spineless no wing bristles female fly is mated to a male that is claret dark eyes and hairless no thoracic bristles .Phenotypically wild-type FS1U1B11 S1U1B0female progeny were mated to fully homozygous mutant males,and the following progeny 1000 total were observed: - What are the map distances between hairless and spineless?

Accepted Answer

The answer of In the fruit fly,Drosophila melanogaster,a spineless no...

Question 4

In the fruit fly,Drosophila melanogaster,a spineless no wing bristles female fly is mated to a male that is claret dark eyes and hairless no thoracic bristles).Phenotypically wild-type FS1U1B11 S1U1B0female progeny were mated to fully homozygous mutant males,and the following progeny 1000 total were observed: - What is the gene order?

Accepted Answer

The answer of In the fruit fly,Drosophila melanogaster,a spineless no...

Question 5

Methods for determining the linkage group and genetic map in humans involve which of the following?

Accepted Answer

Methods for determining the linkage group and genetic map in humans typically involve the use of DNA markers, such as microsatellites or single nucleotide polymorphisms (SNPs). These markers can be tracked through families to determine their relative positions on chromosomes, allowing for the construction of a genetic map. Methods such as twin spots, tetrad analysis, and bromodeoxyuridine are typically used in model organisms such as yeasts and fruit flies, while chiasma-type and classical analyses refer to observations of meiotic recombination events and chromosome structure in a variety of organisms. Zygotene and pachytene refer to specific stages of meiotic prophase, which can be useful for studying chromosome behavior but are not typically used for constructing genetic maps.

Question 6

Assume that a cross is made between AaBb and aabb plants and that the offspring occur in the following numbers: 106 AaBb,48 Aabb,52 aaBb,94 aabb.These results are consistent with which scenario described?

Accepted Answer

The offspring ratios suggest that the dominant alleles (A and B) are linked on one chromosome and the recessive alleles (a and b) are linked on the other chromosome in the AaBb parent, leading to a non-Mendelian ratio that is best explained by linkage, as seen in choice D.

Question 7

What relatively recent scientific advancement has made mapping by linkage or classical genetic mapping approaches virtually obsolete?

Accepted Answer

The genome sequence of a species has made mapping by linkage or classical genetic mapping approaches virtually obsolete because it allows for more precise and comprehensive mapping using techniques such as SNP mapping and comparative genomics.

Question 8

Assume that investigators crossed a strain of flies carrying the dominant eye mutation Lobe on the second chromosome with a strain homozygous for the second chromosome recessive mutations smooth abdomen and straw body.The F₁Lobe females were then backcrossed with homozygous smooth abdomen,straw body males,the following phenotypes were observed: Give the map units between Lobe and straw.

Accepted Answer

The answer of Assume that investigators crossed a strain of...

Question 9

The phenomenon in which one crossover decreases the likelihood of crossovers in nearby regions is called________ .

Accepted Answer

Positive interference refers to the phenomenon where the occurrence of a crossover event in one region of a chromosome reduces the probability of another crossover happening nearby. This leads to a lower than expected overall rate of recombination in those regions.

Question 10

Assume that two genes are 80 map units apart on chromosome II of Drosophila and that a cross is made between a doubly heterozygous female and a homozygous recessive male.What percent recombination would be expected in the offspring of this type of cross?

Accepted Answer

The distance between the two genes is 80 map units, which means that there is a 80% chance of recombination occurring between them. This corresponds to a recombination frequency of 0.8, or 80%. Since the two genes are on the same chromosome, the probability of recombination is independent for each gene pair, which means that the cross will produce 50% recombinant offspring. Therefore, the answer is C) 50% recombination.

Question 11

What two terms apply to the fusion of cultured human and mouse cells that produces cell lines that are useful in assigning a gene to a particular human chromosome?

Accepted Answer

The two terms that apply to the fusion of cultured human and mouse cells are "heterokaryon" and "synkaryon". Heterokaryon refers to the initial stage of the fusion where two nuclei are present in the same cell but have not yet fused together. Synkaryon then refers to the later stage where the two nuclei have fused together into a single nucleus. These fused cells are useful in assigning a gene to a particular human chromosome through techniques such as somatic cell hybridization. Choice A (forward and reverse) and choice C (positive and negative) do not apply to this process. Choice B (lod score and pod score) refers to statistical methods used in genetic linkage analysis, which is not relevant to this question. Choice D (single and double) does not accurately describe the process of fusion of cells.

Question 12

In a three-point mapping experiment,how many different genotypic classes are expected?

Accepted Answer

In a three-point mapping experiment, eight different genotypic classes are expected due to the various combinations of parental and recombinant types for three genes.

Question 13

__________occurs when a crossover in one region of a chromosome reduces crossovers in nearby regions.

Accepted Answer

Positive interference occurs when a crossover in one region of a chromosome increases the likelihood of crossovers in nearby regions. In contrast, negative interference occurs when a crossover in one region of a chromosome reduces crossovers in nearby regions. The coefficient of coincidence measures the degree to which observed double crossovers deviate from the expected frequency. Triple crossover refers to the occurrence of three crossovers in a single meiosis event. Negative supercoiling describes the twisting of DNA in a way that makes it more difficult to separate the strands during replication or transcription.

Question 14

The coefficient of coincidence reflects the frequency of observed double crossovers compared to the frequency of expected double crossovers.What is the relationship between the coefficient of coincidence and interference?

Accepted Answer

Interference is defined as the degree to which crossovers at one position inhibit crossovers at nearby positions. The coefficient of coincidence is the ratio of observed double crossovers to expected double crossovers. Interference is calculated as 1- (observed double crossovers/expected double crossovers). Therefore, D is the correct choice as interference is equal to one minus the coefficient of coincidence.

Question 15

In Drosophila,assume that the gene for scute bristles s)is located at map position 0.0 and that the gene for ruby eyes r)is at position 15.0.Both genes are located on the X chromosome and are recessive to their wild-type alleles.A cross is made between scute-bristled females and ruby-eyed males.Phenotypically wild-type F₁ females were then mated to homozygous double mutant males,and 1000 offspring were produced.Give the frequency of scute expected.

Accepted Answer

The answer of In Drosophila,assume that the gene for scute...

Question 16

Three loci,mitochondrial malate dehydrogenase that forms a and b MDHa,MDHb ,glucuronidase that forms 1 and 2 GUS1,GUS2 ,and a histone gene that forms + and - H⁺,H^- ,are located on chromosome #7 in humans.Assume that the MDH locus is at position 35,GUS at position 45,and H at position 75.A female whose mother was homozygous for MDHa,GUS2,and H⁺and whose father was homozygous for MDHb,GUS1,and H^- produces a sample of 1000 egg cells.Give the expected numbers of the MDHa GUS2 H^-cells she would produce.Assume no chromosomal interference.

Accepted Answer

Given the loci positions on chromosome #7 (MDH at 35, GUS at 45, and H at 75), we can calculate the recombination frequencies between these loci. The distance between MDH and GUS is 10 map units, and between GUS and H is 30 map units. Without chromosomal interference, the expected proportion of recombinants between MDH and GUS is 10% (0.1), and between GUS and H is 30% (0.3). The female is heterozygous at all three loci, so to get MDHa GUS2 H^-, recombination needs to occur between GUS and H but not between MDH and GUS. The probability of no recombination between MDH and GUS is 90% (0.9), and the probability of recombination between GUS and H is 30% (0.3). Multiplying these probabilities gives 0.9 * 0.3 = 0.27 or 27% of the gametes. Since she produces 1000 egg cells, 27% of 1000 is 270. However, since recombination can happen on either chromosome, only half of these recombinants will have the desired combination, leading to 270 / 2 = 135 egg cells with the MDHa GUS2 H^- combination.

Question 17

Which of the following best describes why mapping is most accurate when genes are close together on a chromosome?

Accepted Answer

Double crossover events can create a result that looks like no crossover or only one crossover occurred in a two gene mapping experiment if the genes are far apart on a chromosome. This can lead to inaccurate mapping results. However, if the genes are close together, the chance of such an event occurring is reduced and mapping becomes more accurate.

Question 18

If interference is complete,what would be the frequency of double crossovers?

Accepted Answer

If interference is complete, it means that there will be no double crossovers occurring. Therefore, the frequency of double crossovers would be zero.

Question 19

Which of the following is an example of positive interference?

Accepted Answer

Positive interference refers to the phenomenon where a crossover in one region of a chromosome reduces the likelihood of another crossover occurring nearby, thus inhibiting additional crossovers in adjacent regions.

Question 20

What is a physical map of a chromosome?

Accepted Answer

A physical map of a chromosome describes the base pair difference between two loci, providing a detailed representation of the physical distances between genes or genetic markers on the chromosome.

Quiz 5: Chromosome Mapping in Eukaryotes