Exam 7: Linkage and Chromosome Mapping in Eukaryotes

Mendel predicted that some genes would be carried in the same chromosome.

Diagram chromosomal events that will ultimately result in the segregation of alleles (A and a)during meiosis II rather than meiosis I.

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

Assume that the genes for tan body and bare wings are 15 map units apart on chromosome II in Drosophila.Assume also that a tan-bodied,bare-winged female was mated to a wild-type male and that the resulting F₁ phenotypically wild-type females were mated to tan-bodied,bare-winged males.Of 1000 offspring,what would be the expected phenotypes,and in what numbers would they be expected?

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 arrangement of genes?

Three loci,mitochondrial malate dehydrogenase that forms a and b (MDHa,MDHb),glucouronidase 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 genotypes and expected numbers of the various types of cells she would produce.Assume no chromosomal interference.

If two gene loci are on nonhomologous chromosomes,genes at these loci are expected to assort independently.

Methods for determining the linkage group and genetic map in humans involve ________.

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 F₁ female progeny were mated to fully homozygous (mutant)males,and the following progeny (1000 total)were observed: Phenotypes Number Observed spineless 321 wild-type 38 claret, spineless 130 claret 18 claret, hairless 309 hairless, claret, spineless 32 hairless 140 hairless, spineless 12 (a)Which gene is in the middle? (b)With respect to the three genes mentioned in the problem,what are the genotypes of the homozygous parents used in making the phenotypically wild-type F? heterozygote? (c)What are the map distances for the three genes? A correct formula with the values "plugged in" for each distance will be sufficient. (d)What is the coefficient of coincidence? A correct formula with the values "plugged in" will be sufficient.

Assume that a cross is made between AaBb and aabb plants and that all the offspring are either AaBb or aabb.These results are consistent with ________.

Linkage (viewed from results of typical crosses)always occurs when two loci are on the same chromosome.

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?

In the early 1900s,two scientists noted that there were many more genes than chromosome pairs,thus setting the stage for the suggestion that some gene loci might be linked during meiotic processes.Who were these two scientists?

Under what circumstance might two loci be on the same chromosome but behave as if independently assorting in crosses?

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 F₁ Lobe females were then backcrossed with homozygous smooth abdomen,straw-body males,and the following phenotypes were observed: smooth abdomen,straw body 820 Lobe $\quad$$\quad$$\quad$$\quad$$\quad$$\quad$$\quad$$\quad$ 780 smooth abdomen,Lobe $\quad$$\quad$ 42 straw body $\quad$$\quad$$\quad$$\quad$$\quad$$\quad$ 58 smooth abdomen $\quad$$\quad$$\quad$$\quad$ 148 Lobe,straw body $\quad$$\quad$$\quad$$\quad$ 152 (a)Give the gene order and map units for these three loci. (b)What is the coefficient of coincidence?

Given that loci A and B in Drosophila are sex-linked and 20 map units apart,what phenotypic frequencies would you expect in male and female offspring resulting from the following crosses? (Assume that A and B are dominant to a and b,respectively.)

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?

What is the relationship between the degree of crossing over and the distance between two genes?

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

At what stage of the meiotic cell cycle and during what chromosomal configuration does crossing over occur?