Exam 4: Mapping Eukaryote Chromosomes by Recombination

Three genes of corn,R,D,and Y,lie on chromosome 9.The map of the three genes,using map units is as follows: R 10 D 5 Y In a testcross of a trihybrid plant (R D Y/r d y)arising from true-breeding R D Y/R D Y and r d y/r d y parents,how many R d Y/- d - plants would be expected in a progeny of 1000 if there were no interference? If the coefficient of coincidence were 0.3,how would this change your answer?

In a fungal cross of A B X a b (the loci are linked)in meiosis in which there has been a four-strand double crossover between the loci,the percent of meiotic products that will be recombinant for A and B for that ascus is:

Holliday junctions are:

Two genes,A and B,are linked.An individual of genotype A b/A b is crossed to one that is a B/a B.The F₁ is selfed and an F₂ is obtained.Which of the following ratios most likely represents the phenotypic ratio observed in this F₂?

Ascospores from a cross a $\times$ b c (all auxotrophic markers)are analyzed.If genes b and c are linked,10 map units apart,but gene a is located on a different chromosome,what proportion of ascospores is expected to be a⁺b⁺c⁺?

In maize,two plants that are both heterozygous for the recessive alleles a and b are crossed.What frequency of double-mutant progeny will appear if a and b are 7.2 map units apart,and both parents carry a and b in repulsion (trans)?

A cross was made between two Neurospora strains with genotypes + b and a +.The genes are linked.The genotypes of the spores in one ascus from this cross are listed in order,from left to right: $+ b ; + b ; + + ; + + ; a b ; a b ; a + ; a +$ a)Is the ascus a parental ditype (PD),a tetratype (T),or a nonparental ditype (NPD)? b)Do the alleles of the a gene show first or second division segregation? c)Do the alleles of the b gene show first or second division segregation? d)Show how the above tetrad of spore pairs was derived by diagramming the chromosomes at pachytene of meiosis I.Indicate with a line between appropriate chromatids,the simplest crossover that can yield this tetrad. e)If recombination between a and b is 10% and 2/100 asci are NPD,how many tetratype asci would you expect to find? How many PD? f)Diagram how a tetrad with the same spore pairs,as shown above,would arise if the genes showed independent assortment.

Ascospores from a cross of c $\times$ d e (all auxotrophic markers)are analyzed.If genes c and d are linked,10 map units apart,but gene e is located on a different chromosome,what proportion of ascospores is expected to be c⁺d⁺e⁺?

Dihybrid AaBb female Drosophila were testcrossed with aabb males.The following offspring genotypes were obtained. AaBb $\quad$ 99 Aabb $\quad$ 83 AaBb $\quad$ 86 Aabb $\quad$ 92 A $\chi$ ² test was performed to determine whether the data supported the original hypothesis that the genes are unlinked.What is the correct value of $\chi$ ²?

In Drosophila,the two genes w and sn are X-linked and 25 map units apart.A female fly of genotype w⁺sn⁺/w sn is crossed to a male from a wild-type line.What percent of male progeny will be w⁺sn?

In maize,the genes W and D are so tightly linked that virtually no crossovers occur between them.A dihybrid W d/w D is testcrossed to w d/w d.The percent of progeny with W-/D- phenotype will be:

Four possibly different albino strains of Neurospora were each crossed to wild type.All crosses resulted in 1/2 wild-type and 1/2 albino progeny.Crosses were made between the first strain and the other three with the following results: 1 $\times$ 2: 975 albino : 25 wild type 1 $\times$ 3: 1000 albino 1 $\times$ 4: 750 albino : 250 wild type Which mutations represent different genes and which genes are linked? How did you arrive at your conclusions?

Consider the following experimental results. (M = mapping function and MU = M x 50) OBSERVED RF 18.5\% 0.46 23.1 6.4\% 0.137 6.8 Based on these results,would you advocate the use of mapping functions? In which situations would you recommend using them?

In Neurospora,an Irish strain makes big as well as green-colored colonies (genotype b g).It is crossed to a wild-type strain,which makes small and orange-colored colonies (genotype: + +).The following ordered tetrads of asci were found,with the numbers of each type indicated below each tetrad. I II III IV V VI VII bg b+ bg bg bg b+ bg ++ +g ++ +g b+ b+ bg +g b+ bg b+ +g +g ++ b+ +g ++ ++ ++ +g ++ 4 1 24 1 91 2 177 a)Classify the asci above (I-VII)according to type. (i) those which are PD (ii) those which are NPD (iii) those which are $\mathrm{T}$ b)Calculate the percent recombination between b and its centromere. c)Calculate the percent recombination between g and its centromere. d)What is the most likely map of the two genes in relation to each other and to the position of their respective centromere(s)?

Three crosses involving three linked genes were performed,with the resulting "shorthand" designation of phenotypes indicated: What is the best map of these three genes?

A plant of genotype C/C ^. d/d is crossed to c/c ^. D/D,and the F₁ is testcrossed.If the genes in question are linked,the percentage of double homozygous recessive individuals in the offspring of the testcross will be:

The fungal cross a ^. b⁺ $\times$ a^{+ .} b yields 30 parental ditype,27 nonparental ditype,and 18 tetratype asci.The two genes,a and b,are:

Ascospores from a cross leu-2 * ad-7 met-3 (all auxotrophic markers)are plated on minimal medium (no adenine,leucine,or methionine).If the three genes are unlinked,what proportion of ascospores is expected to grow into a colony on this medium?

Two genes,A and B,are linked.An individual of genotype A b/A b is crossed to one that is a B/a B.The F₁ is testcrossed.Which of the following ratios most likely represents the phenotypic ratio observed in the progeny of this testcross?

In a fungal cross $a b * + +$ ,gene a is on chromosome 1 and gene b is on chromosome 6.It is known that the two loci are so closely linked to their centromeres that there is never any crossing over in either centromere to locus region.What proportion of linear tetrads will be: a)M_I M_I PD? b)M_I M_I NPD? c)M_I M_II T? d)M_II M_I T? e)M_II M_II PD? f)M_II M_II NPD? g)M_II M_II T?