Unit 5: Linkage, chromosome mapping and chromosomal variations in genetics - Practice Quiz

1 What is genetic linkage?

2 Which observation suggests that two genes are linked?

3 How is recombination frequency calculated?

4 What is the maximum observable recombination frequency between two genes?

5 During which stage of meiosis does crossing-over normally occur?

6 Crossing-over usually involves an exchange between which structures?

7 What is a chiasma?

8 What is a major genetic result of crossing-over?

9 What information does a chromosome map primarily show?

10 Which unit is commonly used for genetic map distance?

11 Two genes have a recombination frequency of . What is their approximate map distance?

12 Which pair of genes is expected to have the lowest recombination frequency?

13 Which chromosomal variation involves the loss of a chromosome segment?

14 Which chromosomal variation produces an extra copy of a chromosome segment?

15 What occurs during a chromosomal inversion?

16 What is aneuploidy?

17 Which notation represents trisomy?

18 How many chromosome sets does a haploid cell contain?

19 Why are haploid organisms useful for studying recessive mutations?

20 What is a major advantage of double haploids in plant breeding?

21 A testcross produces 800 offspring, of which 84 and 76 belong to the two recombinant classes. What is the estimated recombination frequency between the genes?

22 An individual with genotype is testcrossed, and the most frequent offspring classes are and . What does this indicate about the arrangement of its alleles?

23 Two genes show a recombination frequency close to . Which conclusion is most appropriate?

24 At a proposed recombination fraction of , the LOD score for two genes is . How should this result be interpreted?

25 During which meiotic stage are homologous chromosomes fully synapsed and crossing-over primarily occurs?

26 A single crossover occurs between two nonsister chromatids of a homologous chromosome pair. What chromatids are expected at the end of meiosis?

27 A mutation prevents the formation of programmed double-strand DNA breaks during early meiosis. Which outcome is most likely?

28 What products commonly result from a crossover within the inverted segment of a paracentric inversion heterozygote?

29 In a three-point testcross, the most frequent classes are and , while the least frequent classes are and . Which gene lies in the middle?

30 A three-point testcross gives the following offspring: , , , , , , , and . What is the best map?

31 Two adjacent map intervals are 10 cM and 20 cM. The observed double-crossover frequency is . What is the interference?

32 Genes , , and occur in that order, with distances of 12 cM between and and 8 cM between and . If crossovers occur independently, how many double-crossover offspring are expected among 5,000 progeny?

33 A recessive allele becomes phenotypically expressed in a heterozygous individual because the homologous chromosome carries a deletion covering the normal dominant allele. What is this phenomenon called?

34 Why does a reciprocal translocation heterozygote often show reduced fertility despite having no net loss of genetic material?

35 A phenotypically normal person has 45 chromosomes because two acrocentric chromosomes have fused. Which condition best explains this observation?

36 A diploid species has . How many chromosomes would be present in a nullisomic individual and in a tetrasomic individual, respectively?

37 An interspecific hybrid is sterile because its chromosomes lack homologous partners during meiosis. Which treatment is most likely to restore fertility?

38 A haploid plant produced by anther culture has genotype . Its chromosome complement is then doubled. What genotype will the resulting doubled haploid have?

39 A tetraploid crop has . A dihaploid is produced from it through haploidization. What chromosome number is expected in the dihaploid?

40 Why are haploid organisms especially useful for identifying recessive mutations?

41 A testcross produces observed progeny , , , and . The class has viability relative to all other classes, whose viability is . What is the viability-corrected recombination fraction?

42 An individual with linked loci in repulsion phase, , is testcrossed. If the recombination fraction is and reciprocal classes are equally frequent, what fraction of progeny will be ?

43 Two loci are physically on the same chromosome, but a very large testcross yields a recombination fraction near . Which conclusion is most defensible?

44 A linkage estimate from reciprocal testcrosses is through females but through males. Cytology confirms that the loci are separated physically. Which explanation best fits the result?

45 A programmed meiotic double-strand break is repaired using the homolog, producing gene conversion but no reciprocal exchange of flanking markers. Which pathway most directly explains this outcome?

46 In an ordered fungal octad, a locus displays a allele pattern while distant flanking markers retain their parental association. What event most directly produced the unusual segregation?

47 Crossovers occur in two adjacent intervals with single-interval frequencies and . Among meioses, 120 double crossovers are observed. What is the coefficient of coincidence?

48 A mutant forms meiotic double-strand breaks and invades homologous DNA normally, but cannot resolve double Holliday junctions through the crossover-biased pathway. Which phenotype is most likely?

49 A three-point testcross gives: , , , , , , , and . Which gene order and interval distances are supported?

50 For two loci, the observed recombination fraction is . Using Kosambi's mapping function , what is the estimated map distance?

51 In ordered asci, a marker shows second-division segregation in of asci. Assuming standard ordered-tetrad interpretation, what is its distance from the centromere?

52 A testcross records recombination between and , between and , but only between and . What best explains why the outer-locus estimate is below ?

53 A single crossover occurs within a paracentric inversion loop in a heterozygote. Which products are expected at anaphase I?

54 A phenotypically normal reciprocal-translocation heterozygote has high semisterility. Which meiotic segregation pattern most directly produces balanced gametes?

55 A recessive allele is unexpectedly expressed in a heterozygote, and cytology reveals a deletion spanning the homologous wild-type locus. What is this phenomenon?

56 For a centromere-linked marker, a disomic gamete from a heterozygous parent contains both alleles and . Which error most directly generated this gamete?

57 A locus near the end of an autotetraploid chromosome shows excess homozygosity relative to random chromosomal segregation. Which polysomic-meiosis feature best explains this pattern?

58 A doubled-haploid population is produced from an F1 with recombination fraction . Ignoring selection, what fraction of lines should be fixed as ?

59 A cultivated autotetraploid is crossed to a haploid inducer to recover plants with two chromosome sets from the cultivar. Why are these progeny called dihaploids?

60 A recessive embryo-lethal allele cannot be recovered as a viable doubled-haploid line, although linked markers predict its transmission. Which stage most likely removes the genotype?