Chapter 5 Flashcards
To construct a mapping cross of linked genes, it is important that the genotypes of some of the gametes produced by the heterozygote can be deduced by examining the phenotypes of the progeny.
- True
- False
False.
(To construct a mapping cross of linked genes, it is important that the genotypes of all of the gametes produced by the heterozygote can be deduced by examining the phenotypes of the progeny, taking into consideration that the homozygote produced only recessive gametes. Gametes and their genotypes can never be observed directly.)
In a three point mapping experiment for the genes y-w-ec, the following percentages of events are observed: NCO events: 65%; SCO events between y and w: 15%; SCO events between y and ec: 17%; DCO events: 3%. What is the map distance between y and ec?
- 18 map units
- 17 map units
- 15 map units
- 20 map units
20 map units
(The map distance between any two genes is the sum of the percentages of all detectable recombination events between them, so 17+3 = 20)
For linked genes A, B, and C, the map distance A-B is 5 map units and the map distance B-C is 25 map units. If there are 10 double crossover events out of 1000 offspring, what is the interference?
- 0.2
- 0.0125
- 0.01
- 0.8
0.2
The coefficient of coincidence is 0.01/0.0125 = 0.8, so the interference is 1-0.8 = 0.2
What essential criteria must be met in order to execute a successful mapping gross?
- The genotypes of the offspring must be readily determined from their phenotypes.
- One parent should be heterozygous and the other homozygous recessive for all the genes being mapped.
- One parent should be homozygous dominant and the other homozygous recessive for all the genes being mapped.
- Recombination must occur in the sex that displays genetic heterogeneity.
- The genotypes of the offspring must be readily determined from their phenotypes.
- One parent should be heterozygous and the other homozygous recessive for all the genes being mapped.
- Recombination must occur in the sex that displays genetic heterogeneity.
The cross GE/ge x ge/ge produces the following progeny: GE/ge 404, ge/ge 396, gE/ge 97, Ge/ge 103. From these data, one can conclude that there are 20 map units between the G and E loci.
- True
- False
True
Assume that the genes for tan body and bare wings are 15 map units apart on chromosome 2 in Drosophila. A tan-bodied, bare-winged female was mated to a wild-type male and the resulting F1 phenotypically wild-type females were mated to tan-bodied, bare-winged males to obtain an F2 generation. What phenotypes would be expected in the F2 offspring?
- Wild-type
- Tan body
- Bare wings
- Tan body, bare wings
All of the above.
Because crossing over occurs at the ______ of the cycle, notice that each single crossover involves only ____ of the ____ chromatids.
- Four strand stage
- Two
- Four
Crossing over during prophase I of meiosis occurs between alleles on sister chromatids.
- True
- False
False.
Crossing over during meiosis occurs between alleles on nonsister chromatids.
Which of the following statements about gamete formation during meiosis is false?
- Parental gametes contain the same combinations of linked genes as found in the parent cell.
- Complete linkage results in the formation of only parental gametes.
- Recombinant gametes contain combinations of alleles not found in the parent cell.
- Parental gametes can be formed only if there is no crossing over during meiosis.
-Parental gametes can be formed only if there is no crossing over during meiosis.
(This statement is false. If crossing over occurs, half of the gametes formed are parental and the other half are recombinant.)
Two genes that are separated by 10 map units show a recombination percentage of 10%.
- True
- False
True.
(One map unit is equal to 1% recombination between two genes; 10 map units would be equal to 10% recombination between the genes.)
Genes X, Y, and Z are linked. Crossover gametes between genes X and Y are observed with a frequency of 25%, and crossover gametes between genes Y and Z are observed with a frequency of 5%. What is the expected frequency of double crossover gametes among these genes?
- 1.25%
- 5%
- 50%
- 30%
1.25%
(The probability of a double crossover is the product of the probabilities of the single crossovers: 0.25 x 0.05 = 0.0125, or 1.25%.)
Assume that the genes from the previous example are located along the chromosome in the order X, Y, and Z. What is the probability of recombination between genes X and Z?
- 50%
- 30%
- 20%
- 5%
30%
(Recombination frequencies between linked genes along a chromosome are additive, so the recombination frequency between genes X and Z is 25 + 5 = 30.)
