Chapter 5 - Recombination/Maps Flashcards
How was it established experimentally that the frequency of recombination (crossing over) between 2 genes is related to the distance between them along the chromosome?
How do we know that specific genes are liked on a single chromosome, in contrast to being located on separate chromosomes?
How do we know that crossing over results from a physical exchange between chromatids?
Q1. How do we know that sister chromatids undergo recombination?
Q2. When designed matings can’t be conducted in an organism (in humans) how do we learn that genes are linked? and how do we map them?
Q3. Describe the cytological observation that suggests that crossing over occurs during the 1st meiotic prophase.
Q4. Why does more crossing over occur between two distantly linked genes than between 2 genes that are very close together on the same chromosome?
Q5. Explain why 50% recovery of single-crossover products is the upper limit, even when crossing over always occurs between 2 linked genes?
Q6 Why are double-crossover events expected less frequently than single crossover events?
Q7 What is the propose basis for positive interference?
Q8: What 2 essential criteria must be met in order to execute a successful mapping cross?
Q9: The genes dp, cl, and ap are linked on chromosome 2 of Drosophila. In a series of 2-point mapping crosses, the following genetic distances were determined. What is the sequence of the 3 genes?
Q10. Coloured aleurone in the kernals of corn is due to the dominant allele R. The recessive allele r, when homozygous, produces colourles aleurone. The plant colour (not the kernal colour) is controlled by another gene with two alleles, Y and y. The dominant Y allele results in green colour, whereas the homozygous presence of the recessive allele causes the plant to appear yellow.
In a test cross betwwen a plant of unknown genotype and phenotype and a plant that homozygous recessive for both traits, the following progeny were obtained:
- coloured, green 88
- coloured, yellow 12
- colourless, green 8
- colourless, yellow 92
Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise arrangement of the alleles on the homologs.
Q11. In the cross shown here, involving two linked genes, ebony (e) and
claret (ca), in Drosophila, where crossing over does not occur in
males, offspring were produced in a
2 + :1 ca : 1 e phenotypic ratio:
These genes are 30 units apart on chromosome III. What did crossing over in the female contribute to these phenotypes?
Q12. In a series of two-point mapping crosses involving five genes
located on chromosome II in Drosophila, the following recombinant (single-crossover) frequencies were observed: (see pic below)
(a) Given that the adp gene is near the end of chromosome II (locus 83), construct a map of these genes.
b) In another set of experiments, a sixth gene, d, was tested against b and pr:
- d-b 17%
- d-pr 23%
Predict the results of 2-point mapping between d and c, d and vg, and d and adp.
Q13. Two different female Drosophila were isolated, each heterozygous for the autosomally linked genes b (black body), d (dachs tarsus), and c (curved wings). These genes are in the order d–b–c,
with b being closer to d than to c. Shown here is the genotypic arrangement for each female along with the various gametes formed by both: (see pic below)
Identify which categories are noncrossovers (NCOs), single cross-overs (SCOs), and double crossovers (DCOs) in each case. Then, indicate the relative frequency in which each will be produced.
Q14. In Drosophila, a cross was made between females—all expressing
the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F1, all females were wild type, while all males expressed all three mutant traits.
The cross was carried to the F2 generation, and 1000 offspring were counted, with the results shown in the following table. (see pic below)
No determination of sex was made in the data.
(a) Using proper nomenclature, determine the genotypes of the P1and F1 parents.
(b) Determine the sequence of the three genes and the map distances between them.
(c) Are there more or fewer double crossovers than expected?
(d) Calculate the coefficient of coincidence. Does it represent positive or negative interference?
Q15. Another cross in Drosophila involved the recessive, X-linked genes
yellow (y), white (w), and cut (ct). A yellow-bodied, white-eyed female with normal wings was crossed to a male whose eyes and
body were normal but whose wings were cut. The F1 females were wild type for all three traits, while the F1 males expressed the
yellow-body and white-eye traits. The cross was carried to an F2 progeny, and only male offspring were tallied. On the basis of the
data shown here, a genetic map was constructed (see pic below).
(a) Diagram the genotypes of the F1 parents.
(b) Construct a map, assuming that white is at locus 1.5 on the
X chromosome.
(c) Were any double-crossover offspring expected?
(d) Could the F2 female offspring be used to construct the map?
Why or why not?
Q16. In Drosophila, Dichaete (D) is a mutation on chromosome III with
a dominant effect on wing shape. It is lethal when homozygous. The genes ebony body (e) and pink eye (p) are recessive mutations on chromosome III. Flies from a Dichaete stock were crossed to homozygous ebony, pink flies, and the F1 progeny, with a Dichaete phenotype, were backcrossed to the ebony, pink homozygotes. Using the results of this backcross shown in the table,
(a) Diagram this cross, showing the genotypes of the parents and offspring of both crosses.
(b) What is the sequence and interlocus distance between these three genes?
Q18. In Drosophila, two mutations, Stubble (Sb) and curled (cu), are linked on chromosome III. Stubble is a dominant gene that is lethal in a homozygous state, and curled is a recessive gene. If a female of
the genotype:
Sb + cu +
is to be mated to detect recombinants among her offspring, what male genotype would you choose as a mate?
Q19. If the cross described in Problem 18 were made, and if Sb and cu are 8.2 map units apart on chromosome III, and if 1000
offspring were recovered, what would be the outcome of the cross, assuming that equal numbers of males and females were
observed?
Q20. Are mitotic recombinations and sister chromatid exchanges effective in producing genetic variability in an individual? in the offspring of individuals
Q21.What possible conclusions can be drawn from the observations
that in male Drosophila, no crossing over occurs, and that during meiosis, synaptonemal complexes are not seen in males but are observed in females where crossing over occurs?
Q25. DNA markers have greatly enhanced the mapping of genes in
humans. What are DNA markers, and what advantage do they confer?
Q26. In a certain plant, fruit is either red or yellow, and fruit shape is either oval or long. Red and oval are the dominant traits. Two plants, both heterozygous for these traits, were testcrossed, with the following results: (see pic below)
Determine the location of the genes relative to one another and the genotypes of the two parental plants.
Q30. In laboratory class, a genetics student was assigned to study an unknown mutation in Drosophila that had a whitish eye. He
crossed females from his true-breeding mutant stock to wild-type (brick-red-eyed) males, recovering all wild-type F1 flies. In the
F2 generation, the following offspring were recovered in the following proportions:
wild type 5/8,
bright red 1/8,
brown eye 1/8,
white eye 1/8
The student was stumped until the instructor suggested that perhaps the whitish eye in the original stock was the result of homozygosity for a mutation causing brown eyes and a mutation causing bright red eyes, illustrating gene interaction (see Chapter
4).
After much thought, the student was able to analyze the data, explain the results, and learn several things about the location of
the two genes relative to one another. One key to his understanding was that crossing over occurs in Drosophila females but not
in males. Based on his analysis, what did the student learn about the two genes?
Q32. In Drosophila, a female fly is heterozygous for three mutations, Bar
eyes (B), miniature wings (m), and ebony body (e). Note that Baris a dominant mutation. The fly is crossed to a male with normal eyes, miniature wings, and ebony body. The results of the cross are
as follows.
Interpret the results of this cross. If you conclude that linkage is involved between any of the genes, determine the map distance(s) between them.
