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Chapter 6, Genetic recombination in eukaryotes

Specific Reading Assignment. pp 148-158;160-167 (Also skip Interference). Crucial Figures are 2, 3, 4, 5, 6, 7, 9, 10, 11, 12, and all the text-figures within the assigned pages. Important Figures are 8, 16, 17c, 15. Skip Figures: Outside the assigned pages and 17ab.

INDEPENDENT ASSORTMENT (pp. 150-156)

Objectives. To learn: 1) How the behavior of homologous chromosomes in meiosis explains the inheritance of two or more genes, each controlling a simple trait. 2). The types and frequency of meiotic products from an individual heterozygous for two genes. 3) How to determine the types and frequency of gametes produced by an individual heterozygous for two genes (the testcross). 4) How to determine the types and frequency of offspring produced when an individual heterozygous for two genes is selfed or crossed to another of the same genotype (F1 X F1 cross). 5) How to determine the types and frequency of offspring produced when an individual heterozygous for more than two genes is selfed or crossed to another of the same genotype (branch diagram method).

1a) Apply Fig. 6-2 to a higher diploid organism and identify the various processes and cell types.

1b) It appears that each n cell in Fig. 6-2 has two alleles, how come there are no AA, Aa, Bb, etc. Aoutput@ products?

1c) What is the cellular process where recombination takes place? What are the two forms of recombination?

1d) In the dihybrid (or double heterozygote) A/a B/b shown in p. 151, what determines whether the gametes it produces from one meiocyte will be A B and a b or A b and a B ?

1e) Let us start with one pre-meiotic cell from an individual who is doubly heterozygous for the genes A and B. Assuming absence of crossing over, how many types of gametes will be produced by that meiocyte? Diagram that meiosis.

1f) In light of 1e), how do you explain that there are four types of gametes produced by a dihybrid?

2a) Draw a testcross for an individual who is a double heterozygous for the genes C and D.

END OF CHAPTER PROBLEMS: 1, 2,

3a) Compare the four four-block quadrants in Fig. 6-7, what can you say about them?

3b) With respect to genotypes vs phenotypes, how does the products of a selfcross compare to the products of a testcross.

3c) In a generic dihybrid selfcross, what does the 9:3:3:1 ratio represent in phenotypic terms?

END OF CHAPTER PROBLEMS: 3, 10 (select as many as you wish), 11 (select as many as you wish), 12, 23, 24, 28,

Answers to INDEPENDENT ASSORTMENT

1a) In a higher diploid organism the yellow cells on top, the INPUT are gametes; the dark green 2n cell represents the zygote and all somatic and germline cells that derive from it; meiosis occurs in the gonial cells; the yellow and blue cells (OUTPUT) represent the gametes produced by the organism.

1b) It is misleading to think of these cells as having two alleles, they actually have one allele for each gene that is represented. So, there has to be one of A-a and one of B-b.

1c) The cellular process where recombination takes place is meiosis. The two forms of recombination are Aindependent assortment@, for genes in different chromosomes, which comes about because of the alternative chromosome alignements of the metaphase plate of meiosis I; and Acrossing-over@ for genes in the same chromosomes, which comes about because of the breakage and reunion of homologous chromatids.

1d) The alignment of the chromosomes in the metaphase I plate (See Fig. 6-4).

1e) Two types of gametes. See answer to Orientation question 5d) in Week 4.

1f) There are four types of gametes overall because some meiocytes produce two types (let=s say the parental types) and some produce the other two types (the recombinant types).

2a)

3a) The four quadrants are different with respect to round-wrinkled phenotype, but within each quadrant there is a ratio of 1:2:1 for Y/Y:Y/y:yy. That is to say, the inheritance of R or r does not affect the inheritance of Y or y. This is another way to visualize the independent inheritance of the two traits.

3b) In a testcross the phenotypes reflect the genotypes directly (because one parent contributes only the recessive alleles); in the selfcross, some individuals have the same phenotype but different genotypes.

3c) The 9 are those who show both dominant traits, 3 are dominant for the first trait and recessive for the second, 3 are the other way around, and 1 are recessive for both traits. Another way of expressing this is: 9 A/C;B/C : 3 A/C;b/b : 3 a/a;B/C : 1 a/a;b/b.

CROSSING OVER (pp. 156-158)

Objectives. To learn: 1) The types and frequency of meiotic products from an individual heterozygous for two genes in the same chromosomes. 2) The concept of crossing-over.

4a) Contrast by means of short definitions the concepts of locus, gene, and allele.

4b) How many chiasmata are there in the red figure at the bottom of p. 159.

4c) Draw meiosis as you did in exercise 5d of Week 4 with the following modifications: place two genes A and C on the long chromosome and make the cell heterozygous AC/ac. Indicate that a crossing over occurs between the two genes in meiosis I, and proceed accordingly. NOTE that Abetween the two genes@ means Asomewhere in the section of chromosome that separates the two genes@.

4d) Compare Figs. 6-6 and 6-11. What is the most significant difference?

Answers to CROSSING OVER

4a) Locus is the position occupied by a gene along the chromosome; gene can be defined many ways, but a convenient definition for our purposes is as the segment of DNA that encodes a certain polypeptide. Allele is one of several possible forms of the gene; alleles may differ from each other by base substitutions, deletions, etc.

4b) There appear to be five. Note that while we can tell that the chromatids involved are homologous (i.e., non-sister) it is not possible to specify whether it is the same or different chromatids involved in the various crossing-overs.

4c) The crossing over is highlighted in yellow in step 4. Note that two of the meiotic products are now recombinant: Ac and aC. That is to say, they have a combination of alleles that is different from that of the parental chromosomes (AC and ac)

4d) When genes are in different chromosomes, the frequency of recombinant products is 50%. When they are on the same chromosome, unless they are very far apart, the frequency of recombination is less than 50%, and this is the definition of linkage.

LINKAGE MAPS (pp. 158, 160-163)

Objectives. To learn: 1) The concepts of frequency of crossing-over and genetic map distance. 2) To measure genetic distances using dihybrid testcrosses. 3) To measure genetic distances using a three-point testcross. 4) The concept of genetic map

5a) Refer to Recombination Frequency RF (p. 158); what would its value be in Fig. 6-11? Can RF be predicted for genes on the same chromosome as it can for genes on different chromosomes?

5b) What is the genetic map distance between two genes?

END OF CHAPTER PROBLEMS: 4, 7, 8, 9, 14,

END OF CHAPTER PROBLEMS: 15 (a & b), 16 (a&b), 17 (as many as you like),

7a) Observe Fig. 6-16, consider the distance between genes and the total length of chromosomes. Compare to Fig. 6-17c. What is the most striking difference?

7b) If you crossed a fly heterozygous for roughoid eyes and claret eyes: ro ca/ + + to the homozygous recessive (double mutant), what kinds of phenotypes and in what proportions would be recovered in the progeny? (See the map in Fig. 6-16 for the positions of these genes).

Answers to LINKAGE MAPS

5a) RF would have some value less than 50%, which will indicate that there is linkage. RF cannot be predicted for linked loci; it must be determined experimentally, but for any pair of genes (A and B, for example) once it is determined, any repeat of the experiment will produce approximately similar results.

5b) For genes that are relatively near each other, 5-10 map units, the distance is defined by the frequency of recombination. For genes that are very far apart see question 7b.

7a) There are many more genes mapped in Drosophila, and this a very partial list of the total, which numbers in the thousands of genes.

7b) These two genes are on chromosome 3 and approximately 100 map units apart. Such a large distance can be obtained by measuring shorter distances of genes in between the two extremes and then adding them together. If we were to measure directly the map distance (i.e., the frequency of recombination) between ro and ca, we would obtain an apparent distance of 50 map units: they would recombine as often as if they were in different chromosomes and assorting independently. The reason for this apparent contradiction is that when genes are so far apart, for each crossing over that creates recombinant gametes there is a double crossing over that generates parental type gametes. Thus, these genes are not linked, they assort independently, even if they are on the same chromosome (See questions 4d) and 5a)).

PROBLEMS: 20, 26, 28, 30, 31a

GRADED QUESTIONS

Guide and Orientation Question 1f.

In corn, colored kernels is due to the dominant allele R; the recessive allele r produces white corn. The plant color is controlled by another gene, with alleles Y (dominant) for green plants and y (when homozygous) for yellowish plants. In a back cross to the double homozygous recessive a plant of unknown phenotype and genotype produced the following progeny:

Colored grain, green plants: 850

Colored grain, yellowish plants: 125

White grain, green plants 98

White grain, yellowish plants 900

What are the genotype and phenotype of the unknown plant? Are the two traits linked or do they assort independently? If they are linked, what is the genetic distance?

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