GENETICS CHAPTER 5

Syntenic genes can assort independently when __________.

they are far apart on a chromosome and crossing over occurs frequently between the genes

The alleles of linked genes tend to __________.

segregate together during gamete production

Genetic linkage leads to the production of a significantly greater than expected number of gametes containing chromosomes with __________.

parental combinations of alleles

The syntenic genes A and Z are linked. A cross between two parents AAZZ and aazz, produces F1 progeny with the AaZz genotype. What are the possible arrangements of alleles on the F1 progeny's chromosomes?

AZ/az

In a dihybrid cross exhibiting complete genetic linkage what would you expect?

two equally frequent gametes containing only parental allele combinations and no recombinant gametes

In a two-point test-cross analysis a dihybrid F1 fly is crossed to a __________.

pure-breeding mate with a recessive phenotype (aabb)

In fruit flies red eyes (pr+_) are dominant to purple eyes (prpr) and normal wings (vg+_) are dominant to vestigial wings (vgvg). The genes are located on the same chromosome. A purebreeding red-eyed fly with vestigial wings was crossed with a pure-breedi

pr+vg/prvg+

In fruit flies red eyes (pr+_) are dominant to purple eyes (prpr) and normal wings (vg+_) are dominant to vestigial wings (vgvg). The genes are located on the same chromosome. A purebreeding red-eyed fly with vestigial wings was crossed with a pure-breedi

5%

If you are given a recombination frequency of 34% between genes X and Y and 27% between X and Z can you predict the order of the three genes?

No; based on this data alone, the order could be X-Z-Y or Z-X-Y.

What type of test would you use to determine whether observed data constitute evidence of genetic linkage or are simply a case of chance variation from expected values?

chi-square analysis

You perform a test cross of the dihybrid AaBb and score the phenotypes of 1000 progeny. Assuming independent assortment how many of the progeny do you expect to display the dominant phenotype for both the A and B genes?

250

Incomplete genetic linkage of three genes in a trihybrid produces eight genetically different gamete genotypes. How many different gamete genotypes are produced in a four-gene cross with incomplete genetic linkage?

16

Assuming three genes are linked how many recombinant genotypes would you expect and at what frequency?

six recombinant genotypes, less frequent than expected by independent assortment

For a given cross the expected number of double recombinants is 20 and the observed number of double recombinants is 15. What is the coefficient of coincidence (c)?

0.75

For a given cross the expected number of double recombinants is 20 and the observed number of double recombinants is 15. What is the interference calculation (I)?

0.25

Genes A and B are located 10cM from each other on a chromosome. Gene C is located 25cM from gene A and 15cM from gene B. Assuming that I = 0 what is the probability that the trihybrid ABC/abc will produce an ABC gamete?

38.25%

Genes A and B are located 10cM from each other on a chromosome. Gene C is located 25cM from gene A and 15cM from gene B. What is the probability that the trihybrid ABC/abc will produce any kind of recombinant gamete?

23.5%

Where does crossing over occur?

at nodules that occur randomly at "hotspots" along the synaptonemal complex

When does recombination occur?

at the four-strand stage, when each member of a homologous chromosome pair is composed of two sister chromatids, and after DNA replication

Which type of crossover event produces no recombinant chromosomes?

two-strand double crossover

Which type of crossover event produces two parental and two recombinant chromosomes in gametes?

three-strand double crossover

Which type of crossover event produces all four recombinant chromosomes?

four-strand double crossover

For a given haplotype the frequencies of alleles for gene Y are Y = 0.65 and Y' = 0.35, and the frequencies at gene Z are Z = 0.25 and Z' = 0.75. What is the predicted frequency of the Y'Z' haplotype?

0.26

Intragenic recombination can be detected __________.

based on recombination between homologs with different mutant alleles and recovery of both wild-type and double-mutant chromosomes

After analysis of 100 pedigrees the Zmax from the analysis of linkage between a disease gene D and the DNA marker P was found to be 3.5 at ? = 0.25. Which of the following is the best interpretation of these results?

The evidence supports linkage of D and P at 25 cM.

The Zmax from the analysis of a disease gene N and the DNA marker H was 2.5 at ? = 0.1. Which of the following is the best interpretation of this result?

No conclusion can be made regarding the linkage of N and H.

1)What is observed when syntenic genes are close enough to one another that they are unable to assort independently?

genetic linkage

2)What is the term for chromosomes that do not reshuffle the alleles of linked genes?

parental chromosomes

3)Recombination analysis allows for the mapping of genes because the frequency of crossing over is proportional to what distance?

distance between genes

4)What organism exhibits complete genetic linkage, meaning there is no recombination between homologous chromosomes?,

male Drosophila or Diptera

5)Incomplete genetic linkage is far more common than complete linkage. What is the term for gametes produced when recombination shuffles the alleles of linked genes?

nonparental or recombinant gametes

6)Two genes, A and X, exhibit incomplete linkage. The frequency of each parental gamete (AX and ax) is 45%. What is the approximate frequency of the Ax gamete?

, 5%

7)There are three genes located in the gene order A-B-C on a chromosome. Would you expect the recombination frequency to be higher between A and B or A and C?

higher between A and C (they are farther apart)

8)Morgan realized that Bateson and Punnett had detected genetic linkage but they couldn't explain their results because they performed the wrong cross. What type of cross did Morgan use for the analysis of genetic linkage of autosomal genes in Drosophila?

two-point test-cross analysis

9)In a two-point test-cross analysis, what are the "two points" being tested?,

the two genes

10)What unit of physical distance between genes on a chromosome provides a convenient way to relate the recombination frequencies for linked genes with their positions and order along a chromosome?

map unit (m.u.) or centimorgan (cM)

11)What type of test would you use to determine the relative order of more than two genes?

three-point test-cross

12)In most tests of genetic linkage, the number of double crossovers is less than the number expected due to what effect, which limits the number of crossovers that can occur in a short length of chromosome?,

interference (I)

13)Refer to this map to answer the questions: (check review; table and multiple questions)

a) 12 cM/ b)92/ c)88%/ d)(0.92)(0.88) = 0.8096/ e)(0.5)(0.8096) = 0.4048/ f)8%/ g)12%/ h)(0.08)(0.88)(0.5) = 0.0352/ i) (0.12)(0.92)(0.5) = 0.0552/j) (0.08)(0.12)(0.5) = 0.0048/ k)0.4048 + 0.0048 + 0.0352 + 0.0352 + 0.0552 + 0.0552 + 0.0048 + 0.0048 = 1

14)When constructing a genetic map of Zea mays, Creighton and McClintock used genetic markers as well as structural differences in the homologous copies of chromosome 9 that can be seen under the microscope. What are these structural differences called?,

cytological markers

15)What is the highest possible frequency of recombination between linked genes that can be generated by any type of crossover event?

50%

16)Recombination frequency between linked genes increases as the distance between genes gets larger or gets smaller?

larger

17)Recombination frequency differs between males and females. Which sex has a higher rate of recombination, heterogametic (males) or homogametic (females)?,

homogametic (females)

18)If there is a maximum Lod score of 4.2 at ? = 0.31, what can you say about the linkage and the distance between two genes?

, linked and 31 cM apart

19)If too little time has passed for crossing over to randomize haplotypes or if natural selection favors certain haplotypes, what would you expect to see?,

linkage disequilibrium

20)Ascospores in yeast are not arranged in a particular order. Thus, the four haploid ascospores contained within a yeast ascus are known as what?,

unordered tetrad

1)A chromosome with a different combination of alleles than parental that is created by crossing over between homologous chromosomes is termed ________.

recombinant chromosome

2)Alleles of linked genes usually segregate together during meiosis. When they don't, it is because ________ has occurred between them.,

crossing over

3)Genetic linkage can be spotted by comparing the observed frequencies of ________ with the frequencies expected (assuming independent assortment).

gamete genotypes/progeny phenotypes

4)5% recombination is equal to ________ map unit(s) (m.u.) or centimorgan(s) (cM) of distance between linked genes.

5

5)The specific array of alleles making up a set of linked genes on a single chromosome is called a ________.

haplotype