Linkage, Recombination & Gene Mapping

Question 1

Interchromosomal Genetic Variation

Answer 1

Random segregation of DIFFERENT chromosomes during anaphase 1 of meiosis.

Question 2

Intrachromosomal Genetic Variation

Answer 2

Meiotic recombination between loci on the same chromosome

Question 3

Frequency of Germline Mutation

Answer 3

Relatively low compared to interchromosomal and intrachromosomal

Question 4

How do you distinguish between unlinked and linked genes?

Answer 4

Using a “two point” or dihybrid testcross.

Question 5

Dihybrid or two point cross

Answer 5

Tracks the segregation of alleles at TWO loci.

Question 6

Monohybrid cross

Answer 6

Tracks the segregation of alleles at ONE locus.

Question 7

For single crossovers, the frequency of recombinant gametes is half the frequency of crossing over because

Answer 7

Each crossover takes place between only two of the four chromatids of a homologous pair.

Question 8

Calculating recombination requency

Answer 8

Recombination frequency = (No. recombinant progeny/Total No. of progeny) X 100%

Question 9

We can use genetic data from testcrosses (in model organisms) or from preexisting pedigrees (in humans) to

Answer 9

1. quantitatively estimate distances between loci

2. establish simple genetic maps of the entire genome of organisms - ABCD or ACBD

Question 10

What is a centiMorgan (cM) or map unit (m.u.)?

Answer 10

a unit for measuring genetic linkage. It is defined as that distance between loci for which the recombination frequency in a single generation is 0.01or 1%.

Question 10

What does the number of genotypes from a dihybrid cross reveal?

Answer 10

The type of alleles at each of the two loci

Question 10

what is coupling (cis configuration)

Answer 10

wild type alleles are found on one chromosome; mutant alleles are found on the other chromosome.

• –A——–B—
• –a———b—

Question 11

Repulsion (trans configuration)

Answer 11

wild type allele and mutant allele are found on the same chromosome.

• –A———b—
• –a———B—

Question 12

Steps in determining gene order in a three-point cross

Answer 12

1. identify the nonrecombinant progeny (two most-numerous phenotypes)
2. Identify the double-crossover progeny (two least-numerous phenotypes).
3. Compare the phenotype of double-crossover progeny with the phenotype of nonrecombinant progeny. They should be alike in two characteristics and differ in one.
4. The characteristic that differs between the double crossover and the nonrecombinant progeny is encoded by the middle gene.

Question 13

What is the middle locus if the resulting nonrecombinant progeny are A+B+C+ and ABC and the double crossover progeny are ABC+ and A+B+C?

Answer 13

C

Question 14

Variation

Answer 14

generated by both interchromosomal combinations and intrachromosomal recombinations.

Question 15

Coupling and repulsion

Answer 15

refer to the arrangement of alleles at two loci on homologous chromosomes. This affects the numbers of progeny of each genotype (and phenotype) in a dihybrid testcross.