Ch. 5 - Linkage and Gene Mapping

Question 1

Give a brief important description of Thomas Hunt Morgan (1866 - 1945).

Answer 1

nobel prize winnter 1933

pioneered use of drosophila melanogaster in genetic studies by searching for mutants (took 2 years to find 1st one)

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Question 2

Figure: (a) independent assortment: two genes on two different homologous pairs of chromosomes.

See figure in notebook and describe what is going on.

Answer 2

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Question 3

Figure: (b) linkage: two genes on a single pair of homologs; no exchange occurs.

Answer 3

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Question 4

Figure: (c) linkage: two genes on a single pair of homologs; exchange occurs between two nonsister chromatids.

Answer 4

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Question 5

What is complete linkage?

Answer 5

no crossing over between two genes

produces parental (non-crossover) gametes

Question 6

When does crossing over occur?

Answer 6

occurs between two nonsister chromatids

both parental and recombinant (crossover) gametes are produced

Question 7

What is linkage ratio?

Answer 7

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Question 8

What is linkage group?

Answer 8

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Question 9

What is map unit (mu)?

Answer 9

1 percent recombination between two genes on chromosome

also called centimorgans (cM)

relative distances, not exact ones

Question 10

Describe the process of a single crossover (SCO).

Answer 10

SCO is used to determine distance between two linked genes.

Question 11

Describe the process of a double crossover (DCO).

Answer 11

will be a rare event

Question 12

What is point crosses for mapping genes on a chromosome.

Answer 12

Ex: 1 point crosses, 2 point crosses, 3 point crosses.

Question 13

How do you determine gene order?

Answer 13

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Question 14

How do you determine recombination frequency?

Answer 14

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Question 15

What is interference?

Answer 15

in inhibition of further crossover events

refers to the inhibition of crossovers by another crossover event nearby

reduces expected number of multiple crossovers

Question 16

How do you calculate interference?

Answer 16

using coefficient of coincidence (C)

C is used to quantify disparities that result from interference

C = observed DCO é expected DCO

interference:
I = 1 - C

Question 17

Interference: when is it complete?

Answer 17

when no double crossovers occur

Question 18

Interference: when is it positive?

Answer 18

fewer double-crossover events than expected occur é is a positive number

Question 19

Interference: when is negative?

Answer 19

more double-crossover events than expected occur é is a negative number

Question 20

What occurs when genes are close together?

Answer 20

positive interference occurs

accuracy of mapping is high (mostly SCOs occur)

Question 21

What happens to interference and accuracy of mapping when distance between genes increases?

Answer 21

interference decreases.

accuracy of mapping decreases because multiple crossover events (some of them undetectable at the phenotypic level) are occurring.