L9: Linkage & chromosome mapping in eukaryotes

Question 1

What 2 important mechanisms is genetic diversity in eukaryotes driven by ?

Answer 1

• Indepedent assortment of chromosomes
• Homologous recombination during meiosis

Question 2

What do homologous pairs of chromosomes segregate at to form haploid gametes (sex)

Answer 2

Segregate at meiosis

Question 3

Describe homologous recombination

Answer 3

Homologous chromosomes pair up in metaphase I, non-sister chromatids cross over to exchange genetic material

Question 4

What does crossing over involve?

Answer 4

Reciprocal exchange of nearly identical segments between non-sister chromatids

Question 5

What do chiasmatas ensure?

Answer 5

Precise reciprocal exchange without loss/gain of nucleotides

Question 6

How many chiasmatas form between non-sister chromatids during meiosis?

Answer 6

1-3

Question 7

Recombination is more likely if…

Answer 7

the 2 genes are far apart on the chromosome

Question 8

Are genes closer together more/less likely to have recombination between them?

Answer 8

Less likely

Question 10

Define genetic linkage

Answer 10

When genes are inherited together

Question 11

How can the no. of recombination events between 2 linked genes be used?

Answer 11

To estimate their physical distance from each other

Question 12

What is recombination frequency (RF) used to estimate?

Answer 12

Distance between genes on a chromsome

Question 13

How to calculate recombination frequency (RF)?

Answer 13

(Number of recombinants / Total progeny) x 100

Question 14

Features of trihybrid test cross

Answer 14

• 3 genes to establish linear order
• Phenotypes with least progeny represent double recombinants

Question 15

z

Feautre of double recombinant events

Answer 15

• Middle gene is swapped between chromatids

Question 16

Does chromosome mapping in humans begin on the X or Y?

Answer 16

X

Question 17

What does the double recombinants in Trihybrid cross tell us?

Answer 17

Which gene is in the middle

Question 18

How can double recombinants help determine gene order in a trihybrid cross?

Answer 18

By revealing which gene lies in the middle

Question 19

What is the expected ratio of phenotypes in a dihybrid cross under independent assortment?

Answer 19

9:3:3:1

Question 20

What challenges are faced when mapping chromosomes in humans compared to Drosophila?

Answer 20

Humans have fewer progeny

Question 21

What role does homologous recombination play in genetic diversity?

Answer 21

It generates new combinations of alleles on a chromatid

Question 22

What is the importance of Thomas Hunt Morgan’s work in genetics?

Answer 22

Establishing Drosophila as a genetic model and creating the first genetic map

Question 23

How do true breeding mutants differ from wild type organisms in genetic experiments?

Answer 23

They are homozygous for the mutant allele

Question 24

What are the two less frequent phenotypes referred to in the Drosophila dihybrid test cross results?

Answer 24

Recombinant phenotypes

Question 25

How does the distance between genes affect recombination frequency?

Answer 25

Greater distance increases recombination frequency

Question 26

What are the implications of having fewer progeny in human genetic studies?

Answer 26

Complicates linkage analysis and gene mapping

Question 27

What is the relationship between crossing-over events and genetic variation?

Answer 27

Crossing-over increases genetic variation by creating new combinations of alleles

Question 28

What is a recombinant chromosome?

Answer 28

A chromosome resulting from genetic recombination

Question 29

What is a linkage map?

Answer 29

A map of the relative positions of genes on a chromosome determined by recombination frequencies

Question 30

What is the relationship between independent assortment and genetic diversity?

Answer 30

Independent assortment increases genetic diversity by creating new combinations of alleles

Question 31

How does the presence of mutant phenotypes inform us about gene linkage in Drosophila?

Answer 31

They suggest genes are linked, with recombinant frequency indicating distance

Question 32

Use of recombination-based mapping techniques

Answer 32

Developed to use polymorphic genetic markers along a chromosome to track a disease phenotype through generations of a family