Linkage and Mapping in Eukaryotes

Question 1

Independent segregation of genes on different chromosomes

Answer 1

equally likely to get all 4 combinations of gametes
can get every combination present in the gametes
happens because of meiosis

Question 2

Independent segregation on the same chromosome

Answer 2

Will just get 1 combination because of linkage
unless recombination occurs between genes in meiosis 1
combinations of alleles that parents have
inherited through generations

Question 3

What is linkage?

Answer 3

Two alleles linked together on the same chromosome
both are inherited together
passed on together through the gametes

Question 4

How was linkage discovered?

Answer 4

in certain crosses a greater no of parental genotypes generated than expected
expected the 1/4 ratio as double recessive crossed with heterozygous

Question 5

What indicates two genes are linked?

Answer 5

greater number of parental offspring than expected
non parental genotypes present due to crossing over in meiosis 1
create recombinants due to crossing over

Question 6

Why do you test cross with a recessive?

Answer 6

it can only produce one type of gamete
can then look at offspring and tell what type of crossing over has occurred

Question 7

What are linkage maps?

Answer 7

shows the frequency of crossing over and the distance between genes
further away the linked genes are, higher likelihood of crossing over

Question 8

Thomas Hunt Morgan

Answer 8

studied genetics of the fly Drosophila melanogaster

Question 9

What terminology did Morgan introduce?

Answer 9

genes as 2 letter abbreviations
e.g. pr = purple eye
pr+ red eye (wild type)
+ indicates the wild type

Question 10

cis and trans configuration of genes

Answer 10

cis = ++
trans = +-
dash shows linked together on same chromsome

Question 11

Test cross in the felt results?

Answer 11

pr+pr+vg+vg+ X prprvgvg
very low numbers of recombinants

Question 12

How to work our % of recombinants

Answer 12

recombinants / total
1% = 1 centimorgan cM

Question 13

How to measure linkage maps

Answer 13

linear and additive
measure distance between lines
can formulate hypotheses as to distance between an unknown

Question 14

How to calculate if offspring are mapped using genetic crosses

Answer 14

1. Calculate distances
2. add the number of recombinants
3. divide by total number of offspring produced

if separated by 5 map units- unlinked & on different chromosomes
inherited by normal Mendelian processes

Question 15

Chromosomes mapped using 3 point crosses

Answer 15

add up different they do not add up properly
3 classes of offspring = linked genes

most common = parental types
least common = double recombinant types
rare values count twice as 2 different crossover events

Question 16

How do you deduce gene order?

Answer 16

middle must be the only one to undergo gene recombination
double recombinants not found alongside in parents

Question 17

What are the points for looking at test cross problems? (4)

Answer 17

1. look at patterns - if 3 classes then 3 linked genes
2. determine parental and double recombinant types
3. Deduce gene order
4. Calculate distances between pairs of genes