Gene Mapping

Question 1

what does a gene map show us?

Answer 1

the relative order of genes on the chromosome
and the distance between them

Question 2

what are the 3 types of maps

Answer 2

physical map - based on the DNA sequence of the chromosome (but there’s diff types)

cytogenetic map - based on banding patterns after staining chromosomes with diff dyes

genetic/linkage map - based on distance between genes and the nuber of crossing over events between them

Question 3

why are gene maps useful

Answer 3

identify genes reponsible for diseases or traits

can help design experiments

combine effective trains in plant of animal breeding

compare genome organisation between organisms (we tend to find blocks of chromosomes where the gene order is the same between species)

Question 4

what is conserved/shared synteny

Answer 4

blocks of chromosomes where the gene order is the same between species
(can show how chromosomes have evolved during evolution)

Question 5

what process can we use to locate the disease associated gene on a specific chromosome

Answer 5

positional cloning
via linkage analysis

Question 6

physical maps - what is the restriction enzyme map

Answer 6

map made using restriction enzymes that cut DNA at specific sites

see onenote for the map that shows the section that codes for CFTR

Question 7

what is G banding and how is it used in cytogenetic maps

Answer 7

proteolyitc digestion of chromosomes and staining with Giesma
gives us chromosomes that show charateristic banding

G-light and G-dark banding corresponding to light and heavy elements
responds to the A-T and G-C residues in the chromosome

is identical in each human

see onenote

Question 8

after determining where the centromere is, there is usually a shorter and longer arm of the chromosome. what are these called?

Answer 8

P arm - short (petite)
Q arm - long (jus cuz q comes after p)

Question 9

how are genes located in a cytogenetic map

Answer 9

first they’re assigned to either a P or Q arm
then to a region
then a band
then a sub-band

see one note

Question 10

what is linkage mapping

Answer 10

basically, figuring out the distance between chromosomes depending on hwo often they partake in crossing over

the closer the are, the less frequently they’ll cross over

to be able to measure frequency:
after crossing over, recombinant genes are formed
the proportion of these recombinant genes can be related to the distance between the genes on the chromosome

Question 11

what other process can also produce recombinant genes and why is this something we need to consider in linkage mappinh

Answer 11

independent assortment
because genes on DIFFERENT chromosomes can be independently assorted during prophase 1 in different combinations
can give recombinant genes depending on how they were assorted

Question 12

considering both crossing over and independent assortment, what do recombination frequencies tell us?

Answer 12

IF there’s 50% recombinant genes formed after a cross:
- either the 2 genes are on 2 different chromosomes and the recombinants are formed as a result of IA
OR
- they are so far apart on the SAME chromosome, that everytime meiosis happens, it’ll partake in crossing over and form recombinant

IF there’s less than 50% recombinant genes formed after a cross:
- genes are linked on the same chromosome (so independednt assortment aint it)
and the smaller the recombinant frequency, the closer the genes are to each other

Question 13

what can a test cross help determine

Answer 13

from the phenotype of the offspring produced, we can work out the genotypes of the gametes (from one parent in the cross)

Question 14

what is the first step to do a test cross?

Answer 14

1: make a double heterozygote
using one homo dom and another homo dom
the double hetereozygot = F1

Question 15

what gametes can the double heterozygote F1 make?

Answer 15

w/ no crossover = A B and a b parental gametes

w/ one crossover = A b and a B recombinant gametes (50%)
plus, A B and a b parental gametes (50%)

Question 16

what is the main property a tester strain needs to have?

Answer 16

must be homozygous for the recessive allele

Question 17

what is the second step to do a test cross

Answer 17

2: cross the double heterozygote formed from before with a tester strain

this forms

F2 - with no crossing over =
1: A B and a b
2: a b and a b
(the phenotype being A B for 1 and a b for 2)

w/ crossing over

1: A b and ab
2: aB and ab
(the phenotype being Ab for 1 and aB for 2)

Question 18

what does the phenotype of F2 from the test cross tell us

Answer 18

phenotype of F2 = the genotype of gamete from the double heterozygote

so this is how u find the genotype of the parent from the pehnotype of the offspring

see onenote for test cross diagram

Question 19

what is formula for recombinant frequency

Answer 19

no. of recombinant progeny/total no. of progeny x100

units are in centiMorgans (cM)

e.g. 10% RF = 10cM

Question 20

what is two-point linkage mapping

Answer 20

dealing with 2 genes on the same chromosome

Question 21

example of two-point linkage mapping

Answer 21

see onenote

Question 22

Why might it be difficult to correctly identify the parental genotypes from the double heterzygote

Answer 22

cuz there’s two ways to form a double heterzygote
One is the normal AABB x aabb
But also
AAbb x aaBB

these form different parental and recombo types later on, see OneNote for diagram I can’t explain it smh

Question 23

how do we combine map distances to make a linakge map?

Answer 23

gotta do it relative to each other
jus go look at onenote pls

Question 24

why is the map non-linear (so the distance don’t rly add up)

Answer 24

1. when the distance is realllyyyyy far
   sometime 2 crossing over events can happen
   which kinda just reforms it back to the original parental gametes being formed
   so
   these events aren’t gonna be accounted for
   which can UNDERESTIMATE the true distance
2. crossover events aren’t always independent
   so one crossing over event could stop or stimulate crossing over event from occuring (negative and postive interferance)
3. frequency of crossing over varies in different regions of the genome (this is more to do on a larger scale than looking at small scale maps)