Gene Mapping Flashcards
what does a gene map show us?
the relative order of genes on the chromosome
and the distance between them
what are the 3 types of maps
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
why are gene maps useful
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)
what is conserved/shared synteny
blocks of chromosomes where the gene order is the same between species
(can show how chromosomes have evolved during evolution)
what process can we use to locate the disease associated gene on a specific chromosome
positional cloning
via linkage analysis
physical maps - what is the restriction enzyme map
map made using restriction enzymes that cut DNA at specific sites
see onenote for the map that shows the section that codes for CFTR
what is G banding and how is it used in cytogenetic maps
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
after determining where the centromere is, there is usually a shorter and longer arm of the chromosome. what are these called?
P arm - short (petite)
Q arm - long (jus cuz q comes after p)
how are genes located in a cytogenetic map
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
what is linkage mapping
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
what other process can also produce recombinant genes and why is this something we need to consider in linkage mappinh
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
considering both crossing over and independent assortment, what do recombination frequencies tell us?
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
what can a test cross help determine
from the phenotype of the offspring produced, we can work out the genotypes of the gametes (from one parent in the cross)
what is the first step to do a test cross?
1: make a double heterozygote
using one homo dom and another homo dom
the double hetereozygot = F1
what gametes can the double heterozygote F1 make?
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%)
what is the main property a tester strain needs to have?
must be homozygous for the recessive allele
what is the second step to do a test cross
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)
what does the phenotype of F2 from the test cross tell us
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
what is formula for recombinant frequency
no. of recombinant progeny/total no. of progeny x100
units are in centiMorgans (cM)
e.g. 10% RF = 10cM
what is two-point linkage mapping
dealing with 2 genes on the same chromosome
example of two-point linkage mapping
see onenote
Why might it be difficult to correctly identify the parental genotypes from the double heterzygote
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
how do we combine map distances to make a linakge map?
gotta do it relative to each other
jus go look at onenote pls
why is the map non-linear (so the distance don’t rly add up)
- 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 - crossover events aren’t always independent
so one crossing over event could stop or stimulate crossing over event from occuring (negative and postive interferance) - 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)
