Genetic Mapping Flashcards
What is the purpose of Genetic Mapping?
Genetic Mapping: the key to understanding what are the genes and proteins made that are responsible for human genetic diseases.
(used to determine the location of genes on chromosomes; using genome sequencing and computer analysis )
Explain how genes sort for two unlinked genes (according to Mendel’s Laws)
If there are two Unlinked genes, Genes sort INDEPENDENTLY (according to Mendel’s laws)
if both parents are heterozygous for both genes, the alleles of genes will assort independently which provide 9:3:3:1 ratio
(9:3:3:1 ratio occurs because of gametes having 4 equal types (4 types made by mom and 4 types made by dad)
What was important about Morgan’s study of fruit flies? What rare flies were discovered?
Morgan’s study of fruit flies broke open process of studying animal genetics in many ways.
He looked at lots of genes that showed abnormal types of flies:
found rare flies with unique triats
ex: Purple eye gene (pr1/pr1; recessive mutation)
cause purple colored eyes
-Vestigial wings (Vg1/vg1); where these flies had barely any wings, and could not fly.
hence Morgan saw 2 different genes each controlled by recessive mutations
What abnormal ratio for flies did Morgan discover for fruit flies? What occured in the crosses for P0, F1 and F2? Why was this significant ?
When he did crosses with flies with two different genes, they noticed abnormal ratios (that were NOT mendel’s laws
Cross:
P0: wild type male (pr+/pr+ vg+/vg+) x homozygous Purple, vestigial female (pr1/pr1 vg1/vg1)
-ALL progeny in F1 were wild type (pr+/pr1 vg+/vg+) since both genes are recessive
When take F1 flies and cross males with purple vestigial strain, got ODD ratios
F1: pr+/pr1 vg+/vg1 (female) x pr1/pr1 vg1/vg1 (male)
you got a lot of flies that are wild type, and purple, vestigial and few amount of purple flies (normal wings) and vestigial flies (normal colored eyes)
Morgan set up experiment like this because he wanted to see what kind of gametes the mother was making and had one parent to be homozygous to have SIMPLER ratios, instead of regular 16 possibilities, He got 4 possibilities instead
(all flies get purple allele and vestigial from dad, meanwhile 4 possible combos from mom)
Differentiate between expected and observed ratios in Morgan’s experiment. Why is this important? What could be discusses about the gametes ?
if genes are assorting independently, you would have Expected all combos to be equal in number
but by setting up experiment where only one parent makes different types of gametes instead of both parents, it was Easy for Morgan to conclude observed ratios are far away from what is expected.
-Morgan also knew which gametes were made by mother
Morgan could predict all of gametes made by heterozygote (mother)
F1: pr+/pr1 vg+/vg1 x pr1/pr1 vg1/vg1 (purple, vestigial father)
would have F2 gametes :
wild type: get pr+ vg+ both in eggs
purple, vestigial: pr1 vg1 (get two mutant forms in egg)
purple : pr1 vg+ get purple mutant and wild type vestigial in egg
vestigial: pr+ vg1 (good copy of purple gene, and mutant vestigial in egg)
You can predict which genes (mutated or wild type were in eggs ( from mom) since dad gave both mutated copies of genes to children
What did Morgan observe to be common gametes?
parental gametes were MORE COMMON than recombination ones
The gametes that mother from F2 got from her parental (P0) with genotypes pr+ vg+ (wild type) and pr+ vg+ (purple, vestigial) were found to be more common as gametes than Recombinants: pr1 vg+ (purple) and pr+ vg1 vestigial
Explain why Morgan did a complementary cross with fruit flies? What was the point ?
Morgan did complementary cross to show that it had same results
complementary cross (make recombinants the parental in P0) ;
P0 : pr1 vg+ (purple) x pr+ vg1 (vestigial) will lead to –>
F1: pr+ vg1/pr1 vg+ (wild type) x pr1 vg 1/pr1 vg1 (purple, vestigial) –>
F2: gametes will be few wild type, and purple, vestigial that are RECOMBINANT, and purple, and vestigial that are PARENTALS
**Complementary Cross gives SAME results (of parental being more common that recombinants) **
even if we switch who is parental or recombinant, parental still more common
What was later found about purple and vestigial genes ? Why is this important?
The Purple and vestigial genes are physically LINKED on the same chromosome
Because they are on the SAME chromosome, alleles of pr and vg usually segregate together into gametes.
———–pr————-vg———-
Sometimes the chromosome breaks and the pieces recombine (Crucial process)
What occurs to chromosomes in Meiosis I and Meiosis II?
meiosis I: 2 sets of chromosomes line up with each other and exchange pieces of DNA (important for knowing that all parts of meiosis are completed)
-so at end of Meiosis I: can either pass on normal chromosome 1 or chronometer with recombination
meiosis II: pass on normal chromosomes or recombinant ones
Who was Alfred Sturtevant? What did he discover?
Alfred sturtevant, a student of Morgan’s lab at Columbia
they were waking with pencils, slide rules, paper.
They gathered data from fruit fly crosses, and saw crosses that did not follow Mendel’s laws and ones that did.
Alfred said: “ I suddenly realized that the variation in strength of linkage, offered possibility of determining sequences in linear dimension of a chromosome”
**He realized the variation in how many parental and recombinant genes, offered possibilities of determining how far apart genes were on chromosome ***
Alfred also created FIRST Chromosome MAP (that included sex-linked genes in order) and principles to build map i
What is the equation for recombinant frequency? What does it measure? Who proposed this equation ?
Recombinant frequency: number of recombinant gametes/number of total games
= 151(purple) + 154 (vestigial) /2839 (total)
= 10.7%
-Morgan and Sturtevant proposed that recombination frequency is a measure of DISTANCE
10.7 map units= 10.7 centiMorgans (cM)
(hence he said if two other genes are 20 % apart, they are father away from each other; whereas if two other genes are 1% apart, they are closer to each other)
How can you create genetic map using distance theory with genes.
you can put individual distances together to make two-factor distances
-then you can add the smaller distances together to make larger distances and map these genes in order from (closer to farthest)
Differentiate between Two factors and three-factor crosses.
You can look at more than two genes at time using three-factor crosses.
Three-factor crosses can be used to determine gene order
Two-factor crosses can be IMPRECISE for ordering nearby gene
How is the three factor cross used?
After finding more mutant genes in fruit flies:
-Vermillion eyes (v1/v1) (flies make bright red eyes)
-Crossveinless (cv1/cv1) (flies lack cross veins)
-Cut wings (ct1/ct1) (flies have cuts in their wings)
you can do three factor cross with the genes
Described the three factor cross that occured with the new mutants genes in fruit flies. What were the gametes?
Three Factor Cross:
Vermillion (male) x cut, crossveniless (female); to get all
WILD TYPE in F1 generation
cross wild type (female) x Vermilion cut, crossveinless (males) and get gametes.
gametes: 8 kinds
Most frequent gametes were Vermilion and Cut, crossveinless (Parental genes)
three other classes:
-vermillion, crossveinless, cut mutants (these types are similar in size (either all mutant or all wild type)
-Vermillion, cut, crossveinless, and wild type
- vermilion, cut
- crossveniless
These last two types were least common
( parental genes are more common; first two types)
Assign genotypes of each of eggs made by mom
since dad provides homozygous recessive mutations to all children
group progeny to classes.
Most common classes are parental (same chromosomes that parents had)
-other classes involve single recombinant between 2 of 3 genes (two different forms of classes) , OR two different recombinant events (Rare)
Use can use these classes two define order of genes on chromosome
-these three genes must be on same chromosome since, parental is more common than any other gene rearrangement
How do you know which gene is in the correct order? Discuss example with three mutant genes in fruit flies
Predicting gene order
-information about single and double recombinant will let us know what the correct gene order is.
when look at rare double recombinants, determine how it was formed.
A mother who is heterozygous for 3 genes: have 3 possible ways to order gene: the genes that are cross veinless, cut gene or vermilion gene could be in middle (3 possibilities)
-however, we know
double recombinant events needed to make rare classes of vermillion cut or vermillion crossveinless progeny.
so ask, which one of arrangements would give us two kinds of flies by double cross over event?
How many chromosomes do Drosophila flies have? What can you notice about the Drosophila Genetic Map?
Flies have 4 chromosomes (1 of them is tiny and not really important) 3 chromosomes are big and are on maps
if two genes are on different chromosome, you get rearrangements only half the time.
you had to build map:
** smallest pieces are the most ACCURATE pieces**
-small recombinant frequencies are one with Highest confidence
if two genes are on same chromosome, closer they are to each other, less frequency there is a break that recombines (and determines which alleles switch)
