lecture 5 Flashcards
are genes on the same chromosome
Sometimes
for drosophila element most likely on sae chromosomes since it has very little chromosomes
describe why morgan got his findings - for indep ass
segregate independently from each other
only 10% of parental since traits on same chromosome
not very precise for single meiotic event
homologous chromosomes together and cross over = recombinant
when does crossover take place during meiosis, before or after dna rep? could there be multiple crossovers in a single meiosis event
hard to answer since looking at end product
what is tetrad
four spores produced by meiosis of a haploid organism eg fungi
can mate so each haploid to produce diploid
what is ascus
keeps tetrad together which can be isolated to study single meiotic event
describe tetrad - informally
when become diploid = can be mieotic event
ascus = keeps tetrad together
product of single meiotic event is kept in single physical structure = ascus
can dissect and isolate and grow and analyze 4 colonies = result of single event and can answer questions
describe cross between 2 haploids
AB x ab
2 chromosome stage = happens before dna rep, end product = only recombinant phenotype in single ascus
4 chromatid stage = cross over must happen after dna rep to get expected result
describe multiple cross overs between 2 haploids = ABC x abc
3 diff traits on same chromosome
can visualize what happened
combinatorial shuffling
2 chromatids = can predict what happened during meiotic event by looking at products, must be exchange = cross over events, 4 phenotypes see 2 parental and 2 recombinant = must be multiple cross overs
describe cross over and physical distance of genes
farther apart the genes are = more likely to crossover and produce recombinants
frequency obtained = very varied
product of many diff meiotic events
why is the frequency obtained very varied
cross over events = randomly
if 2 genes further apart = more likely for recombination event to happen like if cross over events happen every 1-4 genes ish
what is relative distance of 2 genes reflected in
recombination frequency
describe genetic map unit
student of morgan found it
centimorgan
1 genetic map unit = m.u. = frequency at which one out of 100 meiosis products is recombinant
recomb total/total x 100
pr and vg = 10.7% = 10.7 mu = 10.7Cm
in theory RF of 2 genes cannot exceed 50%, recombination frequency
what can RFs generate
RFs between linked genes can generate map of genome
describe 3 point testcross - traits
vermillion
cross veinless
cut
3 genes linked
describe 3 point testcross - cross
double mutant to single mutant = create hetero for all 3 genes, looks like wild type tho
recessive for all genes = mutant
F1 x tester (recessive)
2^3 = 8 possible combos
how to generate map
add up all recombinant ones and divide by total to get RF
v and cv = 18.5
v and ct = 13.2
cv and ct = 6.4
so map =
v 13.2 ct 6.4 mu
but 13.2 + 6.4 = 19.6 anf not 18.5 = WHYYY
WHY don’t the RFs add up? for the 3 point testcross
parental genotype = set up in a way, but then mutant changes places on the map = mislead since didnt know relative position of gene
must be 2 cross over events so counts as 2 events
parental type hella important
2 recomb events between v and cv = 2(3+5) + 268 = 284 recombination events so 284/1448 x 100 = 19.6 = sum of the 2 recombination events
describe map of drosophila genes
things that are nearby on chromosomes tend to segregate together = get low numbers for recombinant types
didnt know structure of dna or sequence of genes but could figure out with genetics
mapped based on RF
