Week 5 Flashcards
Linkage, Chi Square Analysis, and Mapping (40 cards)
what does it mean for genes to be genetically linked
they are in close proximity to one another and tend to assort together
what does departure from a 1:1:1:1 ratio of F1 gametes indicate
two genes are on the same chromosome and not completely independent
how can we track genes of chromosomes
looking at the F2 offspring of a testcross
why are drosophila used to study genetics
- ~75% or known human disease genes have a similar gene match in the genome of fruit flies
- ~50% of fly protein sequences have a similar protein match in mammals
what are drosophila used to study
- neurodegenerative diseases: Parkinson’s, Huntington’s, Alzheimer’s
- research on processes of aging, diabetes, drug abuse
what indicates that genes are linked
parental combinations outnumber recombinant types
how do we represent the genotype of the F1 to keep track of parental allelic combinations
vgb/vg+b+ = alleles from father/alleles from mother
how are recombinant phenotypes formed
crossing over between non-sister chromatids
what does recombination frequency depend on
the distance between the gene pair
what does a higher recombination frequency indicate
the genes are further apart
what is an example of linkage in humans
Nail-patella syndrome gene is linked to gene I or ABO blood group
is linkage 100%
it is never 100%
what is the expected ratio of parental to recombination class number in unlinked genes
1:1:1:1
what is the chi square test used for in terms of genetic linkage
used to evaluate if two genes assort independently or are genetically linked
what is the chi square test method (in terms of genetics)
1) use the data obtained from a breeding experiment to determine the total number of offspring, the number of different classes of offspring and the number of offspring observed in each class
2) calculate the number of offspring that would be expected for each class if the null hypothesis were correct (% predicted x total number of offspring)
3) x2 = ∑(number observed - number expected)2/number expected
4) determine the degrees of freedom (df) = number of classes - 1
5) find the chi-square value and number of degrees of freedom on a chi square table, to determine a p value: the probability that a deviation from the predicted numbers at least as large as that observed in the experiment will occur by chance
6) evaluate the significance of the p value. the convention is that a 0.05 p value is the boundary between accepting and rejecting the null hypothesis. very small p values indicate a high degree of significant difference
what does a high p value from a chi square test indicate
the null hypothesis is correct; the deviation from expected is due to chance, the difference is not significant, and the genes are not linked
what is the null hypothesis for the chi square analysis for gene linkage
the genes are not linked (50% should be parent, 50% should be recombinant)
does sample size matter for the chi square test
yes, a large sample size is needed, the larger the better
what is the evidence of recombination
physical markers: cytologically visible abnormalities that make it possible to keep track of specific chromosome parts from one generation to the next
who proved recombination in 1931
Harriet Creighton and Barbara McClintock
what species did Creighton and McClintock use to study recombination
corn
who studies recombination with drosophila
Curt Stern
who first proposed that chiasmata were the points of crossing over that resulted in recombination
Thomas Hunt Morgan
who proposed that recombination frequency (RF), could be used as a gauge of the physical distance between any two genes on the same chromosome
A. H. Sturtevant
