Module 8 - Gene Mapping Flashcards
1
Q
What are linkage groups?
A
- if the recombination frequency between two loci equals 50%, then they belong to different linkage groups. Two loci on the same chromosome are said to belong to the same linkage group, and the recombination frequency is less than 50%. They don’t assort independently
2
Q
What is the Chi-Square test (x^2 test)?
A
- the Chi-Square test indicates whether the difference between the observed and expected is due to chance or not. If probability is less than 5%, then the difference between the observed and expected IS considered significant
- if probability is greater than 5%, then the difference between the observed and expected is NOT considered significant
- formula: X2 = (O-E)^2/(E)
3
Q
Steps in Chi-Square test
A
- Start with a null hypothesis. The null hypothesis states that the observed results match the expected results. (if P < 5% it IS considered significant, reject the null hypothesis, and it doesn’t assort independently)
- (if P is > 5% then it is NOT considered significant, accept the null hypothesis, and it can assort independently)
2. calculate the expected results using the total number of progeny and the cross - determine the number of degrees of freedom
- use the table
4
Q
Notation for crosses with linkage
A
- in analyzing crosses with linked genes, you need to know the genotype and the arrangement of genes on chromosomes
5
Q
What are nonrecombinant gametes in regard to complete linkage?
A
- if no crossing over occurs because genes are linked, gametes that only contain combinations of alleles that were present in the original parent are called nonrecombinant gametes
6
Q
What are nonrecombinant progeny in regard to complete linkage?
A
- the same combinations of traits that were present in the original parents are called nonrecombinant progeny
7
Q
Unlinked genes and independent assortment in regards to nonrecombinant gametes and progeny
A
- if genes are unlinked and show independent assortment, nonrecombinant and recombinant gametes are produced in equal proportions
- 1/2 of the progeny are nonrecombinant and 1/2 are recombinant
8
Q
What is incomplete linkage?
A
- genes that are linked but are far enough apart that some crossover occurs
- crossing over takes place in meiosis I
- only crossing over between non-sister chromatids that carry different alleles will produce recombinant gametes
- the number of recombinant gametes formed depends on the frequency at which crossing over takes place
- if a single crossover takes place at EVERY meiosis then there would be 50% nonrecombinant and 50% recombinant gametes formed raising two possibilities; genes are on different chromosomes or genes are far apart on the same chromosome
- if crossing over takes place at SOME meioses (gene is incompletely linked) then some recombinant gametes would form that are less than 50%, but the majority that is greater than 50% would be nonrecombinant gametes that would form
9
Q
Recombination Frequency and Genetic Map Distances
A
- recombination frequency = number of recombinant progeny/total number of progeny x 100
- recombination frequency can be converted into genetic map distance with units of m.u or cM
- 1 m.u or cM equals 1% recombination
10
Q
Predicting Outcomes of Crosses With Linked Genes
A
- if you are given a genetic map (or recombination frequency) you can use that information to make predictions about types and proportions of progeny in a cross
- predicted frequencies of progeny are determined using the multiplication rule
11
Q
Genetic Mapping and Double Crossovers
A
- a double crossover arises when two separate events take place between the same two loci
- a double crossover between two linked genes only produces nonrecombinant gametes
- if a double crossover goes undetected, it causes an underestimation of the true map distance between two linked genes
- genetic maps based on shorter distances are generally more accurate than maps based on longer distances
