Genetic linkage and gene mapping Flashcards
1
Q
Who was the first to associate a specific gene with a specific chromosome?
A
- Morgan and students
- They used the common fruit fly (which feed on fungi growing on rotten fruit)
- Noted that the male and female have certain anatomical differences
2
Q
What terminology did Morgan use?
A
- Referred to the normal phenotype as the ‘wild type’ = (+)
- Referred to the mutant phenotype as the ‘alternate trait to wild type’
- The gene in his terminology takes its symbol from the first mutant
- For example, the eye colour in fruit flies: 1st mutant = white eye allele (w), wild type = red eye allele (w+)
3
Q
Outline Morgan’s theory
A
- Sex linkage hypothesis
- His F2 and F1 observations make sense if the eye colour gene locus is on the X chromosome
- In fruit flies sex is determined by XY
- Males are ‘hemizygous’ and possess only ONE allele of any sex-linked gene
4
Q
Outline genetic linkage
A
- Genes on any given chromosome that are physically linked to each other
- Tend to be inherited together as a linkage group
- Linked genes DO NOT conform to Mendelian patterns of inheritance (whenever two genes are linked because of their location on a chromosome, their alleles will not segregate independently during gamete formation)
5
Q
Outline the pattern of inheritance for two linked genes
A
- Disproportional number of parental phenotypes
- Alleles in a linkage group tend to be inherited together, but a smaller number of recombinant phenotypes is present due to crossing over
6
Q
Describe crossing-over
A
- Occurs during prophase of meiosis I
- DNA in homologous chromosomes (HCs) has duplicated (so HCs are composed of identical sister chromatids)
- HCs pair precisely, aligning gene by gene
- They form a tetrad structure (a synaptonemal complex - a protein structure that forms between homologous chromosomes)
-Homologous portions of two non-sister chromatids exchange places, giving rise to individual chromosomes that contain both maternal and paternal DNA
7
Q
Where does crossing over occur?
A
- The chiasma (an x-like structure inside the tetrad)
8
Q
Explain the mechanism of genetic recombination within the tetrad
A
- Non-sister chromatids physically exchange DNA at chiasma
- Tension results in DNA duplexes breaking and re-joining clockwise
- Multiple cross over events can occur (double x-over, triple x-over, etc)
9
Q
Explain the difference between genetic recombination and crossing over
A
- The main difference is that recombination is the production of different combinations of alleles in the offspring whereas crossing over is the exchange of genetic material between non-sister chromatids, the event which produces recombination
10
Q
How would you work out recombination frequency?
A
Recombination frequency = number of recombinants/total number of offspring, x100
11
Q
Where does recombination frequency lie?
A
- Between 0% and 50% (o% < RF < 50%)
12
Q
What does RF 50% mean?
A
- It is when genes are unlinked
- also when genes are far apart on the same chromosome
13
Q
What does RF (near to) 0% mean?
A
- When genes are very tightly linked
- I.e., very close together
14
Q
Outline genetic mapping (for genetic linkage)
A
- First maps were the sequence of genes along a chromosome and the relative distance between them
- Calculating RFs allowed for genes to be ordered (giving an indication of the distances between them)
- It is assumed that there is an equal chance of crossing over along a length of chromosome
- Therefore, if two genes = far apart, they are more likely to cross over (high RF prediction)
- And, if two genes = very close, they are less likely to cross over (low RF)
15
Q
What was the first gene map?
A
- An analysis of three mutant traits on fruit flies, drosophila (yellow body, white eye, miniature wing)
- Linkage map was calculated
- One map unit = a centimorgan (cM)
- 1cM = 1% recombination between two genes
-Linkage map units (cM) do not have an absolute size, but as a general rule in humans, 1cM = around 1x10^6bp
