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What is a locus?
physical location of a gene on a chromosome
What are alleles?
- Homologous pairs of chromosomes often contain alternative forms of a given gene
- Different alleles of the same gene segregate at Meiosis I
- Alleles of different genes assort independently in gametes
What is linkage?
Genes on the same chromosome exhibit linkage: inherited together
How do you determine map units?
- Gene mapping determines the order of genes and the relative distances between them in map units
- 1 map unit = 1 cM (centimorgan
How to determine the relative distance between alleles?
- Gene mapping methods use recombination frequencies between alleles in order to determine the relative distances between them
- Recombination frequencies between genes are inversely proportional to their distance apart
- Distance measurement: 1 map unit = 1 percent recombination (true for short distances)
What does recombination result from?
results from crossing-over between linked alleles
What is a genetic map?
The linkage of the genes can be represented as a genetic map, which shows the linear order of the genes along the chromosome spaced so that the distances between genes is proportional to the frequency of recombination between them.
What are linked genes?
Genes with recombination frequencies less than 50 percent are on the same chromosome = linked
What is a linkage group?
Linkage group = all known genes on a chromosome
What are unlinked genes?
Two genes that undergo independent assortment have recombination frequency of 50 percent and are located on nonhomologous chromosomes or far apart on the same chromosome = unlinked
What is the map distance?
The map distance (cM) between two genes equals one half the average number of crossovers in that region per meiotic cell
How to measure recombination?
¨The recombination frequency between two genes indicates how much recombination is actually observed in a particular experiment; it is a measure of recombination
What does not occur in a short interval on a genetic map?
- Over an interval so short that multiple crossovers are precluded (~ 10 percent recombination or less), the map distance equals the recombination frequency because all crossovers result in recombinant gametes
- Over the short interval genetic map = linkage map = chromosome map
What are noncrecombinant chromosomes?
Two exchanges taking place between genes, and both involving the same pair of chromatids, result in nonrecombinant chromosomes
Genetic vs physical distance discrepancies:
- Map distances based on recombination frequencies are not a direct measurement of physical distance along a chromosome
- Recombination “hot spots” overestimate physical length
- Low rates in heterochromatin and centromeres underestimate actual physical length
very little recombination takes place, a small distance in the genetic map corresponds to a large distance on the chromosome.
what is a morphological marker?
- RECOGNIZABLE FEATURES IN FIELD OR LAB OFTEN BASED ON PHENOTYPE
- LIMITED OPPORTUNITIES BUT VERY USEFUL
What is a genetic marker?
- A Genetic Marker is any difference (or uniqueness) in DNA sequence, which can be tracked from generation to generation.
- May include genes for recognizable traits or just DNA sequences with no known function.
- If the marker is based on DNA sequence per se it is called a DNA Marker
what can dna markers be used for?
- Used to track genetic differences among individuals
- DNA Markers can be used to identify the position of “Normal or Wildtype” genes, mutant genes, chromosome breaks, inversions, etc. on a chromosome
What is a polymorphism?
- a genetic difference that is relatively common in a population
- Many polymorphisms do not occur in DNA sequences that code for proteins
- These polymorphisms conversely can be useful in determining the presence of two or more forms of a gene or DNA sequence = Marker