Gene Mapping Flashcards
What is a gene map?
A gene map shows the relative order of genes on the chromosome and the distance between the genes.
It is a tool to navigate genes in a genome to show the location and orientation of genes along the chromosome
Why are gene maps useful?
- Identify genes linked to diseases or traits (positional cloning)
- Aid in studying gene function
- Support plant/animal breeding programs
- Compare genome structures (to see how genes have evolved) across species (conserved/shared synteny)
What is the approximate conversion of 1 centiMorgan (cM) in humans?
1 cM is approximately 1 Mb (megabase) in humans.
Name the three types of gene maps.
- Genetic/Linkage map - based on crossover events in the genome between different kinds of species (numbers tell us the relative order and distance of genes on the chromosome)
- Cytogenetic map - refers to genomic location based on banding patterns (light & dark regions)
- Physical map - based on DNA sequences found
What is a physical map?
A physical map is created using restriction enzymes that cut DNA at specific sites and provides the DNA sequence of human chromosomes.
What is the significance of G-banding in cytogenetic maps?
G-banding involves staining chromosomes to produce a characteristic pattern of light and dark bands, with each band assigned a specific number.
What occurs during genetic linkage?
Alleles of genes on the same chromosome will segregate together in gametes during meiosis unless crossing-over occurs between them.
What is the relationship between crossing-over frequency and gene distance?
The frequency of crossing-over between two gene loci is proportional to the physical distance between them on the chromosome.
What does a recombination frequency (RF) of less than 50% indicate?
Genes are linked on the same chromosome.
What is a testcross?
A testcross is designed to determine the genotypes of gametes from one parent based on the phenotypes of the offspring they produce.
How is genetic distance calculated in linkage mapping?
Genetic distance is calculated using the formula
RF = (number of recombinant progeny x 100) / total number of progeny.
Genetic distance = Recombination Frequency (RF)
So 10% RF = 10 cM
What is the unit of measurement for genetic distance?
Map distances are given in centiMorgans (cM).
What is the expected percentage of recombinant gametes when genes are on different chromosomes?
50% recombinant gametes.
What is the problem with measuring genetic distances over large distances?
Double (or multiple) crossover events can underestimate the true distance between genes.
True or False: Map distances are additive over long distances.
False. Map distances are additive over short distances but not over long distances.
What is the significance of the frequency of crossing over in linkage mapping?
It gives a measure of the distance between genes.
What happens during independent assortment?
Independent assortment produces recombinant gametes from genes on different chromosomes or genes that are far apart on the same chromosome.
Fill in the blank: A genetic map shows the _______ order of genes on the chromosome.
[relative]
What is the genetic distance if the RF is 10%?
10 cM.
In a two-point mapping experiment, what are the parental types?
The parental types are the most frequent offspring phenotypes.
What is an example of a genetic trait linked to a gene?
Cystic fibrosis is linked to a mutation in the CFTR gene.
How many G-dark bands are visible on the 23 human chromosomes?
About 850 G-dark bands.
What is the expected outcome of a testcross with a double heterozygote?
It produces both parental and recombinant gametes.
What is the role of positive or negative interference in crossover events?
Interference affects the probability of crossover events in adjacent regions, influencing genetic mapping.
What are the principles of linkage mapping?
- Crossing over occurs at ‘random’ positions along each chromosome
- The frequency of crossing over between 2 gene loci is proportional to the physical distance between them on the chromosome
- The further apart the genes are, the higher the chances of crossing over events and recombinant gametes being produced
- The closer the gene, the lower the frequency of crossing over events and less recombinant gametes being produced
What do recombinant frequencies tell us?
50% recombinant gametes (from crossing over or independent assortment) = genes on different chromosomes or far apart on the same chromosome
Less than 50% recombinant gametes = genes are linked on the same chromosome
The smaller the RF, the closer the genes
What are the steps in making a testcross?
- Make a double heterozygote
- Cross the double heterozygote to a tester strain which is homozygous recessive
Why aren’t genetic distances additive?
- Over large genetic distances, double (or multiple) crossover events underestimate the true distance
- Crossover events are not always independent - a crossover in one region can influence the probability of a crossover in an adjacent region = positive or negative interference
- The frequency of crossing over varies in different regions of the genome
