Chapter 5: Chromosome Mapping in Eukaryotes Flashcards
What are the units of transmission in meiosis?
Chromosomes, NOT genes
What are linked genes?
Linked genes are genes that are likely to be inherited together because they are physically close to one another on the same chromosome.
Chromosomes have the potential to recombine, If genes are close together, the chances of being recombined are higher than if they are far away from each other.
The frequency of crossing over on a single chromosome is proportional to…
Distance between them
What does crossing over result in?
Recombination
What do chromosome maps indicate?
The relative location of genes on a chromosome
Meiotic Consequences: independent assortment
No linkage exhibited
Meiotic Consequences: Linkage without crossing over
Complete linkage
Meiotic Consequences: Linkage with crossing over
Generates recombinant gametes
What is complete linkage?
0% recombination
Parental combinations of alleles always segregate together into the same gamete
Extremely rare
For this to occur, the alleles at the two loci are physically attached to the same chromatid and will nearly always segregate together into the same gamete.
No crossing over between two genes
Produces parental nonrecombinant gametes
What is crossing over?
-Occurs between two nonsister chromatids
-Both parental and recombinant (crossover) gametes are produced
What is the linkage ratio?
Complete linkage between two genes due to close proximity
-Results in unique F2 phenotypic ration results
What is a linkage group?
-Genes on the same chromosome are part of a linkage group
-Number of linkage groups should correspond to haploid number of chromosomes
Chiasmata
-Synapsed chromosomes in meiosis wrap around each other
-CHIASMATA: the point at which two non homolog pairs touch when crossing over
-X shaped intersections with points of overlap
-Points of genetic exchange
The percentage of offspring resulting from recombinant gametes depends on distance between two genes on ________ chromosome
(same or different?)
Same
Two genes that are located close to each other along a chromosome are less likely to have a ____________
Chiasma
Sturtevant (this is the name of the scientist dude)
-He compiled data from crosses
-Recombination frequencies between linked genes are ADDITIVE
Frequencies of exchange is an estimate of relative ….
distance between two genes
Map unit (mu)
1 percent recombination between two genes on a chromosome
Also called the centi-Morgan (cM)
RELATIVE distances, not EXACT
Single Crossover (SCO)
-Occurs between two nonsister chromatids
-Recombination is observed in 50 percent of gametes
-In genes that are 50 cM apart, crossing over can be expected between 100 percent of tetrad
When a single crossover event takes place in the tetrad stage, what gametes are produced?
-Two noncrossover (parental) gametes are produced
-two crossover (recombinant) gametes are produced
What are single crossovers used to determine?
the distance between two linked genes
What are double corssovers, and what do they help determine?
-It is the double exchanges of genetic material
-Used to determine the distance between THREE linked genes
-In this case, genes must be heterozygous for two alleles
What are the three criteria for three-point mapping?
-Parent must be heterozygous for all three genes under consideration
-Phenotypic class must reflect the genotype of gametes of parents
-Sufficient number of offspring must be produced for a representative sample
What is a Three-point cross?
-Determining the distance and order of three genes on one chromosome (doing it at three genes at a time)
Noncrossover F2 phenotypes occurs…
in the greatest proportion of offspring
Double crossover (DCO) phenotypes occurs…
in the smallest proportion
Reciprocal Classes of phenotypes
-F2 phenotypes compliment each other
-Derived from heterozygote
-Have wild type and mutant for all three genes
??
Determining gene sequence: two methods?
Method 1: based in the three possible arrangements of genes
Method 2: uses three possible arrangements as basis, BUT it also considers the double-crossover event
There are a large number of mutants in organisms like…
These organisms allow for ….
-Drosophila
-Maize
-Mice
Allows for the construction of extensive chromosome mapping
What are DNA markers ?
What are the earliest examples of DNA markers?
-Short segments of DNA with known sequence and location
-Useful landmarks for mapping
-Earliest examples of DNA markers:
-RFLPs and microsatellites
Another example: Single-nucleotide polymorphisms
What are RFLPs (what does it stand for, etc.)?
-Restriction fragment length polymorphisms
-polymorphic sites
-generated when specific DNA sequences are recognized and cut by restriction enzyme
What are microsatellites?
-Short repetitive sequences
-Found throughout the genome
What are single-nucleotide polymorphisms? provide an example of what they screen?
-Found throughout the genome
-Used by geneticists to identify and locate related genes
-Used to screen for diseases
Example: Cystic fibrosis
What is cystic fibrosis? Give a description and relate it to DNA markers
-Gene located by using DNA markers
-Life-shortening autosomal recessive exocirne disorder
-Gene causing disorder found on chromosome 7
During which phase in meiosis does crossing over take place in?
Prophase I of meiosis
