Chapter 7 Flashcards
_______ student of T.H Morgan, generated the first map of chromosome based recombination frequency. He postulated that frequency of recombination could be used to determine the physical distance separating the genes on a chromosome
- Alfred Henry Sturtevant
______ suggested that genes on same chromosomes segregated together, also those closely linked were rarely subject to recombination
- T.H Morgan
Principle of segregation
- each diploid individual possesses 2 alleles that separate during meiosis, one allele to each gamete
- each allele is at the same position on a homologous chromosome
Principle of Independent assortment
- two alleles separate independently of alleles at other loci
Chromosome theory of heredity
- Sutton, genes are found on chromosomes
Linked Genes
- those that are close together on the same chromosome travel together during meiosis and do not usually sort independently
What is responsible for recombination?
- the switching of genes from one homologous chromosome to another
- genes that are normally linked now sort independently
Single Crossover leads to what
- half recombinant half nonrecombinant
Crossing over can occur in in what type of chromosomes?
- nonsister/nonhomologous chromatids
- if occurred between sister chromatids no genetic difference would occur in products
__________ maize, cytological markers and 2 linked genes. Showed crossing over results from a physical change between homologs
- Harriet Creighton and Barbara McClintock
Recombination frequency formula
=(# of recom progency)/(total #of progency) x100
Coupling (cis)
- wild type alleles on one chromosome/mutant on other
Repulsion(trans)
- each chromosome has one wt and one mutant
If genes are completely linked (no crossing over) what results?
- only nonrecombinant are produced
If genes assort independently what results?
- 1/2 are recombinant, 1/2 nonrecombinant
With linked genes and some crossing over what is predominant?
- nonrecombinant
Independent Assortment allows genes to be
- on different chromosomes, combine randomly
Completely linked
- on same chromosome, close together so crossing over is rare
Incomplete linkage
- genes on same chromosome, but some distance apart allows for some recombination
Interchromosomal recombination
- between genes on different chromosomes, arises from independent assortment on sister chromatids anaphase 1 of meiosis
Intrachromosomal recombination
- between genes on the same chromosome, crossing over between nonsister chromatids in prophase 1 of meiosis
Genetic maps
- use recombination frequency to make chromosome maps
- distances are in terms of map units (1% of recomb freq, or centimorgans cM)
Gene mapping with recomb frequency rules
- recomb freq cannot be > 50% for two genes
- 50% is the freq of recombination for genes on different chromosomes, so impossible to discern between two genes that are far apart on same chromosome and those on different chromosomes
- genes that are far apart can undergo DOUBLE crossovers (its as if the crossover never occured)
Two point test cross
- test cross between 2 genes
Linkage groups
- distant markers can be tied together by intermediate markers
- intermediate markers allow a linkage group to span greater than 50cM distance
Why are three point crosses better than 2 point crosses?
- order of genes can be established in a single cross
2. double crossovers can be detected
What does a double crossover yield?
- recombinant chromosomes with altered middle genes
what is a map unit equal to
- 1% recombination
- so if 10%=10 map units
Nonrecombinants are the ____ numerous
- most
Double crossovers are the ____ numerous
- least
How do we compare observed numbers to expected ?
- the probability of double cross occurring can be calculated by the probability of two single crossovers multiplied together
Exp=(RF1xRF2) x total # observations
-decimals not percents
Interference
- when the incidence of a crossover affects another crossover
Negative interference
- increases the chances of another crossover
- More observed DOCS occurred than expected
Positive interference
- reduces the chances of another crossover taking place
- less observed DOCs occurred than expected
coefficient of coincidence formula
COC= #obs crossovers/# expected crossovers
Interference formula
Intf=1-COC x100
- answer means that whatever % you get of double crossover progeny expected will NOT be observed
If interference is 1, how many double crossovers will you get?
- zero crossovers
Recombination hot spots
- areas where recombination occurs more than expected
- more recombination in females bc no crossing over occurs in males
Restriction fragment length polymorphism (RFLP)
- changes in DNA sequence that modify restriction enzyme recognition sites
Variable number of tandem repeats (VNTR)
- differences in copy number
- SSLP, minisatellite, microsatellite
Single Nucleotide polymorphism (SNP)
- a single base change
Types of molecular markers
- Restriction fragment length polymorphism (RFLP)
- Variable number of tandem repeats (VNTR)
- Single Nucleotide polymorphism (SNP)
Microsatellite markers
- di, tri, tetra nucleotide repeats
- polymorphic repeats of 2-4 base pairs
- codominant
If two chromosome genes are completely linked what is the expected F2 phenotype? genotype?
- 3:1 phenotype
- 1:1 genotype
If independent assortment occurs what will be the phenotypic ratio?
- 1:1
