LECTURE 4: Linkage and Recombination Flashcards
WHAT is the cornerstone in UNDERSTANDING genetic phenomena
and what does it presupposes?
over time, what new information says about this?
Mendelian Laws
“each gene pair resides in a pair of homologous chromosome”
– the number of genes per species exceeds the number of homologous chromosomes in that species
therefore, each chromosome bears MANY GENES
ALLELIC vs NON-ALLELIC gene
Allelic genes:
- genes that share the same locus on a chromosome
Non-allelic genes:
- different locations on a chromosome, but still affect the same phenotyping
Linkage Group
- made of?
- number of linkage groups?
- all the genes carried in a chromosome
- number of linkage groups = a species’ haploid chromosome number
LINKAGE
- the _______ of ______ genes in the ______ chromosome
- the ______ of ____ to be ______
2 types
- the physical association of non-allelic genes in the same chromosome
- the tendency of genes to be inherited together
- Complete Linkage
- Incomplete Linkage
Independent Segregation
When two __________ are located in two _____________, they are said to be _____________
+ what genotypic ratio of progeny of AaBb parent undergoing testcross
** no linkage
When two DIFFERENT GENES are located in two DIFFERENT HOMOLOGOUS CHROMOSOMES, they are said to be INDEPENDENTLY SEGREGATING
*this means 2 different genes from 2 different PAIRS of homologous chromsomes
1 AaBb: 1 aabb: 1 Aabb: 1aaBb
50% parental types
50% recombinant types
Complete Linkage
- definition
- how is it written
+ what genotypic ratio of progeny of AaBb parent undergoing testcross
When two genes are very close together on the same chromosome, they are completely linked and do not undergo recombination
- for example: EeFf is written as EF(taas)―ef(baba)
1 AaBb: 1 aabb
100% parental types
Incomplete Linkage
- definition
- how is it written
+ what genotypic ratio of progeny of AaBb parent undergoing testcross
When two genes are on the same chromosome but farther apart, crossing over can occur between them, producing some recombinant gametes (less than 50%)
- for example: EeFf is written as EF(taas)―ef(baba)
20% AaBb: 20% aabb: 10% Aabb: 10% aaBb
80% parental types
20% recombinant types
In incomplete linkage, why are its recombinant types significantly less than 50%?
kasi once umabot ng 50%, indepdnent segregation na
does CROSSING OVER in complete linkage occur?
no
does CROSSING OVER in incomplete linkage occur?
yes
special case of linkage?
sex linkage
In humans, _____ is hemizygous for ___-linked genes
In other animals like ___, _____ is hemizygous for ___-linked genes
- Male
X-linked - Females
Z-linked
What is hemizygous?
having only one copy of a gene or chromosome, instead of the usual two
In birds, what is the genotype of male and female?
male: ZZ
female: ZW
if the no. of genes exceeds the no. of chromosomes, what is present?
linkage
LINKAGE
- based on what phenotype system?
- created by ? in what year?
- what experiment was done?
- what theory was confirmed
- wild-type and mutant phenotype system
- Thomas Hunt Morgan (1910)
- X-linked traits in Drosophila
- confirmed the Chromosome Theory
LINKAGE IN DROSOPHILA
Wild-type vs Mutant
+ how are alleles written?
WT = phenotype most commonly observed (dominant)
M = phenotype rarely observed (recessive)
alleles are written using small letters from MUTANT
wild-type is denoted with +
E.G.
m+ (wild-type)
m (mutant)
LINKAGE IN DROSOPHILA
- what phenotype had an X-linked trait?
- F2 phenotypic ratio
wild-type = red
mutant = white
2 red-eyed female: 1 red-eyed male: 1 white-eyed male
SEX-LINKED GENES
- genes located in the??
- 2 types
- sex chromosomes
- X-linked genes - located in the X chromosome
- Y-linked genes - located in the y chromosome
X-LINKED INHERITANCE
- states that?
X-linked genes can be dominant or recessive
Y -LINKED INHERTICANCE
- states that?
- example
- states that y-linked traits are HOLANDRIC (affected fathers will pass to affected sons)
HYPERTRICHOSES - excessive hair growth in ears
2 types of configuration in LINKAGE
- cis configuration (a+b+c+ / abc)
- trans configuration (a+bc+ / ab+c)
CROSSING OVER vs GENETIC RECOMBINATION
CO = process during prophase 1 wherein non-sister chromatids of homologous chromsomes exchange DNA segments
GR = production of new combination of traits/alleles not found on the parents
what is the % of the RECOMBINANT OFFSPRING
+ formula
RECOMBINATION FREQUENCY
(recombinants / total offspring) x 100
GENETIC MAPS VS LINKAGE MAPS
Genetic maps - ordered lists of genes along particular chromosome
Linkage maps - genetic maps constructed from recombination frequency data.
- 3 students of morgan in what year?
- studied the phenomenon of?
- studied the use of ___ as tool for?
- used WHAT as a measure of distance between genes in drosophila?
- reason for ^
- 3 units of measurement used?
Alfred Sturtevant
Herman Muller
Calvin Bridges
(1915)
- phenomenon of crossing over
- used linkage as tool for chromosome mapping
- recombination frequency (% recombinant)
- recom freq is dependent on the distance of genes in a chromosome
- 1 map unit = 1% recombination = 1 cM (centiMorgan)
STEPS in LINKAGE MAPPING
- Test cross (heterozygous x homozygous recessive)
- Identify parentals and double cross overs (DCO)
- Determine gene order/sequence
- Find SCO1 and SCO2
- Compute for the distance between genes (solve for CO1 and CO2)
Purpose of test cross in linkage mapping?
to determine if linkage is present and what type if ever
KEY NOTE for identifying parents and double cross overs
parentals = 2 highest phenotypic data (most frequent)
DCO = 2 lowest phenotypic data (least frequent)
How to determine gene order/sequence
- Identify the 2 genes that are side-by-side in all parentals and DCOs
- Make the isolated gene be the middle gene
- If no. 1 does not happen, the gene order is already correct and does not need to be revised
2 PARAMETERS related to linkage?
- coefficient of coincidence (cc)
- measure of strength of linkage (or interference in linkage) - Interference
- measure of how strong a crossover in 1 region interferes w crossover in an adjacent region
wherein
i = 1 : complete interference (stronger linkage/genes are nearer)
i = 0 : no interference (weaker linkage/genes are farther)
Interpretation of INTERFERENCE in terms of occurrence of DCO
If I = 0, then DCOs are occurring in the population as expected.
If I = 1, then DCOs are not occurring in the population as expected
FORMULAS
- I
- cc
- DCO
- SCO 1
- SCO 2
- Parentals
- CO1
- CO2
I = 1 - cc
cc = (DCO / N) / [(CO1)(CO2)]
DCO = cc x CO1 x CO2 x N
SCO 1 = (CO1 * x N) - DCO
SCO 2 = (CO2 * x N) - DCO
CO1 = [(total SCO1 + total DCO) / N ] x 100
CO2 = [(total SCO2 + total DCO) / N ] x 100
*should not be times to 100
if genes are closer to _____, there is LESS CHANCE for crossing over to occur
centromere
How to determine:
a——b———-c
- cross over between a and b
- cross over between b and c
- cross over between a and c
- SCO 1
- SCO 2
- DCO
