GENE LINKAGE Flashcards
WHAT IS GENE LINKAGE?
the genes are located on the same chromosome and their alleles have a tendency to be inherited together
-exception to Mendelian genetics (against independent assortment)
-always more genes than chromosomes, therefore, not all genes undergo independent assortment
DEFINE HAPLOTYPE
combination of alleles which are inherited together
-every human has 2 haplotypes of any autosomal region (1 from mother, 1 from water)
-a whole single haplotype is always passed to gamete
EG: example - Y chromosome or MHC complex on 6p21.3 (codominant gene in complete linkage)
-men are hemizygotes for characteristics coded on non-homologous regions of Y
chromosomes
-Y-haplotype is identical from father to son, which is used is genealogic studies à we
can examine the inheritance of Y chromosome through centuries
EG: MHC complex on chromosome 6 (6p21.3)
-divided into 3 classes
-very polymorphic
-codes for glycocalyx recognizing non-self structures
WHAT HAPPENS DURING COMPLEX LINKAGE?
genes are so close to each other that they dont segregate during crossing over
-only combinations of alleles which are also present in parents
WHAT ARE THE POSSIBILITIES OF P IN A DIHYDRIDE CROSS?
- cis (coupling) AABB x aabb
- trans (repulsion) AAbb x aaBB
EXPLAIN THE F1 GENERATION OF A DIHYDRIDE CROSS?
form only two types of gametes with non-recombined alleles
-1:1
EXPLAIN THE F2 GENERATION OF A DIHYDRIDE CROSS?
genotypic ratio 1:2:1
phenotypic ratio is 3:1 or 1:2:1
EXPLAIN THE RESULT OF A DIHYDRIDE BACKCROSS?
always having a phenotypic and genotypic ration of 1:1 = LINKAGE
- F1 (heterozygote) x recessive homozygote: AaBb x aabb -> phenotypic and genotypic ratio is the same
WHAT IS INCOMPLETE LINKAGE?
individual genes are on the same chromosomes but so far apart that they can be separated through recombination / crossing over
-maximal probability of recombination is 50%
-frequency of crossing-over is directly proportional to the distance between genes
WHAT IS THE PURPOSE OF CROSSING OVER DURING PROPHASE 1?
gametes with a set of alleles that are in gene linkage, and which is different from the parental set, are created when there have occurred a recombination between the alleles
DISCUSS THE MONOHYBRID PATTERN OF LINKED GENES
HOW DOES F1 OF DIHYBRID IMCOMPLETE LINKAGE LOOK?
HOW DOES F2 OF DIHYBRID INCOMPLETE LINKAGE LOOOK?
WHAT ARE THE POSSIBLE PARENTAL AND RECOMBINANTS OF DIHYBRID CROSS?
IS THERE A DIFFERENCE BETWEEN CIS AND TRANS RATIOS IN B1?
yes
INCOMPLETE LINKAGE
HOW IS GENE STRENGTH MEASURED?
based on the number of recombinant and non-recombinant offspring in the analytical (test) backcross – B1
WHAT IS MORGANS NUMBER?
p = recombinant frequency / whole population
p = 0 (complete linkage)
p = 0.5 (free assortment)
1cM = 1% frequency of recombination
WHAT IS BATESONS NUMBER?
measures how many times more probable it is for the formation of non-recombinant gametes when compared to recombinant ones
c = non recombinant / recombinant
always = 1 (independent assortment) < c < infinity (complete linkage)
IS IT POSSIBLE TO HAVE MORE RECOMBINANT GAMETES THAN PARENTAL GAMETES?
no
there are always more non-recombined gametes than recombined
WHAT ARE THE FACTORS THAT AFFECT GENE LINKAGE STRENGTH?
1- multiple crossing over
2 - interference
3 - absolute linkage
4 - pseudoallelism
WHAT IS THE RATIO OF A B1 CROSS THAT HAS NO LINKAGE?
1:1:1:1
HOW MANY cM EQUALS COMPLETE LINKAGE?
0
HOW MANY cM EQUALS NO LINKAGE?
50
HOW MANY cM EQUALS INCOMPLETE LINKAGE?
any other number
