Genetic Linkage Flashcards
What is Mendel’s principle of segregation?
… states that for a diploid individual. the two alleles at a particular gene locus separate during meiosis and up in different gametes.
What is Mende’s principle of independent assortment?
… states that segregation of alleles at a particular genes locus occurs independently of the segregation of alleles at other loci.
What are the conditions for Mendel’s principle of independent assortment?
a) the genes in question are on separate chromosomes.
b) the genes are widely separated on the same chromosome.
Mendel’s second rule DOES NOT apply to many dihybrid crosses.
Where are genes located?
On chromosomes
Do homologous pairs segregate independently of other homologous pairs?
Yes!
How can Mendel’s principle of independent assortment be violated?
When genes are present on the same chromosomes ,i.e., they show a strong tendency to stay together and therefore co-inherited.
What are linked genes?
Genes locates close together on the same chromosome
What is a linkage group?
All of the genes on a single chromosome. They do not assort independently but tend to be co-inherited belong to a linkage group.
(rather than independently assorting during cell division, they move as a unit)
What are Syntenic genes?
Genes that reside on the same chromosomes
What is Linked Gene Inheritance?
Genes that DO NOT assort independently.
(Syntenic genes show linked gene inheritance)
How does Recombination occur?
(Hint: from P to F1)
P generation: AABB x aabb
(gamete formation)
Gametes AB ab
Fertilization
F1 generation AaBb
(gamete formation)
Gametes AB ab Ab aB
(parental combination (new combination
-nonrecombinant -recombinant
alleles) alleles)
What is the expected phenotypic ratio in F2 progeny after selfing the F1 generation in a dihybrid cross for a digenic trait? Is this seen in linkage?
The expected phenotypic ratio is- 9:3:3:1
This is NOT seen in linkage/ non-independent assortment of digenic traits.
Instead there is a predominance of progeny showing the parental type.
What are recombinant gametes?
New combinations of alleles that are not seen in the parental gametes.
What are non-recombinant gametes?
Original combinations of alleles that are found in the parental gametes.
What is the expected test cross ratio in a dihybrid cross for a digenic trait? Is this seen in linkage?
The expected test cross ratio is- 1:1:1:1
This is NOT seen in the linkage/ non-independent assortment of digenic traits.
Where does crossing over occur?
Prophase I of Meiosis I
Would crossing over occur in linkage/ do linked genes undergo crossing over?
NO! This is because the genes have to be close together on the SAME chromosome for them to be linked!
If the genes are on different chromosomes or far apart on the same chromosome they are NOT linked! Therefore, crossing over in homologous chromosomes does not occur in linked genes.
Where does Independent Assortment occur
Metaphase I of Meiosis I
AND
Metaphase II of Meiosis II
Would independent assortment occur in linkage/ do linked genes undergo independent assortment?
NO! This is because unlinked genes are the only kind of genes that can be independently assorted as they are either on different chromosomes or far apart on the same chromosome.
*accounts for genetic variation
How do we know if the genes were independently assorted?
Test cross= 1:1:1:1
(AaBb x aabb)
What is the chiasma?
The structure that forms between homologous chromosomes by crossover recombination and physically links the homologous chromosomes during meiosis.
What is Chiasma Frequency?
The frequency of chiasmata formation between two genes of interest during meiosis
What is Recombination Frequency?
The frequency of recombinant type gametes formed
Recombination Freq.= 1/2 (chiasma freq.)
Why is the Recombinant Frequency half the chiasma frequency?
Every time a chiasma forms only two of the four gametes will be recombinant types.
What is Linkage Notation?
How genes in homologous chromosomes are written.
When genes are linked, only one slash is used- NB: the slash stands for a pair of homologous chromosomes.
AB/AB or ab/ab or AB/ab
NB: Genes in coupling would have the dominant genes together on one side of the slash and recessives on the other side (AB/ab). Repulsion would represent the other arrangement (Ab/aB).
What are the types of Linkage?
Complete Linkage
AND
Incomplete Linkage
What is Complete Linkage?
Occurs between two genes when the genes are very close together on the same chromosome.
All gametes will contain original parental combinations of alleles, i.e., there will be no recombinants.
What is Incomplete Linkage?
Occurs when the association between two linked genes can be broken or disrupted so that the genes have the ability to assort independently.
This disruption is caused by crossing over between homologous chromosomes and produces recombinants
What are the Limits of Recombination?
Case 1: Complete Linkage
The two genes are tightly linked in such a way that there will be no chiasma formation between them- the chiasma freq. and recombination freq.= 0
Only parental type gametes are formed
Testcross ratio = 1:1
What are the Limits of Recombination?
Case 2: Independent Assortment
The genes are far apart on the chromosome, so that chiasma formation is 100%.
Hence recombination freq.= 0.5 or 50%
PT (parent) = RT (recombinant)
Testcross ratio = 1:1:1:1
What are the Limits of Recombination?
Case 3: Incomplete Linkage
The linked genes are sufficiently apart to allow some chiasma formation between them
- r < 0.5 and PT > RT
Testcross ratio of 1:1:1:1 NOT obtained.
Results in a distorted ratio.
What is Coupling phase?
(cis- configuration)
When the two wild type alleles are on the same chromosome and the two mutant alleles are on the other
(AB/ab) (p+b+/pb)
Parents: AABB & aabb Parents: p+p+b+b+ & ppbb
A B p+ b+
x x
a b p b
F1
Parental: AB & ab Parental: p+b+ & pb
Recombinant: Ab & aB Recombinant: p+b & pb+
What is Repulsion phase?
(trans-configuration)
When each chromosome contains both a wild-type and mutant allele
(Ab/aB) (p+b/pb+)
Parents: AAbb & aaBB Parents: p+p+bb & ppb+b+
A b p+ b
x x
a B p b+
F1
Parental: Ab & aB Parental: p+b & pb+
Recombinant: AB & ab Recombinant: p+b+ & pb
How do you calculate the gametic frequency if Coupling and Repulsion Linkage?
Parental Gametes: (1-r)/2
Recombinant Gametes: r/2
What is the Recombination %?
RC% = (RT gametes/ Total) x 100
(in test crosses)
How do you calculate the frequency of genotype?
The frequency of genotype (aabb) = f(ab) x f(ab) = f(ab)^2
Therefore, f(ab)= ./ f(aabb)
(in dihybrid crosses)
