Genetic linkage

Question 1

What is linkage analysis?

Answer 1

A method that is used in establishing the carrier

status of female ‘at-risk’ and for prenatal diagnosis.

Question 2

What is the principle of independent assortment?

Answer 2

The alleles on different chromosomes are distributed randomly
(OR independently assorted) of each other to individual gametes. thus, genes are NOT linked

Question 3

What are recombinant gametes?

Answer 3

New combination of alleles generated

from recombination

Question 4

Define the coupling phase (cis)

Answer 4

when the dominant A and B alleles (test alleles/markers) are on the same chromosome
Linkage phase is written as: AB/ab

Question 5

Define repulsion phase (trans)

Answer 5

when the dominant A and B allele are on the homologous
chromosomes
Linkage phase is written as: Ab/aB

Question 6

How will the gametes be arranged if A and B alleles are in coupling phase?

Answer 6

Majority of the gametes will have either two dominant
alleles or two recessive alleles on the same
chromosome.

Question 7

How will the gametes be arranged if A and B alleles are in repulsion phase?

Answer 7

Gametes containing one dominant and one recessive

allele will be most abundant.

Question 8

How would knowing the linkage phase help a geneticist?

Answer 8

In clinical setting, knowing the linkage phase would allow geneticists to predict the risk of a couple to produce an “affected” child

Question 9

What is recombination frequency?

Answer 9

The proportion of recombinant gametes produced varies depending on how often crossovers occur between the two genes.

It can be calculated as No.of recombinant progeny/total progeny * 100

Question 10

What is Map distance?

Answer 10

This is the percentage of recombinant offspring produced based off the distance between two genes

Question 11

what is 1 map unit?

Answer 11

Distance between two loci that will generate 1% recombinant phenotype between them

E.g 12% recombination = 12mu

Question 12

What are the limitations of a 2 point test cross?

Answer 12

Difficult to determine gene order if two genes are too close or too
far apart
Actual distances between genes do not always add up (double
crossing-overs, COs)
Pairwise (test) crosses are time and labor consuming, not
amenable for all species

Question 13

What are the advantages of a 3 point tests cross?

Answer 13

Faster and more accurate way to map
genes

 Simultaneous analysis of three markers

 Information on the position of three genes
relative to each other can be obtained
from one mating rather than two
independent matings.

 Being able to include double crossovers
is the reason three point crosses are
more accurate than two point crosses