genetic counselling and linkage

Question 1

preimplantation test and prenatal test

Answer 1

• select disease free embryos for implantation

- detect potential genetic disease in foetus

Question 2

screening test and pre-symptomatic test

Answer 2

• population-based test, providing a personalised risk assessment
• individual with no symptoms but has family history

Question 3

carrier test and diagnostic test

Answer 3

• detects genetic variant which increases likelihood offspring will have a disease
• confirm in symptomatic patient a suspected diagnosis

Question 4

societal implications of genetic counselling

Answer 4

• availability
• cost
• education to public
• privacy laws

Question 5

research merit and integrity

Answer 5

potential benefit, well designed, follows principles of research conduct

Question 6

justice

Answer 6

fair recruitment, and distribution and access of benefits

Question 7

beneficence

Answer 7

likely benefit must outweigh the harm

non-maleficence = avoid doing harm

Question 8

respect

Answer 8

autonomy of participants

respect privacy, confidentiality and cultural beliefs

Question 9

what is genetic counselling

Answer 9

1. interpretation of family and medical history
2. education about inheritance, testing, management, prevention
3. counselling to promote informed choices

Question 10

when is genetic counselling needed

Answer 10

• diagnosing
• risk determination of carriers
• plan gestation in carriers
• if prenatal tests reveal abnormality or risk

Question 11

why generate genetic maps

Answer 11

• determine whether mutations affect different genes
• clone genes and their map positions
• predict inheritance patterns.
• genome sequencing projects

Question 12

linkage with no exchange

Answer 12

ratio of 3A_B_:1aabb

Question 13

linkage with exchange

Answer 13

linkage on a single pair of homologs with exchanging occurring between non-sister chromatids.
the linked genes can be separated by recombination, shuffling alleles.
depends on how often recombination occurs and how close the genes are

Question 14

coupling phase

Answer 14

cis. 2 dominant alleles of different genes on same homolog

Question 15

repulsion phase

Answer 15

trans. 2 2 dominant alleles of different genes do not stay together. heterozygous on each homolog

Question 16

recombination frequency

Answer 16

proportion of recombination gametes depending on how often the cross overs occurs
RF = (no. of recombinants / total progeny) x100

Question 17

map distance

Answer 17

percentage of recombinant offspring correlated with the distance between two genes. map units mu or centiMorgan cM.

Question 18

mapping human genes

Answer 18

mapping genes is difficult with small number of progeny. analysis to pedigree where the data from pedigrees can be combined using LOD (logarithm of odds) scores

Question 19

interference

Answer 19

interference between double cross-overs can occur, where one cross-over interferes with another occurring nearby, therefor reducing the chance of recombination nearby.

Question 20

calculating interference

Answer 20

calculate expected number of cross-overs
- chance of c.o. of the two inner genes and multiply them together x the progeny
- add up observed cross overs
- observed/ expected = coefficient of coincidence
Interference = 1- C

Question 21

positive interference

Answer 21

less double crossovers than expected I>0, C<1

Question 22

negative interference

Answer 22

more double crossovers than expected I<0, C>1

Question 23

mitotic recombination

Answer 23

in mitosis homolog chromosomes do not pair to form bivalence, therefore cross over do not occur. however can occur producing new combinations of alleles. giving rise to a path of tissue with characteristics different to the rest of the mitotic clones

Question 24

physical exchange between chromosomes

Answer 24

recombination combinations can be demonstrated by markers so that they can be distinguished cytologically.

Question 25

intragenic recombination

Answer 25

recombinations can occur within genes

e.g. lozenge eye. trans-heterozygote (2 different mutations in same gene) for 2 alleles with mutant phenotype