8: genetic linkage & mapping in eukaryotes

thomas morgan

identified white-eyed mutants & showed it’s a sex-linked trait

alfred sturtevant

• drosophila notation

• generated 1st linkage maps

drosophila notation

homologous chromosome pairing/alignment @metaphase

• y chromosome @anaphase

bateson & punnett

• didn’t observe 9:3:3:1 ratio in dihybrid crosses

• nonindependent assortment

linked genes

• genes on same chromosome don’t sort independently

• travel together in meiosis

• passed onto same gamete

chiasma

salamander chromosomes —> showed chromosome exchange NOT genetic exchange

• evidence of crossover

sturtevant’s concepts

• proposed “crossing over” to describe physical exchange leading to recombination

• determined genes are arranged on chromosome in linear fashion

recombination frequency (cM) =

part (recombinant)/whole

map distance =

% recombination

two point mapping

determine separation of 2 loci on chromosome

three point mapping

ordering of genes

underestimated

distances longer than 10 map units

if one parents is homozygous recessive, calculations are based on the gametes produced by…

the other parent

no crossover during meiosis in male flies due to…

lack of synapse during prophase 1

single crossover

may not occur b/w 2 genes if they are closely linked

double crossover

more than 1 crossover is possible over larger distances

• can measure more accurate map distances

3 point cross: unlinked

8 gametes —> all equally represented due to independent assortment

3 point cross: 2 linked

8 gametes —> NOT equally represented

4 —> parental types

4 —> recombinant types

3 point cross: 3 linked

8 gametes —> NOT equally represented

2 —> parental types

4 —> single crossover recombinant (intermediate)

2 —> double cross recombinant (smallest)

double crossovers are…

NOT always independent

interference (I)

a crossover may influence another crossover nearby

coefficient of coincidence =

observed DCO frequency / expected DCO frequency

observed DCO frequency =

observed DCO / total progeny

expected DCO frequency =

MD R1/100 x MD R2/100

interference =

1 - CoC

if interference is complete & no double crossover…

I = 1.0, mean CoC = 0

CoC > 1

negative interference (more DCO than expected)

CoC < 1

positive interference (less DCO than expected)

CoC = 0

complete interference (no DCO)

CoC = 1

no interference (expectations met)

reciprocal pairs

each gamete type has a reciprocal or inverse partner that is formed during meiosis

• occur @ equal frequencies

order of genes: 3 linked genes

• 2 genes found in both parentals & DCO —> outer 2 genes

• if gene flip —> middle gene

order of genes: 2 linked genes

• genes unlinked if one column is covered up and see 4 dif combinations

• genes linked if one column is covered and see 2 combinations