Linkage and meiotic 

recombination 

-Genes linked together on the same chromosome usually assort together.

-linked genes may become seperated through meiotic recombination

-the frequency with which genes become seperated (recombine) reflects the physical distance between them 

(longer distance, greater recombination)

-Rarely, recombination occurs during mitosis

-In eukaryotes mitotic recombination produces genetic mosaics

Genes of different chromosomes

Ratio 1:1:1:1 gametes 

(Ch 5 pt1 slide 3)

two genes on same chromosome segregate together

(see Ch5 pt1 slide 4)

Lead to separation of linked genes

(see ch5 pt1 slide 5)

-in dihybrid crosses, departures from a 1:1:1:1 ratio of f1 gametes indicate that the two genes are on the same chromosome

X²=∑(observed-expected)²/expected

-measures 'goodness of fit' between observed and predicted results

-pinpoints probability that ratios are evidence of linkage

-ratios alone will never allow you to determin if observed data are significantly different from predicted values. 

-larger the sample, the closer your observed values areexpected to match predicted values. 

-observed values are not different from the expected values

-for linkage studies, no linkage is null hypothesis

-observed values are different from expected values

-for linkage studies, genes are linked

regions in which nonsister chromatids of homologous chromosomes cross over each other

-observed by frans jansens 1909

-thomas hunt morgan suggested these were site of chromosome breakage and change resulting in genetic recombination

for pairs of genes reflect distance between them

1 RF = 1 map unit

crossing over

chiasma are visible signs of crossing over!

Mapping and Locating genes:

Two point crosses

comparisons help establish relative gene positions

-genes arranged on a line along chromosome. 

-difficult to determin gene order if two genes are close together

-actual distance between genes do not always add up

-pairwise crosses are time and labor consuming

-faster method to map genes

-pr must be in the middle because longest distaance is between vg and b

see (ch5 pt1 slide 27)

Measuring Interferences:

coefficient of coincidence =

ratio between actual frequency and double crossover and expected frequency of double cross over

interference= 1 - coefficient of coincidence

-Order of genes is correctly predicted by physical maps

-distance between genes is not always similar on physical maps

-mapping functions compensate for inaccuracies but are often imprecise

-order of genes in a genetic map is the same is the order of those same genes along the DNA molecule of a chromosome.

-physical distance (amount of DNA spersting genes) is not always the same as the genetic distance between genes

-genetic distance between genes correspong to different physical distance between organisms

(human - 1 m.u.= million bp, yeast - 1 m.u. = 2500 bp)

-all four haploid products of meiosis are contained in 'ascus'

-tetrads can be characterized by the number of parental and recombinant spores they contain. 2 genes 2 chromosomes.

Parental Ditypes (PD) outnumbering (NPD)

(see ch5 pt2 slide 24)

       RF=NPD+(^1/₂ T)   x100

    total terads